A Review of β-Ga2O3 Power Diodes

As the most stable phase of gallium oxide, β-Ga2O3 can enable high-quality, large-size, low-cost, and controllably doped wafers by the melt method. It also features a bandgap of 4.7–4.9 eV, a critical electric field strength of 8 MV/cm, and a Baliga’s figure of merit (BFOM) of up to 3444, which is 10 and 4 times higher than that of SiC and GaN, respectively, showing great potential for application in power devices. However, the lack of effective p-type Ga2O3 limits the development of bipolar devices. Most research has focused on unipolar devices, with breakthroughs in recent years. This review mainly summarizes the research progress fora different structures of β-Ga2O3 power diodes and gives a brief introduction to their thermal management and circuit applications.


Introduction
As technology advances, there is an increasing need for electronic devices capable of a high frequency, high voltage, and high power in various applications, such as AI, 5G communication, electric vehicles, rail transportation, new energy power generation, and power transmission [1][2][3][4].Si, being the most mature semiconductor material, is widely used in electronic manufacturing, but its narrow bandgap (1.12 eV) significantly limits its application in high-voltage fields.Si-based power devices are gradually reaching their performance limits but still cannot meet the growing demands for development [5][6][7].Consequently, wide-bandgap semiconductor materials, represented by SiC (3.3 eV) and GaN (3.4 eV), have progressively entered the spotlight.Owing to their larger bandgap, the device's maximum breakdown field strength can be improved, and their higher electron mobility and saturation velocity result in lower conduction losses and switching losses.Hence, wide-bandgap power semiconductor devices have witnessed a rapid development, serving to address the shortcomings of Si-based power devices in the field of high-power electronics [8,9].However, the production cost of SiC and GaN is significantly higher than that of Si wafers, as these materials require epitaxial growth methods to obtain high-quality crystals, which severely limits their large-scale application [10][11][12].
As a quintessential exemplar of ultra-wide-bandgap semiconductors, Ga 2 O 3 manifests in five polymorphic phase structures, encompassing the α, β, γ, δ, and ε phases.Among them, the β phase is endowed with thermodynamic stability.Utilizing melt growth methodologies, it is feasible to fabricate high-quality, large-scale, cost-effective, and dopingcontrollable β-Ga 2 O 3 wafers.Furthermore, several pivotal electrical properties of β-Ga 2 O 3 exhibit superiority over those inherent to SiC and GaN [13][14][15][16][17][18][19].Currently, 6-inch β-Ga 2 O 3 wafers have been successfully fabricated using the edge-defined film-fed growth (EFG) technique.The cost of β-Ga 2 O 3 wafers produced via the Czochralski (CZ) method amounts to only one-third of that for SiC wafers of the same size.Notably, the EFG method proves even more cost-effective, being two times less expensive than the CZ process, thus rendering the overall cost of β-Ga 2 O 3 substantially lower than that of SiC [20,21].Moreover, during Materials 2024, 17, 1870 2 of 35 the production of β-Ga 2 O 3 wafers, the cost and attrition rate of Ir crucibles constitute a significant proportion of expenditure.In April 2022, C&A Corporation (Sendai, Japan) succeeded in growing 2-inch β-Ga 2 O 3 single crystals using a Cu crucible CZ method, potentially reducing costs to one-hundredth of those associated with the EFG method, laying the foundation for the mass application of β-Ga 2 O 3 devices [22].Electrically, β-Ga 2 O 3 possesses a wider bandgap of 4.7-4.9eV and a critical electric field strength of 8 MV/cm, which is 27 times that of Si, 3 times that of SiC, and 2 times that of GaN.The Baliga's figure of merit (BFOM), a crucial parameter for evaluating the suitability of materials for power device fabrication, reaches 3444 for β-Ga 2 O 3 , which is 10 times that of SiC and 4 times that of GaN; this indicates that β-Ga 2 O 3 possesses greater potential for application in power devices [23][24][25][26].
Although β-Ga 2 O 3 offers the advantages of low cost and high performance, similar to other wide-bandgap semiconductor materials, it is challenging to obtain p-type Ga 2 O 3 with a high conductivity [27][28][29][30].Calculations suggest the absence of shallow acceptors in Ga 2 O 3 , and holes exhibit self-trapping effects within the material [31,32].Consequently, current research is primarily focused on unipolar Ga 2 O 3 power electronic devices, including field-effect transistors (FETs) and Schottky barrier diodes (SBDs) [33][34][35].Compared to traditional p-n junction diodes, SBDs exhibit lower turn-on voltages and faster recovery times, making them commonly employed in low-power and high-speed switching applications.However, the breakdown voltage (BV) of conventional Si-based SBDs is generally low, and the introduction of wide-bandgap materials can effectively elevate their breakdown limits.Since Sasaki et al. [36] first reported on β-Ga 2 O 3 SBDs in 2012, β-Ga 2 O 3 SBD devices have been developing towards a higher BV and lower on-resistance (R on ) through improvements in epitaxial layer quality, device structure enhancements, and optimized manufacturing processes.
In recent years, there has been a sharp increase in research publications on Ga 2 O 3 SBDs, leading to significant breakthroughs in device performance.Lateral β-Ga 2 O 3 SBDs have achieved a BV exceeding 10 kV [37,38], and vertical NiO/β-Ga 2 O 3 Schottky heterojunction diodes devices have reached a power figure of merit (PFOM = BV 2 /R on ) of 13.2 GW/cm 2 [39].Significant progress has also been made in commercial applications; in December 2021, Novel Crystal Technology Inc. (Saitama, Japan) announced the release of an ampere-level 1200 V β-Ga 2 O 3 trench SBD device [40].Therefore, it is essential to compile a summary of the current research progress on β-Ga 2 O 3 power SBD devices, as it may provide meaningful guidance for future developments in the field.
This article provides a comprehensive review of the research advancements in variously structured β-Ga 2 O 3 power SBDs up to the year 2024.Section 2 elucidates the material properties of β-Ga 2 O 3 , substrate preparation, and epitaxial processes; Section 3 discusses the metal/β-Ga 2 O 3 contacts, including ohmic and Schottky contacts; Section 4 presents the progress in research on differently structured β-Ga 2 O 3 power diodes, which encompasses vertical structure SBDs, heterojunction structure diodes, and lateral structure diodes; Section 5 offers a brief overview of the surge current robustness and thermal management of β-Ga 2 O 3 SBD devices; Section 6 summarizes the circuit applications and reliability of β-Ga 2 O 3 SBDs; and Section 7 concludes with a summary and prospects.
Materials 2024, 17, x FOR PEER REVIEW 3 of 34 γ-Ga2O3 is principally utilized as a photocatalyst for hydrogen production via water splitting [50][51][52][53].Current research on the cubic perovskite structure of δ-Ga2O3 is comparatively limited.The hexagonal structure of ε-Ga2O3, which exhibits spontaneous polarization, can form a high-density two-dimensional electron gas at heterojunction interfaces and holds the potential for fabricating high-electron-mobility transistors [54,55].Furthermore, there exists an orthorhombic κ-Ga2O3 with a very similar crystal-structure ε-Ga2O3, and they are usually considered as one structure [56,57].Recently, orthorhombic κ-Ga2O3 has been proven to be stable up to 800-1000 °C, and this makes it suitable for the fabrication of reliable devices such as detectors for UV and X-rays [58][59][60][61].Monoclinic-phase β-Ga2O3 belongs to the C2/m space group with a densely stacked anion structure.Its crystal structure and unit cell schematic are shown in Figure 2 [19,62].The unit cell is comprised of two GaO4 tetrahedra and two GaO6 octahedra, containing two different Ga atomic positions, as well as three distinct O atomic sites.The lattice constants of β-Ga2O3 are as follows: a = 12.21 Å, b = 3.03 Å, c = 5.79 Å, with α = γ = 90° and β = 103.8°.The length of the a-axis is four times that of the b-axis, and the length of the caxis is 1.9 times that of the b-axis, which facilitates cleavage along the (100) and (001) directions.Utilizing these crystallographic properties of β-Ga2O3, it is feasible to perform mechanical exfoliation to create quasi-two-dimensional thin-layer materials.This attribute endows β-Ga2O3 with significant potential in the domain of two-dimensional materials and devices [63][64][65][66].[62].[13].
Monoclinic-phase β-Ga 2 O 3 belongs to the C2/m space group with a densely stacked anion structure.Its crystal structure and unit cell schematic are shown in Figure 2 [19,62].The unit cell is comprised of two GaO 4 tetrahedra and two GaO 6 octahedra, containing two different Ga atomic positions, as well as three distinct O atomic sites.The lattice constants of β-Ga 2 O 3 are as follows: a = 12.21 Å, b = 3.03 Å, c = 5.79 Å, with α = γ = 90 • and β = 103.8• .The length of the a-axis is four times that of the b-axis, and the length of the c-axis is 1.9 times that of the b-axis, which facilitates cleavage along the (100) and (001) directions.Utilizing these crystallographic properties of β-Ga 2 O 3 , it is feasible to perform mechanical exfoliation to create quasi-two-dimensional thin-layer materials.This attribute endows β-Ga 2 O 3 with significant potential in the domain of two-dimensional materials and devices [63][64][65][66].
Materials 2024, 17, x FOR PEER REVIEW 3 of 34 γ-Ga2O3 is principally utilized as a photocatalyst for hydrogen production via water splitting [50][51][52][53].Current research on the cubic perovskite structure of δ-Ga2O3 is comparatively limited.The hexagonal structure of ε-Ga2O3, which exhibits spontaneous polarization, can form a high-density two-dimensional electron gas at heterojunction interfaces and holds the potential for fabricating high-electron-mobility transistors [54,55].Furthermore, there exists an orthorhombic κ-Ga2O3 with a very similar crystal-structure ε-Ga2O3, and they are usually considered as one structure [56,57].Recently, orthorhombic κ-Ga2O3 has been proven to be stable up to 800-1000 °C, and this makes it suitable for the fabrication of reliable devices such as detectors for UV and X-rays [58][59][60][61].The length of the a-axis is four times that of the b-axis, and the length of the caxis is 1.9 times that of the b-axis, which facilitates cleavage along the (100) and (001) directions.Utilizing these crystallographic properties of β-Ga2O3, it is feasible to perform mechanical exfoliation to create quasi-two-dimensional thin-layer materials.This attribute endows β-Ga2O3 with significant potential in the domain of two-dimensional materials and devices [63][64][65][66].The growth techniques for β-Ga 2 O 3 single-crystal substrates predominantly encompass the Czochralski method (CZ) [15,18,67], optical floating zone method (OFZ) [68][69][70], vertical Bridgman method (VB) [71][72][73], and edge-defined film-fed growth method (EFG) [17,[74][75][76][77], among which the EFG is the most mature, offering the potential for mass production and cost efficiency.In 2018, Japan's Novel Crystal Technology Inc. (Saitama, Japan) pioneered the fabrication of 6-inch β-Ga 2 O 3 single-crystal substrates using the EFG method.The company has also industrialized the production of 4-inch β-Ga 2 O 3 single-crystal substrates, currently holding an international leading position, thereby laying the foundation for the large-scale commercialization of gallium oxide devices.
As compared to the already implemented SiC and GaN, the β-Ga 2 O 3 material exhibits superior properties, as shown in Table 1 [19,96].The bandgap of β-Ga 2 O 3 ranges from 4.7 to 4.9 eV, with its ultraviolet absorption edge situated between 260 nm and 280 nm, precisely aligning with the solar-blind UV spectrum of 200 nm to 280 nm.This intrinsic property renders β-Ga 2 O 3 a rare and ideally suited material for solar-blind UV detection.Consequently, Ga 2 O 3 -based solar-blind UV photodetectors have been extensively investigated [97][98][99][100].Furthermore, β-Ga 2 O 3 features a maximum critical electric field strength of up to 8 MV/cm and a BFOM of 3444.This implies that β-Ga 2 O 3 devices have the theoretical capability to withstand a higher BV and operate under higher power conditions, making them more suitable for power device applications.Although diamond has superior properties in comparison, it is still far from being commercially viable, due to the limit of its large size and high-quality single crystal preparation, as well as its extremely high production and processing costs [101,102].Interestingly, diamond is capable of effective p-type doping but lacks n-type doping and possesses a very high thermal conductivity.Thus, the complementary formation of a heterojunction diode with β-Ga 2 O 3 can give full play to the advantages of both.A diamond/β-Ga 2 O 3 pn heterojunction diode with on/off ratios of greater than 10 8 at ±10 V and leakage currents of less than 10 −12 A has already been obtained by a direct bonding method [103,104].At present, Ga 2 O 3 materials still lack effective p-type doping, and research applications are basically n-Ga 2 O 3 , typically doping with elements like Si and Sn within a range of 10 15 to 10 19 cm −3 in concentration.In recent years, researchers have begun to address the shortfall of p-type Ga 2 O 3 by using p-type NiO to form a p-n heterojunction with Ga 2 O 3 , presenting a novel approach for the advancement of Ga 2 O 3 devices [39,105].Besides, the primary challenges associated with β-Ga 2 O 3 materials are their relatively low electron mobility and thermal conductivity.β-Ga 2 O 3 exhibits an electron mobility of 300 cm 2 /V•s compared to GaN, rendering it less ideal for the fabrication of high-frequency devices.However, its saturation electron velocity of 2 × 10 7 cm/s offsets this disadvantage [106,107].In response to potential heat dissipation issues due to the limited thermal conductivity of Ga 2 O 3 materials, numerous studies have proposed solutions, such as hetero-bonding, substrate thinning, and junction-side cooling, which will be elaborated in Section 5. Overall, Ga 2 O 3 devices possess considerable practical application value and hold broad prospects for utilization.

Metal/β-Ga 2 O 3 Contact
The fundamental structure of β-Ga 2 O 3 SBDs is composed of β-Ga 2 O 3 and an anodic Schottky contact, as well as a cathodic ohmic contact.The contact between metal and β-Ga 2 O 3 is the principal determinant of interfacial electron transport and significantly impacts the performance of the device.

Schottky Contact
When the metal and semiconductor are in contact, the diffusion of charge carriers equalizes the Fermi levels on both sides of the interface, reaching an equilibrium state.Consequently, after contact, the energy bands in the semiconductor become bent due to the built-in electric field, thereby forming an electron potential barrier at the interface, known as the Schottky barrier, as illustrated in Figure 3 [108].The Mott model posits that the height of the Schottky barrier is solely related to the metal work function and the electron affinity of the semiconductor, and the Schottky barrier height (SBH) is equal to the difference between the two [109].Experimentally, the SBH is commonly determined through current-voltage (I-V) or capacitance-voltage (C-V) measurements.Currently, there are many metals that are capable of forming Schottky contacts with β-Ga 2 O 3 , including Pt, Ir, Pd, Au, Ni, Co, Ru, Cu, Mo, W, Cr, Ti, Ag, etc.; graphite also forms Schottky contacts with β-Ga 2 O 3 .Among these, Ni is widely used, due to its low cost and ability to form excellent Schottky contacts with β-Ga 2 O 3 [110][111][112][113].At present, Ga2O3 materials still lack effective p-type doping, and resea applications are basically n-Ga2O3, typically doping with elements like Si and Sn with range of 10 15 to 10 19 cm −3 in concentration.In recent years, researchers have begun address the shortfall of p-type Ga2O3 by using p-type NiO to form a p-n heterojunc with Ga2O3, presenting a novel approach for the advancement of Ga2O3 devices [39,1 Besides, the primary challenges associated with β-Ga2O3 materials are their relatively electron mobility and thermal conductivity.β-Ga2O3 exhibits an electron mobility of cm 2 /V•s compared to GaN, rendering it less ideal for the fabrication of high-freque devices.However, its saturation electron velocity of 2 × 10 7 cm/s offsets this disadvant [106,107].In response to potential heat dissipation issues due to the limited ther conductivity of Ga2O3 materials, numerous studies have proposed solutions, such hetero-bonding, substrate thinning, and junction-side cooling, which will be elaborate Section 5. Overall, Ga2O3 devices possess considerable practical application value hold broad prospects for utilization.

Metal/β-Ga2O3 Contact
The fundamental structure of β-Ga2O3 SBDs is composed of β-Ga2O3 and an ano Schottky contact, as well as a cathodic ohmic contact.The contact between metal and Ga2O3 is the principal determinant of interfacial electron transport and significa impacts the performance of the device.

Schottky Contact
When the metal and semiconductor are in contact, the diffusion of charge carr equalizes the Fermi levels on both sides of the interface, reaching an equilibrium st Consequently, after contact, the energy bands in the semiconductor become bent du the built-in electric field, thereby forming an electron potential barrier at the interf known as the Schottky barrier, as illustrated in Figure 3 [108].The Mott model posits the height of the Schottky barrier is solely related to the metal work function and electron affinity of the semiconductor, and the Schottky barrier height (SBH) is equa the difference between the two [109].Experimentally, the SBH is commonly determi through current-voltage (I-V) or capacitance-voltage (C-V) measurements.Curren there are many metals that are capable of forming Schottky contacts with β-Ga including Pt, Ir, Pd, Au, Ni, Co, Ru, Cu, Mo, W, Cr, Ti, Ag, etc.; graphite also fo Schottky contacts with β-Ga2O3.Among these, Ni is widely used, due to its low cost ability to form excellent Schottky contacts with β-Ga2O3 [110][111][112][113].However, experimental results indicate that the SBH of β-Ga2O3 weakly depend the metal work function and is more influenced by interface states, as well as inter structures and interactions [114].The Bardeen model suggests that electron transfer f the semiconductor to the metal is mediated by interface states on the contact surf assuming a continuous distribution of surface states defined by a neutral energy level However, experimental results indicate that the SBH of β-Ga 2 O 3 weakly depends on the metal work function and is more influenced by interface states, as well as interface structures and interactions [114].The Bardeen model suggests that electron transfer from the semiconductor to the metal is mediated by interface states on the contact surface, assuming a continuous distribution of surface states defined by a neutral energy level Φ 0 , with the Fermi level position determined by the location of the surface states within the bandgap, which is the Fermi level pinning effect [115].Therefore, the surface states of β-Ga 2 O 3 also significantly affect device performance; experiments have found that treatments such as oxygen plasma and annealing, as well as chemical solution cleaning, can effectively reduce the density of surface states on β-Ga 2 O 3 , enhancing device performance, while etching and plasma bombardment may increase surface state density, leading to poorer device performance [116][117][118][119][120][121].
In practical applications, due to the amount of heat generated by high power, power devices often operate in high-temperature environments, and many studies have investigated the high-temperature performance of different Schottky metal/β-Ga 2 O 3 SBDs [122][123][124].Furthermore, given that metal oxides have a better high-temperature stability and higher SBH compared to metals, β-Ga 2 O 3 SBDs utilizing metal oxides as Schottky electrodes have also been the subject of extensive research [125][126][127].Hou et al. [128] reported that IrO x /β-Ga 2 O 3 Schottky contacts achieved a leakage current of only 2.3 × 10 −9 A/cm 2 (@-3 V) at a high temperature of 350 • C and just 7.5 × 10 −8 A/cm 2 at −100 V, with the rectification ratio exceeding 10 10 at all temperatures.Dela Cruz et al. [129] showed the outstanding high-temperature performance of Pt x Ir (1−x) O y on (201) β-Ga 2 O 3 , with a rectification ratio of 10 9 (±3 V) at 300 • C and 10 6 at 500 • C, demonstrating the application potential of metal oxide Schottky contacts in high-temperature devices.

Ohmic Contact
Typically, an excellent ohmic contact exhibits a low or even no SBH, manifesting a linear I-V curve that minimizes thermal effects caused by contact resistance.This is particularly crucial for β-Ga 2 O 3 , which has a low thermal conductivity.Hence, superior β-Ga 2 O 3 ohmic contacts are a prerequisite for achieving high-performance devices.Currently, almost all Ga 2 O 3 devices use Ti as the ohmic contact and deposit Au as a protective layer to avoid oxidation.The ohmic contacts can be effectively formed by heavy doping, plasma treatment, and post-annealing treatment [110][111][112][113].In 2012, Higashiwaki et al. [130] first reported a field-effect transistor based on β-Ga 2 O 3 , utilizing Ti/Au ohmic contacts, and highlighted the necessity of Reactive Ion Etching (RIE) processing for establishing ohmic contacts.Subsequently, the ohmic contact of Ti on β-Ga 2 O 3 was improved in β-Ga 2 O 3 MOSFETs devices through annealing at 470 • C for 1 min [131].Bhattacharyya et al. [132] reported a record low contact resistivity of 80 mΩ•mm for Ti metal on heavily doped (∼1.8 × 10 20 cm −3 ) β-Ga 2 O 3 , with a specific contact resistivity of 8.3 × 10 −7 Ω•cm 2 .Currently, the formation of high-quality Ti/β-Ga 2 O 3 ohmic contacts is commonly achieved using post-annealing treatment.

β-Ga 2 O 3 Power Diodes
In the last decade, β-Ga 2 O 3 power diodes have been extensively studied due to their exceptional electrical properties.To enhance device performance, researchers have proposed new device structures, which can be categorized into three main groups based on their structure type: vertical-structure SBDs, vertical heterojunction-structure diodes, and lateral-structure diodes.

Vertical-Structure SBDs
SBDs are unipolar devices that benefit from significantly reduced switching losses due to the absence of minority carrier storage effects.Initial β-Ga 2 O 3 SBDs exhibited excellent rectifying behavior, but the device BV was low due to the electric field concentration effect at the electrode edges.To address this, researchers have employed various structures to alleviate electric field concentration, thereby enhancing the BV and overall device performance.These structures include field plates (FPs), edge termination (ET), mesa termination (MT), and trench structure.Beyond the BV, device performance parameters also encompass ideality factors (n), on-state voltage (V on ), on-resistance (R on ), forward current (I F ), and reverse leakage current.Amongst these, the BV and R on , as well as their derived PFOM, are particularly crucial, reflecting the device's potential in power circuit applications.This section summarizes the development of devices with different vertical structures of β-Ga 2 O 3 SBDs.

Simple Structure
Early devices were fabricated on single-crystal substrates and exhibited good rectifying properties.However, their breakdown performance was significantly inferior to bipolar devices, as they relied only on the Schottky barrier formed by the metal/semiconductor contact to control the unidirectional transmission of carriers.To enhance the BV, power SBD structures have incorporated a lightly doped drift layer, as shown in Figure 4.In 2012, Sasaki et al. [36] grew, homogeneously and epitaxially, a 1.4 µm thick epitaxial layer on a β-Ga 2 O 3 substrate using MBE, fabricating for the first time a Pt/β-Ga 2 O 3 Schottky barrier diode with a reverse BV exceeding 100 V, a R on of 2 mΩ•cm 2 , and an I F of 200 A/cm 2 at 1.7 V.In 2015, Higashiwaki et al. [133] first reported a gallium oxide Schottky diode with a 7 µm thick Si-doped n-type drift layer (2.0 × 10 16 cm −3 ) epitaxially grown using the HVPE technique, featuring a R on of 2.4 mΩ•cm 2 , an ideality factor of 1.02, and a BV approaching 500 V, demonstrating the potential for future applications of β-Ga 2 O 3 -based power devices.
forward current (IF), and reverse leakage current.Amongst these, the BV and Ron, as well as their derived PFOM, are particularly crucial, reflecting the device's potential in power circuit applications.This section summarizes the development of devices with different vertical structures of β-Ga2O3 SBDs.

Simple Structure
Early devices were fabricated on single-crystal substrates and exhibited good rectifying properties.However, their breakdown performance was significantly inferior to bipolar devices, as they relied only on the Schottky barrier formed by the metal/semiconductor contact to control the unidirectional transmission of carriers.To enhance the BV, power SBD structures have incorporated a lightly doped drift layer, as shown in Figure 4.In 2012, Sasaki et al. [36] grew, homogeneously and epitaxially, a 1.4 µm thick epitaxial layer on a β-Ga2O3 substrate using MBE, fabricating for the first time a Pt/β-Ga2O3 Schottky barrier diode with a reverse BV exceeding 100 V, a Ron of 2 mΩ•cm 2 , and an IF of 200 A/cm 2 at 1.7 V.In 2015, Higashiwaki et al. [133] first reported a gallium oxide Schottky diode with a 7 µm thick Si-doped n-type drift layer (2.0 × 10 16 cm −3 ) epitaxially grown using the HVPE technique, featuring a Ron of 2.4 mΩ•cm 2 , an ideality factor of 1.02, and a BV approaching 500 V, demonstrating the potential for future applications of β-Ga2O3-based power devices.In addition, in high-power circuits there is not only a large voltage but also a large current.Compared with small-area devices, large-area devices are capable of handling higher currents with lower on-state voltage drops.However, the emergence of new materials and new processes inevitably faces the issue of defects; a larger electrode size also means more defects, which significantly impacts the breakdown performance of devices [134,135].In 2017, Yang et al. [136] fabricated a series of β-Ga2O3 SBDs with varying electrode sizes.Test results demonstrated that as the electrode size increased, the BV gradually decreased and the Ron also reduced.At an electrode diameter of 20 µm, the BV was 1600 V and the Ron was 25 mΩ•cm 2 , whereas at 0.53 mm diameter, the BV fell to only 250 V and the Ron to 1.6 mΩ•cm 2 .
The crystal structure of β-Ga2O3 leads to severe anisotropy, which has an important impact on the performance of the device.In 2017, Fu et al. [137] investigated the electrical properties of β-Ga2O3 SBDs on two distinct crystallographic planes, (2 01) and (010).Findings indicated that the (010) plane had more negative charges and defects on its surface, leading to a more pronounced upward bending of the conduction band.As a result, the Ron, Von, n, and SBH for devices on the (010) plane were higher compared to those on the (2 01) plane, reminding us to consider the anisotropy of the crystal structure in device design.
Furthermore, in 2021, He et al. [138] discovered that β-Ga2O3 exposed to air over prolonged periods would lead to surface enrichment of donor-like impurities, drastically deteriorating device performance.By removing the unreliable surface layer formed in the air with ICP and preserving the sample in alcohol, minimizing contact with air during the In addition, in high-power circuits there is not only a large voltage but also a large current.Compared with small-area devices, large-area devices are capable of handling higher currents with lower on-state voltage drops.However, the emergence of new materials and new processes inevitably faces the issue of defects; a larger electrode size also means more defects, which significantly impacts the breakdown performance of devices [134,135].In 2017, Yang et al. [136] fabricated a series of β-Ga 2 O 3 SBDs with varying electrode sizes.Test results demonstrated that as the electrode size increased, the BV gradually decreased and the R on also reduced.At an electrode diameter of 20 µm, the BV was 1600 V and the R on was 25 mΩ•cm 2 , whereas at 0.53 mm diameter, the BV fell to only 250 V and the R on to 1.6 mΩ•cm 2 .
The crystal structure of β-Ga 2 O 3 leads to severe anisotropy, which has an important impact on the performance of the device.In 2017, Fu et al. [137] investigated the electrical properties of β-Ga 2 O 3 SBDs on two distinct crystallographic planes, (201) and (010).Findings indicated that the (010) plane had more negative charges and defects on its surface, leading to a more pronounced upward bending of the conduction band.As a result, the R on , V on , n, and SBH for devices on the (010) plane were higher compared to those on the (201) plane, reminding us to consider the anisotropy of the crystal structure in device design.
Furthermore, in 2021, He et al. [138] discovered that β-Ga 2 O 3 exposed to air over prolonged periods would lead to surface enrichment of donor-like impurities, drastically deteriorating device performance.By removing the unreliable surface layer formed in the air with ICP and preserving the sample in alcohol, minimizing contact with air during the experimental process, they fabricated a device with a R on of 2.25 mΩ•cm 2 and a BV of 1720 V.The resulting PFOM reached 1.32 GW/cm 2 , making it one of the most outstanding simplestructured β-Ga 2 O 3 SBD devices to date, fully tapping into the potential applications of gallium oxide in power devices.

Field Plate Structure
The field plate structure is to add a layer of dielectric at the edge of the Schottky electrode.By extending part of the electrode, an electric field is formed that acts upon the contact edge to alleviate electric field concentration, thereby effectively increasing the BV of the device.The manufacturing process is simple and controllable, and it is widely applied in SiC and GaN power devices [139][140][141][142].In 2016, Konishi et al. [143,144] first reported the deposition of SiO 2 as a field plate structure on β-Ga 2 O 3 by chemical vapor deposition (CVD), as shown in Figure 5a, where the device's BV exceeded 1kV.In 2018, Yang et al. [145,146] employed plasma-enhanced chemical vapor deposition (PECVD) to deposit SiN x as a field plate.When the Schottky electrode diameter was 150 µm, the BV reached 2.3 kV and the R on was 0.25 Ω•cm 2 .The reverse recovery time (t rr ) measured when switching the device from +2 V to −2 V was 22 ns.For devices with Schottky electrode dimensions of 1×1 mm 2 , the I F exceeded 1 A, and the highest BV was 650 V, showcasing the potential application of β-Ga 2 O 3 SBDs in the high-power domain [147].
V. The resulting PFOM reached 1.32 GW/cm 2 , making it one of the most outstanding simple-structured β-Ga2O3 SBD devices to date, fully tapping into the potential applications of gallium oxide in power devices.

Field Plate Structure
The field plate structure is to add a layer of dielectric at the edge of the Schottky electrode.By extending part of the electrode, an electric field is formed that acts upon the contact edge to alleviate electric field concentration, thereby effectively increasing the BV of the device.The manufacturing process is simple and controllable, and it is widely applied in SiC and GaN power devices [139][140][141][142].In 2016, Konishi et al. [143,144] first reported the deposition of SiO2 as a field plate structure on β-Ga2O3 by chemical vapor deposition (CVD), as shown in Figure 5a, where the device's BV exceeded 1kV.In 2018, Yang et al. [145,146] employed plasma-enhanced chemical vapor deposition (PECVD) to deposit SiNx as a field plate.When the Schottky electrode diameter was 150 µm, the BV reached 2.3 kV and the Ron was 0.25 Ω•cm 2 .The reverse recovery time (trr) measured when switching the device from +2 V to −2 V was 22 ns.For devices with Schottky electrode dimensions of 1×1 mm 2 , the IF exceeded 1 A, and the highest BV was 650 V, showcasing the potential application of β-Ga2O3 SBDs in the high-power domain [147].Compared to a single-layer field plate, a bilayer field plate can combine the advantages of different dielectrics to further mitigate the electric field crowding effect and improve interface states, thus enhancing device performance.In 2019, Yang et al. employed PECVD to deposit a SiO2/SiNx bilayer dielectric as a field plate, as illustrated in Figure 5b, with the SiO2 layer capable of absorbing high electric fields and limiting the difference between the conduction and valence bands, effectively improving the breakdown performance of the device.A series of studies was conducted on large-area devices [148,[151][152][153].For devices with a Schottky electrode diameter of 1 mm, the measured BV was 760 V, and the current could reach 1 A at 2.3 V.The trr from an IF of 1 A to an off-state of −300 V was 64 ns, and it was not affected by a temperature below 150 °C [151].When the Schottky electrode size was 0.4 × 0.4 mm 2 , the device could also achieve a current of 1 A at 1 V, with a BV of 1900 V and a Ron of 0.24 Ω•cm 2 [152].The device functionality under a high reverse voltage of −900 V was first demonstrated, with a trr of Compared to a single-layer field plate, a bilayer field plate can combine the advantages of different dielectrics to further mitigate the electric field crowding effect and improve interface states, thus enhancing device performance.In 2019, Yang et al. employed PECVD to deposit a SiO 2 /SiN x bilayer dielectric as a field plate, as illustrated in Figure 5b, with the SiO 2 layer capable of absorbing high electric fields and limiting the difference between the conduction and valence bands, effectively improving the breakdown performance of the device.A series of studies was conducted on large-area devices [148,[151][152][153].For devices with a Schottky electrode diameter of 1 mm, the measured BV was 760 V, and the current could reach 1 A at 2.3 V.The t rr from an I F of 1 A to an off-state of −300 V was 64 ns, and it was not affected by a temperature below 150 • C [151].When the Schottky electrode size was 0.4 × 0.4 mm 2 , the device could also achieve a current of 1 A at 1 V, with a BV of 1900 V and a R on of 0.24 Ω•cm 2 [152].The device functionality under a high reverse voltage of −900 V was first demonstrated, with a t rr of 81 ns and a reverse recovery current (I rr ) of 38 mA [153].Subsequently, they compared the performance of different bilayer field plate dielectrics (SiO 2 /SiN x , Al 2 O 3 /SiN x , HfO 2 /SiN x ) on β-Ga 2 O 3 FP-SBDs, finding that their forward performance was similar.The Al 2 O 3 /SiNx bilayer field plate exhibited the smallest reverse leakage current, with a BV of 730 V, exceeding that of the SiO 2 /SiN x bilayer field plate at 562 V and the HfO 2 /SiN x bilayer field plate at 401 V [154].In 2022, Guo et al. [155] fabricated β-Ga 2 O 3 FP-SBDs with a 1 mm diameter using an Al 2 O 3 /SiN x bilayer field plate, achieving a current of 2 A at 2 V, with a BV of 467 V.The device's t rr from an I F of 1 A to −100 V was only 8.8 ns, with a reverse recovery charge (C rr ) of 8.33 nC.
With reduced costs and improved material quality, power devices and circuits based on Ga 2 O 3 are expected to have a broad range of applications in the foreseeable future.
Furthermore, the field plate dielectric significantly impacts device performance.Compared to high-k dielectrics, low-k dielectrics have fewer internal charges and the edge electric field diminishes more rapidly within them, hence exerting a smaller influence on the electric field concentration at the electrode edge.Conversely, in high-k materials, the decrease in electric fields is lower, allowing for more effective propagation of the electric field, which better alleviates the concentration at the electrode edge.In 2021, Roy et al. [156] used a β-Ga 2 O 3 wafer with an epitaxial layer of only 1.7 µm to deposit an ultra-high-k dielectric BaTiO 3 (BTO) and BaTiO 3 /SrTiO 3 (BTO/STO) stack as the field plate structure to fabricate β-Ga 2 O 3 FP-SBDs.The devices achieved a low R on of 0.32 mΩ•cm 2 , with the BV increasing from 148 V without field plates to 486 V and 687 V, respectively, and a maximum PFOM reaching up to 1.47 GW/cm 2 .Subsequently, BTO was used as the field plate dielectric on a β-Ga 2 O 3 wafer with an epitaxial layer of 11 µm, resulting in devices with a R on of 6.9 mΩ•cm 2 and a BV of 2.1 kV for SBDs, which is 2.7 times higher than the structures without field plates, demonstrating the potential of high-k materials as field plate dielectrics [157].Liu et al. [149], through TCAD simulation studies, found that, under the same reverse bias, the higher the dielectric constant of the field plate dielectrics and the smaller the angle of the beveled field plate, the higher the device BV.This indicates that high-k dielectrics are more effective in mitigating the concentration of the electric field at the electrode edge, and it also points out that the geometry of the field plate has a significant effect on the electric field at the electrode edge.Smaller geometric discontinuities can further alleviate the electric field crowding effect, as in the small-angle beveled field plate illustrated in Figure 5c.Similar to beveled field plates, stair-shaped field plates can also somewhat reduce the geometric abruptness of the field plate.Sun et al. [150] formed a stair-shaped TiO x field plate via the double thermal oxidation of Ti metal, as shown in Figure 5d, increasing the device BV from 460 V to 950 V, while the R on only increased from 2.7 mΩ•cm 2 to 2.8 mΩ•cm 2 .Kumar et al. [158] fabricated a stair-shaped field plate through PECVD deposition of SiO 2 and etching processes.Compared to conventional field plate structures, device BVs increased from 980 V to 1530 V.In addition, studies on transistors have shown that the introduction of a field plate structure introduces parasitic capacitance within the device, which results in a decrease in the device cutoff frequency and a deterioration in RF performance, and which is exacerbated by an increase in the dielectric permittivity of the dielectric [159][160][161][162].However, there are few studies on the effect of introducing the field plate structure on the parasitic capacitance of β-Ga 2 O 3 diode devices, so it will not be discussed here.
In summary, due to its simple fabrication processes, the field plate structure is widely used in the manufacturing of power devices.By employing composite field plates and highk dielectrics, and reducing the geometric discontinuities of the device, the concentration at the electrode edges can be further diminished, thus fully capitalizing on the benefits of field plate designs.

Edge Termination Structure
Edge termination structure involves the formation of high-resistance regions at the electrode edges through methods such as ion implantation, thermal oxidation, or groovefilling with oxides, resulting in electron isolation, which effectively mitigates electric field crowding at the electrode edges, as shown in Figure 6.Compared to field plate structures, edge terminations reduce geometric discontinuities in devices, but their fabrication processes are more complex.In 2019, Gao et al. [163] reported Ar ion-implanted high-resistance edge-terminated β-Ga 2 O 3 SBDs; compared to conventional structures, the BV increased from 209 V up to 550 V.In the same year, Lu et al. [164] also reported Ar ion-implanted high-resistance edge terminations.The devices had a R on of 4 mΩ•cm 2 and an ideality factor of 1.02; compared to traditional structures, the leakage current was reduced by a factor of 10 3 , with an on/off ratio reaching 10 13 , and the BV was enhanced from 257 V to 391 V. Additionally, the devices exhibited excellent dynamic switching performance.Compared to commercial Si fast recovery diodes, the I rr was reduced by a factor of 12 (38 mA), the t rr by 5.5 times (14.1 ns), and the Q rr was only 1.7% (0.34 nC) that of the commercial Si fast recovery diode, fully demonstrating the potential of edge-terminated β-Ga 2 O 3 SBDs in fast-switching circuits.Zhou et al. [165] achieved β-Ga 2 O 3 SBD devices with a R on of 5.1 mΩ•cm 2 , an on/off ratio of 10 8 -10 9 , a BV of 1.55 kV, and a PFOM of 0.47 GW/cm 2 by using Mg ion implantation, thereby demonstrating the advantages of ion-implanted high-resistance edge terminations.However, ion implantation can cause significant damage to the gallium oxide lattice structure, and the heavier the ion mass, the more severe the damage.As a result, a large number of traps and defects are present in the ion-implanted region, leading to device performance degradation under off-state stress [166].
edge terminations reduce geometric discontinuities in devices, but their fabrication processes are more complex.In 2019, Gao et al. [163] reported Ar ion-implanted highresistance edge-terminated β-Ga2O3 SBDs; compared to conventional structures, the BV increased from 209 V up to 550 V.In the same year, Lu et al. [164] also reported Ar ionimplanted high-resistance edge terminations.The devices had a Ron of 4 mΩ•cm 2 and an ideality factor of 1.02; compared to traditional structures, the leakage current was reduced by a factor of 10 3 , with an on/off ratio reaching 10 13 , and the BV was enhanced from 257 V to 391 V. Additionally, the devices exhibited excellent dynamic switching performance.Compared to commercial Si fast recovery diodes, the Irr was reduced by a factor of 12 (38 mA), the trr by 5.5 times (14.1 ns), and the Qrr was only 1.7% (0.34 nC) that of the commercial Si fast recovery diode, fully demonstrating the potential of edge-terminated β-Ga2O3 SBDs in fast-switching circuits.Zhou et al. [165] achieved β-Ga2O3 SBD devices with a Ron of 5.1 mΩ•cm 2 , an on/off ratio of 10 8 -10 9 , a BV of 1.55 kV, and a PFOM of 0.47 GW/cm 2 by using Mg ion implantation, thereby demonstrating the advantages of ionimplanted high-resistance edge terminations.However, ion implantation can cause significant damage to the gallium oxide lattice structure, and the heavier the ion mass, the more severe the damage.As a result, a large number of traps and defects are present in the ion-implanted region, leading to device performance degradation under off-state stress [166].Research indicates that the high-temperature annealing of β-Ga2O3 in oxygen can facilitate the generation of gallium vacancies, thereby inducing semi-insulating properties [167][168][169][170]. Hence, employing high-temperature oxygen annealing to form high-resistance terminations represents an efficient and low-cost technique.In 2020, Wang et al. [171] utilized SiO2 as a barrier layer, forming high-resistance terminations through thermal oxidation at various temperatures, and deposited a SiO2 layer atop the high-resistance area as a passivation layer.From C-V measurements, the carrier concentration decreased close to a constant (1 × 10 16 cm −3 ) after thermal oxidation at higher than 400 °C.Subsequently, the BV increased to a maximum of 940 V, with a Ron of 3.0 mΩ•cm 2 and a PFOM of 295 MW/cm 2 .In 2022, He et al. [172] formed high-resistance terminations using high-temperature thermal oxidation at 1100 °C, employing polycrystalline silicon as the barrier layer.Experiments revealed that the carrier concentration in the polycrystalline silicon area showed no significant depletion (~1.8 × 10 16 cm −3 ), whereas a pronounced depletion occurred in the exposed region (3.0 × 10 14 cm −3 ), with a depletion depth of 2.4 µm, indicating the ideal barrier capabilities of polycrystalline silicon against the oxygenannealing environment.The resulting device exhibited a Ron of 4.1 mΩ•cm 2 , a BV of 1800 V, and a PFOM up to 0.78 GW/cm 2 , demonstrating the advantages of thermal oxidation high-resistance termination structures.
Although ion-implanted terminations and thermal oxidation terminations can effectively alleviate electric field crowding, ion-implanted terminations introduce a significant number of traps and defects, and thermal oxidation terminations do not form Research indicates that the high-temperature annealing of β-Ga 2 O 3 in oxygen can facilitate the generation of gallium vacancies, thereby inducing semi-insulating properties [167][168][169][170]. Hence, employing high-temperature oxygen annealing to form high-resistance terminations represents an efficient and low-cost technique.In 2020, Wang et al. [171] utilized SiO 2 as a barrier layer, forming high-resistance terminations through thermal oxidation at various temperatures, and deposited a SiO 2 layer atop the high-resistance area as a passivation layer.From C-V measurements, the carrier concentration decreased close to a constant (1 × 10 16 cm −3 ) after thermal oxidation at higher than 400 • C. Subsequently, the BV increased to a maximum of 940 V, with a R on of 3.0 mΩ•cm 2 and a PFOM of 295 MW/cm 2 .In 2022, He et al. [172] formed high-resistance terminations using high-temperature thermal oxidation at 1100 • C, employing polycrystalline silicon as the barrier layer.Experiments revealed that the carrier concentration in the polycrystalline silicon area showed no significant depletion (~1.8 × 10 16 cm −3 ), whereas a pronounced depletion occurred in the exposed region (3.0 × 10 14 cm −3 ), with a depletion depth of 2.4 µm, indicating the ideal barrier capabilities of polycrystalline silicon against the oxygen-annealing environment.The resulting device exhibited a R on of 4.1 mΩ•cm 2 , a BV of 1800 V, and a PFOM up to 0.78 GW/cm 2 , demonstrating the advantages of thermal oxidation high-resistance termination structures.
Although ion-implanted terminations and thermal oxidation terminations can effectively alleviate electric field crowding, ion-implanted terminations introduce a significant number of traps and defects, and thermal oxidation terminations do not form complete insulating regions, resulting in device BVs that are generally below 2 kV.In 2022, Dong et al. [173] utilized a deep trench filled with a thick SiO 2 layer to efficiently block current conduction.Since the bandgap of SiO 2 (8~9 eV) is much larger than that of β-Ga 2 O 3 , it can withstand higher voltages and critical fields, and the relative permittivity of SiO 2 (~4) is smaller; according to Poisson's equation, the electric field of the β-Ga 2 O 3 layer at the interface is less than 1/3 of that of the SiO 2 layer.Test results showed that the device reached a maximum BV of 6 kV and a minimum R on of 3.4 mΩ•cm 2 , achieving a PFOM of 10.6 GW/cm 2 , surpassing the unipolar limit of SiC and GaN devices and confirming the tremendous potential of edge termination-structured β-Ga 2 O 3 SBDs as next-generation high-voltage and high-power electronic components.
Moreover, in SiC and GaN devices, another edge termination method is employed, known as the floating metal ring (FMR) structure, which can be fabricated simultaneously with the Schottky contact.This method is simple and achieves effective reverse blocking characteristics [174,175].Simulation results for β-Ga 2 O 3 SBD devices also indicate that FMR can effectively increase the device BV, but insufficient experimental verification exists, so it is not discussed in detail here.
To summarize, the edge termination structure is a simple and effective method that can be used to increase the BV by decreasing the electron concentration at the electrode's edge and reducing the peak value of the electric field.However, the potential of ion implantation termination and thermal oxidation termination structures needs to be explored further through experiments.Additionally, there is a lack of reports on how devices with large-area edge termination structures perform, which needs to be explored in greater detail.

Trench Structure
Although field plate structures and edge termination structures can effectively alleviate electric field concentration at the anode edges and thereby increase the device BV, they rely solely on the Schottky barrier formed by metal-semiconductor contacts to control the reverse blocking of carriers.Under reverse bias, a high reverse electric field exists near the Schottky contact interface, leading to a large reverse leakage current [176].In an effort to diminish the leakage current and regulate the field strength distribution from the Schottky contact interface to the interior of the device, the trench structure for SBDs can be a promising choice; under reverse bias, the trench's metal oxide semiconductor (MOS) structure depletes surface charges and reduces the surface electric field (RESURF), reducing the leakage current path.At high reverse biases, it can even pinch off the trench channel, effectively decreasing the leakage current and enhancing the device BV [177][178][179].
In 2017, Sasaki et al. [180,181] prepared β-Ga 2 O 3 trench MOS SBDs for the first time on β-Ga 2 O 3 substrates with a 7 µm drift layer (6 × 10 16 cm −3 ), as illustrated in Figure 7a.The experiments demonstrated that the trench MOS SBDs are effective in reducing the reverse leakage current, increasing the BV from 70 V to 240 V. Due to the reduced current channel, the R on increased from 2.3 mΩ•cm 2 to 2.9 mΩ•cm 2 .Subsequently, the reverse recovery characteristics of the devices were examined through a double-pulse test circuit, going from an I F of 1 A to a reverse bias of 100 V.The I rr was measured at 0.42 A, with a t rr of 7.6 ns, and a recovery loss of 0.12 µJ.These performances are nearly on par with commercial SiC SBDs [182].
The device structure and preparation process substantially affect performance, including the epitaxial layer thickness, doping concentration, fin width, fin orientation, and etching processes.In 2018, Li et al. [183] developed β-Ga 2 O 3 trench MOS SBDs on a β-Ga 2 O 3 substrate with a 10 µm drift layer (1-2 × 10 15 cm −3 ), depicted in Figure 7b, achieving a BV of 1.5 kV, and a four-magnitude decrease in the reverse leakage current relative to conventional SBDs.Further experimentation on substrates with drift layer concentrations of 2 × 10 16 cm −3 resulted in similar devices with a BV of 1232 V, indicating that an increased epitaxial layer doping concentration can lead to a reduction in BV.Furthermore, the results indicated that a reduced fin width enhanced the RESURF effect, leading to a lower leakage current and higher BV [184].In addition, they discovered that devices aligned along the (001) direction exhibited the highest forward current, while other orientations, due to interface negative charges, experienced severe sidewall conduction losses, causing shallow turn-on behavior and a substantial reduction in forward current [185].In the same year, during the IEDM conference, they reported β-Ga 2 O 3 vertical trench SBDs with a BV of 2.44 kV by introducing wet-etching after dry-etching to minimize the etching damage; for devices with fin widths of 1-2 µm, the reverse leakage current density remained below 1 µA/cm 2 up to the BV, which consistently occurred at the trench corners, where electric field crowding is prone to happen [186].
shallow turn-on behavior and a substantial reduction in forward current [185].In the same year, during the IEDM conference, they reported β-Ga2O3 vertical trench SBDs with a BV of 2.44 kV by introducing wet-etching after dry-etching to minimize the etching damage; for devices with fin widths of 1-2 µm, the reverse leakage current density remained below 1 µA/cm 2 up to the BV, which consistently occurred at the trench corners, where electric field crowding is prone to happen [186].To mitigate the electric field crowding at the trench bottom corners, Huang et al. [187] introduced a bottom corner structure, as shown in Figure 7c, and optimized its structural parameters through TCAD simulations.Simulation results indicated that the maximum BV could reach 3.4 kV, with a PFOM as high as 2 GW/cm 2 .Besides the trench bottom corners, electric field crowding also occurs at the edges of the trench region.Li et al. [188] introduced a field-plated structure at the trench region edges by twice depositing Al2O3 dielectric, as illustrated in Figure 7d.Compared to trench MOS devices without a field plate structure, the BV increased from 2.4 kV to 2.9 kV, and the PFOM reached 0.95 GW/cm 2 , demonstrating the efficacy of the field plate structure in alleviating electric field crowding at the trench region edges.Otsuka et al. [189] fabricated large-area (1.7 × 1.7 mm 2 ) β-Ga2O3 trench MOS SBDs by introducing a stair-shaped field plate structure via SiO2 deposition and double wet-etching at the edges of the trench region, as depicted in Figure 7e.The devices displayed an IF of 2 A at 2 V, and the reverse leakage current was only 5.7 × 10 −10 A at a reverse bias of −1.2 kV, achieving an on/off ratio surpassing 10 9 .Additionally, Roy et al. [190] used the ultra-high-k dielectric material BaTiO3 as the dielectric and introduced a SiNx-passivated dielectric layer to fabricate trench MOS SBDs of varying areas, as shown in Figure 7f.Small-area devices (200 × 200 µm 2 ) exhibited a BV exceeding 3 kV, and the reverse leakage current remained below 1 µA/cm 2 at 3 kV.For large-area devices, devices measuring 1 × 1 mm 2 and 2 × 2 mm 2 had BVs of 1.8 kV and 1.4 kV, respectively, and could sustain a forward current of 3.7 A and 15 A under a 10% duty cycle pulse test.Moreover, the device characteristics, such as the temperature coefficient of resistance, capacitance, stored charge, and switching energy ratio, were smaller compared to commercially available SiC SBDs of equivalent ratings, showcasing the advantages of β-Ga2O3 trench MOS SBDs for high-power applications.
Dry-etching has been found to introduce a significant number of defects at the trench sidewall interfaces, substantially impacting device performance.Li et al. [191] observed To mitigate the electric field crowding at the trench bottom corners, Huang et al. [187] introduced a bottom corner structure, as shown in Figure 7c, and optimized its structural parameters through TCAD simulations.Simulation results indicated that the maximum BV could reach 3.4 kV, with a PFOM as high as 2 GW/cm 2 .Besides the trench bottom corners, electric field crowding also occurs at the edges of the trench region.Li et al. [188] introduced a field-plated structure at the trench region edges by twice depositing Al 2 O 3 dielectric, as illustrated in Figure 7d.Compared to trench MOS devices without a field plate structure, the BV increased from 2.4 kV to 2.9 kV, and the PFOM reached 0.95 GW/cm 2 , demonstrating the efficacy of the field plate structure in alleviating electric field crowding at the trench region edges.Otsuka et al. [189] fabricated large-area (1.7 × 1.7 mm 2 ) β-Ga 2 O 3 trench MOS SBDs by introducing a stair-shaped field plate structure via SiO 2 deposition and double wetetching at the edges of the trench region, as depicted in Figure 7e.The devices displayed an I F of 2 A at 2 V, and the reverse leakage current was only 5.7 × 10 −10 A at a reverse bias of −1.2 kV, achieving an on/off ratio surpassing 10 9 .Additionally, Roy et al. [190] used the ultra-high-k dielectric material BaTiO 3 as the dielectric and introduced a SiN x -passivated dielectric layer to fabricate trench MOS SBDs of varying areas, as shown in Figure 7f.Small-area devices (200 × 200 µm 2 ) exhibited a BV exceeding 3 kV, and the reverse leakage current remained below 1 µA/cm 2 at 3 kV.For large-area devices, devices measuring 1 × 1 mm 2 and 2 × 2 mm 2 had BVs of 1.8 kV and 1.4 kV, respectively, and could sustain a forward current of 3.7 A and 15 A under a 10% duty cycle pulse test.Moreover, the device characteristics, such as the temperature coefficient of resistance, capacitance, stored charge, and switching energy ratio, were smaller compared to commercially available SiC SBDs of equivalent ratings, showcasing the advantages of β-Ga 2 O 3 trench MOS SBDs for high-power applications.
Dry-etching has been found to introduce a significant number of defects at the trench sidewall interfaces, substantially impacting device performance.Li et al. [191] observed that the density of negatively charged traps at the sidewall interface increases with a rising forward voltage, and the release of captured charges is very slow at room temperature, resulting in sidewall depletion, which leads to current collapse and delayed turn-on behavior.To mitigate the defects caused by dry-etching, Tang et al. [192] treated devices post-dry-etching with self-reactive etching (SRE) in an MBE, where metal Ga reacts with Ga 2 O 3 to form Ga 2 O, and the suboxide Ga 2 O is gas, which can be discharged with the exhaust gas to achieve etching purposes.After SRE treatment, a smooth surface with the same morphology as the original substrate can be obtained, and the extremely low interfacial density of states of the device (2.9 × 10 11 cm −2 •eV −1 ) confirms the repairing effect of the SRE process on the etched surface.Moreover, the device boasted excellent thermal stability, with a forward current density of 1228 A/cm 2 at 3 V and an on/off ratio surpassing 10 10 [193].Following this, Dhara et al. [194] employed a similar approach to fabricate β-Ga 2 O 3 trench MOS SBDs, achieving a BV of 1.45 kV, a R on of 1.20 mΩ•cm 2 , and a PFOM exceeding 2 GW/cm 2 , attesting to the effectiveness of the SRE process.
Overall, although the presence of the trench reduces the device's effectiveness, the conduction area is reduced, resulting in a lower I F , and a larger V on due to the depletion of the negative charge at the trench sidewall interface.However, among the various structures of β-Ga 2 O 3 SBDs, the trench structure has the lowest reverse leakage current, which results in a very low off-state loss and high breakdown voltage, and thus β-Ga 2 O 3 trench MOS SBDs also have a large potential for application in high-power devices.

Mesa Termination Structure
The mesa termination structure, achieved through an etching process that raises the Schottky contact area, reduces the geometrical abruptness of the Schottky contact region, which is transferred to the inside of the β-Ga 2 O 3 material to alleviate the electric field concentration at the electrode edges and improve device performance.The fabrication process of the basic mesa structure is very simple, with only one additional step of the self-aligned etching process compared to simple β-Ga 2 O 3 SBDs.As depicted in Figure 8a, Dhara et al. [195] employed a Pt metal mask to create a 4 µm deep mesa structure using inductively coupled plasma (ICP)-etching, which increased the BV of the device from 350 V to 1150 V when compared to a simple structure.Han et al. [196], utilizing a PtO x Schottky electrode as shown in Figure 8b, demonstrated that, when the mesa etching depth was set at 1.2 µm, the device achieved a BV of 2738 V and a PFOM of 1.02 GW/cm 2 .Moreover, this device maintained a leakage current density of less than 10 µA/cm 2 up to −2000 V, thereby manifestly showcasing the application potential of PtO x Schottky electrodes and mesa structure for β-Ga 2 O 3 power devices.
forward voltage, and the release of captured charges is very slow at room temperature, resulting in sidewall depletion, which leads to current collapse and delayed turn-on behavior.To mitigate the defects caused by dry-etching, Tang et al. [192] treated devices post-dry-etching with self-reactive etching (SRE) in an MBE, where metal Ga reacts with Ga2O3 to form Ga2O, and the suboxide Ga2O is gas, which can be discharged with the exhaust gas to achieve etching purposes.After SRE treatment, a smooth surface with the same morphology as the original substrate can be obtained, and the extremely low interfacial density of states of the device (2.9 × 10 11 cm −2 •eV −1 ) confirms the repairing effect of the SRE process on the etched surface.Moreover, the device boasted excellent thermal stability, with a forward current density of 1228 A/cm 2 at 3 V and an on/off ratio surpassing 10 10 [193].Following this, Dhara et al. [194] employed a similar approach to fabricate β-Ga2O3 trench MOS SBDs, achieving a BV of 1.45 kV, a Ron of 1.20 mΩ•cm 2 , and a PFOM exceeding 2 GW/cm 2 , attesting to the effectiveness of the SRE process.
Overall, although the presence of the trench reduces the device's effectiveness, the conduction area is reduced, resulting in a lower IF, and a larger Von due to the depletion of the negative charge at the trench sidewall interface.However, among the various structures of β-Ga2O3 SBDs, the trench structure has the lowest reverse leakage current, which results in a very low off-state loss and high breakdown voltage, and thus β-Ga2O3 trench MOS SBDs also have a large potential for application in high-power devices.

Mesa Termination Structure
The mesa termination structure, achieved through an etching process that raises the Schottky contact area, reduces the geometrical abruptness of the Schottky contact region, which is transferred to the inside of the β-Ga2O3 material to alleviate the electric field concentration at the electrode edges and improve device performance.The fabrication process of the basic mesa structure is very simple, with only one additional step of the self-aligned etching process compared to simple β-Ga2O3 SBDs.As depicted in Figure 8a, Dhara et al. [195] employed a Pt metal mask to create a 4 µm deep mesa structure using inductively coupled plasma (ICP)-etching, which increased the BV of the device from 350 V to 1150 V when compared to a simple structure.Han et al. [196], utilizing a PtOx Schottky electrode as shown in Figure 8b, demonstrated that, when the mesa etching depth was set at 1.2 µm, the device achieved a BV of 2738 V and a PFOM of 1.02 GW/cm 2 .Moreover, this device maintained a leakage current density of less than 10 µA/cm 2 up to −2000 V, thereby manifestly showcasing the application potential of PtOx Schottky electrodes and mesa structure for β-Ga2O3 power devices.Moreover, Hu et al. [197] introduced F ions on the surface through plasma treatment, to alleviate the electric field concentration by utilizing the strong electronegativity of F ions to attract negative charges to gather near the surface, as shown in Figure 8c.Compared to a device without any terminal structures, the F plasma-treated (FPT) device exhibited an increase in BV from 250 V to 520 V.The incorporation of a beveled F plasma-treated (BFPT) structure further enhanced the BV up to 1050 V, indicating that a mesa structure can effectively alleviate electric field concentration effects.Wei et al. [198] introduced a thermal oxidation treatment (TOT) process within the mesa structure to reduce the surface electron concentration, passivate the oxygen vacancy-type interface states to improve device reliability, and deposit a SiO 2 passivation layer to prepare β-Ga 2 O 3 SBDs with a Schottky contact size of 2 × 2 mm 2 .The devices achieved a BV of 600 V, with reverse leakage currents maintained below 10 µA and a forward current capability of up to 7 A. Exhibiting commendable thermal stability during a high-temperature storage (HTS) test at 450 K, these findings underscore a formidable potential for high-temperature, high-power applications.
Similar to the trench structure, the mesa structure can also improve the surface electric field by introducing a side MOS structure.The inclination angle of the mesa has an important effect on the performance of the device.Chen et al. [199] used SiO 2 and Ni as etching masks to fabricate negative-and positive-beveled mesa structures, respectively.After F plasma treatment and the deposition of bilayer field plate dielectrics, they constructed SBDs as shown in Figure 9a-d.Compared to planar SBDs, the negative-beveled (NB) mesa structure increased the BV from 400 V to 1100 V; the positive-beveled (PB) mesa structure proved even more effective in mitigating the electric field concentration, achieving a BV of 1710 V.However, due to the reduced conduction area, the R on of the PB device increased to 3.6 mΩ•cm 2 , with a PFOM of 0.8 GW/cm 2 .Allen et al. [200] successfully fabricated a small-angle beveled mesa structure with a 1 • tilt, using a dual-mask wet-etching technique.They created β-Ga 2 O 3 SBDs incorporating a bilayer field plate using PECVD-SiO 2 /spin-on glass (SOG), as illustrated in Figure 9e, and compared the BV across devices with simple structures, ordinary field plate structures, 45 • beveled field plate (BFP) structures, and 1 • small-angle beveled field plate (SABFP) structures, which were approximately ~200 V, ~400 V, ~650 V, and ~1100 V respectively.This indicates that SABFP can more effectively alleviate the concentration of electric fields.
Compared to a device without any terminal structures, the F plasma-treated (FPT) device exhibited an increase in BV from 250 V to 520 V.The incorporation of a beveled F plasmatreated (BFPT) structure further enhanced the BV up to 1050 V, indicating that a mesa structure can effectively alleviate electric field concentration effects.Wei et al. [198] introduced a thermal oxidation treatment (TOT) process within the mesa structure to reduce the surface electron concentration, passivate the oxygen vacancy-type interface states to improve device reliability, and deposit a SiO2 passivation layer to prepare β-Ga2O3 SBDs with a Schottky contact size of 2 × 2 mm 2 .The devices achieved a BV of 600 V, with reverse leakage currents maintained below 10 µA and a forward current capability of up to 7 A. Exhibiting commendable thermal stability during a high-temperature storage (HTS) test at 450 K, these findings underscore a formidable potential for hightemperature, high-power applications.
Similar to the trench structure, the mesa structure can also improve the surface electric field by introducing a side MOS structure.The inclination angle of the mesa has an important effect on the performance of the device.Chen et al. [199] used SiO2 and Ni as etching masks to fabricate negative-and positive-beveled mesa structures, respectively.After F plasma treatment and the deposition of bilayer field plate dielectrics, they constructed SBDs as shown in Figure 9a-d.Compared to planar SBDs, the negativebeveled (NB) mesa structure increased the BV from 400 V to 1100 V; the positive-beveled (PB) mesa structure proved even more effective in mitigating the electric field concentration, achieving a BV of 1710 V.However, due to the reduced conduction area, the Ron of the PB device increased to 3.6 mΩ•cm 2 , with a PFOM of 0.8 GW/cm 2 .Allen et al. [200] successfully fabricated a small-angle beveled mesa structure with a 1° tilt, using a dual-mask wet-etching technique.They created β-Ga2O3 SBDs incorporating a bilayer field plate using PECVD-SiO2/spin-on glass (SOG), as illustrated in Figure 9e, and compared the BV across devices with simple structures, ordinary field plate structures, 45° beveled field plate (BFP) structures, and 1° small-angle beveled field plate (SABFP) structures, which were approximately ~200 V, ~400 V, ~650 V, and ~1100 V respectively.This indicates that SABFP can more effectively alleviate the concentration of electric fields.[199]; (e) β-Ga2O3 BFP and SABFP SBDs [200].[199]; (e) β-Ga 2 O 3 BFP and SABFP SBDs [200].
In summary, the simple mesa structure is an effective and straightforward approach for mitigating electric field concentration at the electrode edges, which significantly enhances device performance.Mesa structures equipped with field plates are similar to trench structures and offer a larger conduction area, hence providing superior forward performance.Consequently, they hold considerable potential for application in β-Ga 2 O 3 power devices.

Vertical Heterojunction-Structure Diodes
Although SBDs exhibit a lower V on and faster recovery times compared to p-n junction diodes, and the BV of SBDs is greatly improved due to the wide bandgap of β-Ga 2 O 3 and the introduction of various termination structures, it is still challenging to see their BV exceed 3 kV, and the fabrication processes involved are more complex.P-n junction diodes can not only greatly improve the BV of the device and exhibit lower leakage currents, but they can also effectively reduce the R on through the modulation of electrical conductivity [201].However, the absence of an effective p-type Ga 2 O 3 has hindered the development of β-Ga 2 O 3 p-n homojunction diodes.Researchers have thus turned their attention to constructing β-Ga 2 O 3 p-n heterojunction diodes (HJDs), using other p-type materials to achieve Ga 2 O 3 bipolar power devices.These include Cu 2 O [202], NiO [203,204], GaN [205,206], SnO [207], CuAlO 2 [208], and others.Notably, NiO has a bandgap of 3.8-4 eV, controllable doping characteristics, and a hole mobility of 0.5-5 cm 2 /V•s, which can form a type II band alignment with β-Ga 2 O 3 [209,210].Currently, the majority of β-Ga 2 O 3 p-n heterojunction devices utilize NiO and have achieved significant breakthroughs in performance.This section primarily summarizes the NiO/β-Ga 2 O 3 p-n HJDs, heterojunction barrier Schottky (HJBS) diodes, and junction termination extension (JTE) structures.

Heterojunction Diodes
P-type NiO thin films can be fabricated by several methods, including the sol-gel process [211], electron beam evaporation (EBE) [212], sputtering [213], and thermal oxidation [214].Among these, sputtering is widely used due to its rapid growth rate and ability to control the doping concentration of NiO films by adjusting the inflow ratio of oxygen.In 2020, Lu et al. [201] reported a practical NiO/β-Ga 2 O 3 HJDs by sputtering a 200 nm NiO layer with a net doping concentration of 4 × 10 16 cm −3 onto β-Ga 2 O 3 , as shown in Figure 10a, exhibiting a R on of 3.5 mΩ•cm 2 , lower than that of the Ni/β-Ga 2 O 3 SBDs (4.2 mΩ•cm 2 ).The BV was improved from 500 V to 1059 V, with a leakage current below 1 µA/cm 2 before breakdown.Subsequently, Gong et al. [215] fabricated HJDs with a bilayer NiO structure, as demonstrated in Figure 10b, where the high-doped layer provides a high hole concentration, and the low-doped layer effectively suppresses electric field crowding.Compared to the single-layer device with a BV of 0.94 kV, the double-layer device achieved a BV of 1.86 kV.Moreover, experiments have shown that post-annealing treatment can significantly reduce the trap density at the NiO/β-Ga 2 O 3 interface and improve device performance [216,217].Hao et al. obtained a device with a BV of 2.66 kV and a PFOM of 2.83 GW/cm 2 through annealing, which also exhibited excellent thermal stability, maintaining a BV of 1.77 kV at 250 • C [218].
Similar to mesa structures, HJDs can also reduce the geometrical abruptness between p-n junctions through a small-angle structure to alleviate the electric field concentration and improve the device's performance.Zhou et al. [219] fabricated a large-area (1 × 1 mm 2 ) small-angle (~11 • ) beveled mesa NiO/β-Ga 2 O 3 HJD, as illustrated in Figure 10c.The device exhibited a static BV of 1.95 kV and a dynamic BV of 2.23 kV, with an I F reaching 20 A and a R on of 1.9 mΩ•cm 2 .The subsequent fabrication of HJDs with an inclined angle of 6 • resulted in a BV of up to 2.04 kV.Under DC (pulsed) conditions, the device's R on was measured at 2.26 (1.45) mΩ•cm 2 , with a PFOM of 1.84 (2.87) GW/cm 2 .The device demonstrated a t rr of 16.4 ns under switching conditions of 800 V/10 A and exhibited high thermal stability at 200 • C, due to thermally enhanced conductance modulation [220].demonstrated a trr of 16.4 ns under switching conditions of 800 V/10 A and exhibited high thermal stability at 200 °C, due to thermally enhanced conductance modulation [220].Experimental and simulation studies indicate that the extension of the NiO layer beyond the edge of the metal contacts, as depicted in Figure 10d, produces a protective ring effect that can disperse the electric field crowding at the edges of the diode [221][222][223].Li et al. [221] fabricated a 100 µm diameter NiO/β-Ga2O3 HJD on a wafer with a 20 µm epitaxial layer (2 × 10 16 cm −3 ).The device achieved a BV of 4.7 kV and a Ron of 11.3 mΩ•cm 2 , with a PFOM reaching 2 GW/cm 2 .Subsequently a large-area HJD, with a 1 mm diameter, demonstrated a Ron of 11.86 mΩ•cm 2 and a BV to 1.76 kV.The device featured a trr of 101 ns when switched from an IF of 1 A to a reverse bias of −550 V [224].
Further, the low-power sputtering of NiO was used to reduce the interfacial damage at the heterointerface on β-Ga2O3 with a doping concentration of the drift layer less than 10 16 cm −3 , while the diameter of the NiO layer was larger than the Schottky electrodes to form a protective ring.With a Schottky electrode diameter of 100 µm, p-NiO/β-Ga2O3 HJDs with a maximum BV of 8.9 kV were achieved, exhibiting a Ron of 7.9 mΩ•cm 2 and a PFOM surpassing 10 GW/cm 2 .Devices with an area of 1 mm 2 also reached a BV of 4.7 kV and an IF of 4.1 A at 10 V, with a PFOM of 9 GW/cm 2 [225][226][227].The performance of both small-area and large-area devices exceeded the limit of unipolar power devices based on SiC and GaN, fully demonstrating the potential of β-Ga2O3 in future high-power applications.
The performance of HJDs can be further enhanced through the introduction of additional terminal structures.Li et al. [228] prepared a large-area (1 × 1 mm 2 ) NiO/β-Ga2O3 HJD on β-Ga2O3 with a 15 µm drift layer, simultaneously introducing a SiO2/SiNx bilayer field plate structure, as portrayed in Figure 11a.Compared to the device without the field plate structure, the BV increased from 5 kV to 7 kV, and the PFOM improved from 5.7 to 9.2 GW•cm −2 .Wang et al. [229] applied a photoresist reflow technique to introduce a field plate structure with a small angle (~8.5°), as shown in Figure 11b.Compared to the vertical field plate structure, the BV of the device increased from 1945 V to 2410 V, and the device exhibited a Ron of just 1.12 mΩ•cm 2 , enabling a PFOM up to 5.18 GW/cm 2 .Zhang et al. [39] simultaneously used Mg ion implantation terminals and a SiO2 field plate structure to mitigate the effect of the electric field concentration of the NiO/β-Ga2O3 heterojunction, as displayed in Figure 11c.The device achieved a BV of up to 8.32 kV, a Ron of 5.24 mΩ•cm 2 , and a PFOM as high as 13.2 GW/cm 2 .This surpasses the unipolar Experimental and simulation studies indicate that the extension of the NiO layer beyond the edge of the metal contacts, as depicted in Figure 10d, produces a protective ring effect that can disperse the electric field crowding at the edges of the diode [221][222][223].Li et al. [221] fabricated a 100 µm diameter NiO/β-Ga 2 O 3 HJD on a wafer with a 20 µm epitaxial layer (2 × 10 16 cm −3 ).The device achieved a BV of 4.7 kV and a R on of 11.3 mΩ•cm 2 , with a PFOM reaching 2 GW/cm 2 .Subsequently a large-area HJD, with a 1 mm diameter, demonstrated a R on of 11.86 mΩ•cm 2 and a BV to 1.76 kV.The device featured a t rr of 101 ns when switched from an I F of 1 A to a reverse bias of −550 V [224].
Further, the low-power sputtering of NiO was used to reduce the interfacial damage at the heterointerface on β-Ga 2 O 3 with a doping concentration of the drift layer less than 10 16 cm −3 , while the diameter of the NiO layer was larger than the Schottky electrodes to form a protective ring.With a Schottky electrode diameter of 100 µm, p-NiO/β-Ga 2 O 3 HJDs with a maximum BV of 8.9 kV were achieved, exhibiting a R on of 7.9 mΩ•cm 2 and a PFOM surpassing 10 GW/cm 2 .Devices with an area of 1 mm 2 also reached a BV of 4.7 kV and an I F of 4.1 A at 10 V, with a PFOM of 9 GW/cm 2 [225][226][227].The performance of both small-area and large-area devices exceeded the limit of unipolar power devices based on SiC and GaN, fully demonstrating the potential of β-Ga 2 O 3 in future high-power applications.
The performance of HJDs can be further enhanced through the introduction of additional terminal structures.Li et al. [228] prepared a large-area (1 × 1 mm 2 ) NiO/β-Ga 2 O 3 HJD on β-Ga 2 O 3 with a 15 µm drift layer, simultaneously introducing a SiO 2 /SiN x bilayer field plate structure, as portrayed in Figure 11a.Compared to the device without the field plate structure, the BV increased from 5 kV to 7 kV, and the PFOM improved from 5.7 to 9.2 GW•cm −2 .Wang et al. [229] applied a photoresist reflow technique to introduce a field plate structure with a small angle (~8.5 • ), as shown in Figure 11b.Compared to the vertical field plate structure, the BV of the device increased from 1945 V to 2410 V, and the device exhibited a R on of just 1.12 mΩ•cm 2 , enabling a PFOM up to 5.18 GW/cm 2 .Zhang et al. [39] simultaneously used Mg ion implantation terminals and a SiO 2 field plate structure to mitigate the effect of the electric field concentration of the NiO/β-Ga 2 O 3 heterojunction, as displayed in Figure 11c.The device achieved a BV of up to 8.32 kV, a R on of 5.24 mΩ•cm 2 , and a PFOM as high as 13.2 GW/cm 2 .This surpasses the unipolar limit of gallium nitride and silicon carbide, demonstrating its tremendous potential in next-generation power electronics applications.limit of gallium nitride and silicon carbide, demonstrating its tremendous potential in next-generation power electronics applications.Metal-dielectric-semiconductor (MDS) HJDs were fabricated by inserting an insulating dielectric layer between the metal and β-Ga2O3, effectively enhancing the device's reverse characteristics while generally observing a decrease in forward performance, and they will not be discussed in detail here [230-234].

Heterojunction Barrier Schottky Diodes
Although HJDs exhibit a higher BV, their junction capacitance and reverse recovery time are increased due to carrier recombination.To combine the advantages of p-n junctions and SBDs, researchers have proposed heterojunction barrier Schottky (HJBS) diodes.In 2020, Lv et al. [235] reported the first realization of β-Ga2O3 HJBS diodes through the thermal oxidation of p-NiO, as shown in Figure 12a.The diodes with an area of 100 × 100 µm 2 achieved a BV of 1715 V and a Ron of 3.45 mΩ•cm 2 , yielding a PFOM of up to 0.85 GW/cm 2 .Additionally, large-scale HJBS diodes with an area of 1 × 1 mm 2 reached an IF and BV of 5 A/700 V. Yan et al. [236] fabricated HJBS diodes by sputterdepositing p-NiO in a β-Ga2O3 groove, as illustrated in Figure 12b.With a fin width of 3 µm, the device exhibited a Ron of only 1.94 mΩ•cm 2 and a BV of 1.34 kV, leading to a PFOM of 0.93 GW/cm 2 .Furthermore, the sidewall depletion effect of the p-NiO was enhanced with a reduction in fin width, resulting in a decreased reverse leakage current.Metal-dielectric-semiconductor (MDS) HJDs were fabricated by inserting an insulating dielectric layer between the metal and β-Ga 2 O 3 , effectively enhancing the device's reverse characteristics while generally observing a decrease in forward performance, and they will not be discussed in detail here [230-234].

Heterojunction Barrier Schottky Diodes
Although HJDs exhibit a higher BV, their junction capacitance and reverse recovery time are increased due to carrier recombination.To combine the advantages of p-n junctions and SBDs, researchers have proposed heterojunction barrier Schottky (HJBS) diodes.In 2020, Lv et al. [235] reported the first realization of β-Ga 2 O 3 HJBS diodes through the thermal oxidation of p-NiO, as shown in Figure 12a.The diodes with an area of 100 × 100 µm 2 achieved a BV of 1715 V and a Ron of 3.45 mΩ•cm 2 , yielding a PFOM of up to 0.85 GW/cm 2 .Additionally, large-scale HJBS diodes with an area of 1 × 1 mm 2 reached an I F and BV of 5 A/700 V. Yan et al. [236] fabricated HJBS diodes by sputter-depositing p-NiO in a β-Ga 2 O 3 groove, as illustrated in Figure 12b.With a fin width of 3 µm, the device exhibited a R on of only 1.94 mΩ•cm 2 and a BV of 1.34 kV, leading to a PFOM of 0.93 GW/cm 2 .Furthermore, the sidewall depletion effect of the p-NiO was enhanced with a reduction in fin width, resulting in a decreased reverse leakage current.
Materials 2024, 17, x FOR PEER REVIEW 17 of 34 limit of gallium nitride and silicon carbide, demonstrating its tremendous potential in next-generation power electronics applications.Metal-dielectric-semiconductor (MDS) HJDs were fabricated by inserting an insulating dielectric layer between the metal and β-Ga2O3, effectively enhancing the device's reverse characteristics while generally observing a decrease in forward performance, and they will not be discussed in detail here [230-234].

Heterojunction Barrier Schottky Diodes
Although HJDs exhibit a higher BV, their junction capacitance and reverse recovery time are increased due to carrier recombination.To combine the advantages of p-n junctions and SBDs, researchers have proposed heterojunction barrier Schottky (HJBS) diodes.In 2020, Lv et al. [235] reported the first realization of β-Ga2O3 HJBS diodes through the thermal oxidation of p-NiO, as shown in Figure 12a.The diodes with an area of 100 × 100 µm 2 achieved a BV of 1715 V and a Ron of 3.45 mΩ•cm 2 , yielding a PFOM of up to 0.85 GW/cm 2 .Additionally, large-scale HJBS diodes with an area of 1 × 1 mm 2 reached an IF and BV of 5 A/700 V. Yan et al. [236] fabricated HJBS diodes by sputterdepositing p-NiO in a β-Ga2O3 groove, as illustrated in Figure 12b.With a fin width of 3 µm, the device exhibited a Ron of only 1.94 mΩ•cm 2 and a BV of 1.34 kV, leading to a PFOM of 0.93 GW/cm 2 .Furthermore, the sidewall depletion effect of the p-NiO was enhanced with a reduction in fin width, resulting in a decreased reverse leakage current.SBD without edge termination, and a NiO/β-Ga 2 O 3 HJD, as depicted in Figure 12c.The HJBS-2 µm device demonstrated the highest BV of 1.89 kV, a R on of 7.7 mΩ•cm 2 , and a PFOM of 0.46 GW/cm 2 , with the reverse leakage mechanism identified as Poole-Frenkel emission according to J-V-T measurements.
The integration of field plate structures can further enhance the performance of HJBS diodes.Wei et al. [238] fabricated β-Ga 2 O 3 HJBS diodes with dimensions of 3 × 3 mm 2 and 4 × 4 mm 2 through thermal oxidation of p-NiO, incorporating SiO 2 field plates, as shown in Figure 12d.The BV and specific R on were measured at 550 V/500 V and 11 mΩ•cm 2 /15 mΩ•cm 2 , respectively, with the current exceeding 50 A under a forward bias of 6 V. Long-term high-temperature stress tests showed that the devices had good electrical and thermal reliability.Wu et al. [239] introduced a small-angle (~8 • ) SiO 2 field plate structure on the edge of the HJBS diodes, as depicted in Figure 12e, which increased the BV of the small-scale (0.1 × 0.1 mm 2 ) diode from 1895 V to 2395 V, achieving a PFOM of 0.72 GW/cm 2 .For the large-scale (3 × 3 mm 2 ) diode, the BV rose from 685 V to 1060 V, and after packaging, the device exhibited a t rr of 26.8 ns under switching conditions, with a di/dt of 400 A/µs.These findings demonstrate the significant potential for β-Ga 2 O 3 HJBS diodes in various applications.
The distribution of p-NiO also has a significant impact on the performance of HJBS diodes.Zhang et al. [240] compared the performance of 1 mm 2 NiO x /β-Ga 2 O 3 HJBS diodes with stripe and honeycomb anode island layouts, as shown in Figure 13.In comparison to the stripe HJBS diodes, the honeycomb HJBS diodes had a slightly higher V on and R on but a BV that increased from 412 V to 567 V, resulting in a 59% increase in BFOM.Furthermore, the honeycomb HJBS diodes exhibited superior surge current stability when compared to the stripe HJBS diodes, attributed to the superior heat dissipation capability of the honeycomb layout.Simulated results suggested that reducing the size of the honeycomb could further enhance its forward conduction capability.Gong et al. [237] compared five different device structures, including HJBS diodes with field limiting rings (FLRs) of 2 µm or 3 µm, a 2 µm FLR Ni/β-Ga2O3 SBD, a Ni/β-Ga2O3 SBD without edge termination, and a NiO/β-Ga2O3 HJD, as depicted in Figure 12c.The HJBS-2 µm device demonstrated the highest BV of 1.89 kV, a Ron of 7.7 mΩ•cm 2 , and a PFOM of 0.46 GW/cm 2 , with the reverse leakage mechanism identified as Poole-Frenkel emission according to J-V-T measurements.
The integration of field plate structures can further enhance the performance of HJBS diodes.Wei et al. [238] fabricated β-Ga2O3 HJBS diodes with dimensions of 3 × 3 mm 2 and 4 × 4 mm 2 through thermal oxidation of p-NiO, incorporating SiO2 field plates, as shown in Figure 12d.The BV and specific Ron were measured at 550 V/500 V and 11 mΩ•cm 2 /15 mΩ•cm 2 , respectively, with the current exceeding 50 A under a forward bias of 6 V. Longterm high-temperature stress tests showed that the devices had good electrical and thermal reliability.Wu et al. [239] introduced a small-angle (~8°) SiO2 field plate structure on the edge of the HJBS diodes, as depicted in Figure 12e, which increased the BV of the small-scale (0.1 × 0.1 mm 2 ) diode from 1895 V to 2395 V, achieving a PFOM of 0.72 GW/cm 2 .For the large-scale (3 × 3 mm 2 ) diode, the BV rose from 685 V to 1060 V, and after packaging, the device exhibited a trr of 26.8 ns under switching conditions, with a di/dt of 400 A/µs.These findings demonstrate the significant potential for β-Ga2O3 HJBS diodes in various applications.
The distribution of p-NiO also has a significant impact on the performance of HJBS diodes.Zhang et al. [240] compared the performance of 1 mm 2 NiOx/β-Ga2O3 HJBS diodes with stripe and honeycomb anode island layouts, as shown in Figure 13.In comparison to the stripe HJBS diodes, the honeycomb HJBS diodes had a slightly higher Von and Ron but a BV that increased from 412 V to 567 V, resulting in a 59% increase in BFOM.Furthermore, the honeycomb HJBS diodes exhibited superior surge current stability when compared to the stripe HJBS diodes, attributed to the superior heat dissipation capability of the honeycomb layout.Simulated results suggested that reducing the size of the honeycomb could further enhance its forward conduction capability.

Junction Termination Extension Structures and Super Junction SBDs
By introducing p-NiO into various termination structures to replace conventional field plate dielectrics, devices can benefit from the conductance modulation effect to effectively reduce the R on , while lateral expansion of the junction termination can alleviate electric field crowding and enhance device performance.In 2022, Hao et al. [241,242] utilized p-NiO to form junction termination extension (JTE) structures, as shown in Figure 14a.The resulting devices exhibited a R on of 2.9 mΩ•cm 2 , a BV of 2.11 kV, and a PFOM reaching 1.54 GW/cm 2 .Additionally, they fabricated large-area SBDs with an area of 0.78 mm 2 , achieving a forward current density of 180 A/cm 2 at 2 V, a BV of up to 1.3 kV, and measuring a t rr of 15.6 ns and C rr of 15.3 nC, on par with commercial SiC SBDs.Wang et al. [243] constructed stairshaped JTE structures comprising multiple layers of p-NiO, as depicted in Figure 14b; the device showed a R on of 5.9 mΩ•cm 2 , a BV of greater than 2.5 kV, and a PFOM surpassing 1 GW/cm 2 .Xiao et al. [244] introduced a stair-shaped JTE structure on the edge of a NiO/β-Ga 2 O 3 HJD, illustrated in Figure 14c, improving the device's BV from 1770 V to 3590 V, with a PFOM reaching 2.7 GW/cm 2 .These developments convincingly demonstrate the potential for p-NiO termination extension structures in β-Ga 2 O 3 power devices.14b; the device showed a Ron of 5.9 mΩ•cm 2 , a BV of greater than 2.5 kV, and a PFOM surpassing 1 GW/cm 2 .Xiao et al. [244] introduced a stair-shaped JTE structure on the edge of a NiO/β-Ga2O3 HJD, illustrated in Figure 14c, improving the device's BV from 1770 V to 3590 V, with a PFOM reaching 2.7 GW/cm 2 .These developments convincingly demonstrate the potential for p-NiO termination extension structures in β-Ga2O3 power devices.Integrating p-NiO within the edge termination structure also enables the formation of an edge termination extension structure, as depicted in Figure 14d [245].Compared to a simple structure, the BV of the device was increased from 356 V to 1539 V, and p-NiOx effectively passivated the damage caused by dry-etching, yielding ideality factors close to 1 across varying temperatures, albeit with a notable reduction in forward current.As the temperature increases, the hole concentration increases, resulting in a lower leakage current of the SBDs compared to conventional SBDs at high temperatures, and the small polarization transport model in NiOx is used to explain this phenomenon.Yan et al. [246] fabricated p-NiOx edge termination structures with integrated SiO2 field plates, as shown in Figure 14e.For Schottky electrodes with a diameter of 60 µm, the device achieved a maximum BV of 2000 V, a Ron of 3.12 mΩ•cm 2 , and a PFOM of 1.11 GW/cm 2 .The same process was utilized to create large-area SBDs with a diameter of 1240 µm, which exhibited a high IF of 7.13 A at 4.9 V, a BV of 1260 V, and a PFOM of 235 MW/cm 2 .
Qin et al.Integrating p-NiO within the edge termination structure also enables the formation of an edge termination extension structure, as depicted in Figure 14d [245].Compared to a simple structure, the BV of the device was increased from 356 V to 1539 V, and p-NiO x effectively passivated the damage caused by dry-etching, yielding ideality factors close to 1 across varying temperatures, albeit with a notable reduction in forward current.As the temperature increases, the hole concentration increases, resulting in a lower leakage current of the SBDs compared to conventional SBDs at high temperatures, and the small polarization transport model in NiO x is used to explain this phenomenon.Yan et al. [246] fabricated p-NiO x edge termination structures with integrated SiO 2 field plates, as shown in Figure 14e.For Schottky electrodes with a diameter of 60 µm, the device achieved a maximum BV of 2000 V, a R on of 3.12 mΩ•cm 2 , and a PFOM of 1.11 GW/cm 2 .The same process was utilized to create large-area SBDs with a diameter of 1240 µm, which exhibited a high I F of 7.13 A at 4.9 V, a BV of 1260 V, and a PFOM of 235 MW/cm 2 .
Qin et al.In summary, β-Ga 2 O 3 p-n heterojunction devices are the most investigated to date and exhibit superior overall performance, making them highly promising for power circuit applications.In the future, a further advancement in p-n junction devices is anticipated with the effective doping of p-type β-Ga 2 O 3 .

Lateral-Structure Diodes
With the swift advancement of vertical β-Ga 2 O 3 SBDs, lateral β-Ga 2 O 3 SBDs have also seen considerable development.Although lateral structures require a larger area compared to their vertical counterparts, they can be integrated with heterogeneous substrates to significantly reduce costs and improve thermal dissipation, mitigating the self-heating effects due to the low thermal conductivity of β-Ga 2 O 3 materials.This section predominantly summarizes lateral β-Ga 2 O 3 SBDs with a BV exceeding 1 kV.
In 2018, Hu et al. [248] transferred a β-Ga 2 O 3 nano-membrane onto a sapphire substrate via mechanical exfoliation, producing simple-structured lateral SBDs as depicted in Figure 15a.With an anode-cathode spacing (L AC ) of 15 µm, these devices achieved a BV of 1.7 kV, although the breakdown field was only 1.13 MV/cm and the R on was relatively high at 190 mΩ•cm 2 .Even at 150 • C, the on/off ratio remained over 10 7 , demonstrating the cooling advantage of the sapphire substrate.In the same year, by integrating a field plate structure as shown in Figure 15b parameter changes after hundreds of cycles at a peak voltage of 1.7 kV during repetitive UIS tests, validating the device's robust functionality under continuous switching.
In summary, β-Ga2O3 p-n heterojunction devices are the most investigated to date and exhibit superior overall performance, making them highly promising for power circuit applications.In the future, a further advancement in p-n junction devices is anticipated with the effective doping of p-type β-Ga2O3.

Lateral-Structure Diodes
With the swift advancement of vertical β-Ga2O3 SBDs, lateral β-Ga2O3 SBDs have also seen considerable development.Although lateral structures require a larger area compared to their vertical counterparts, they can be integrated with heterogeneous substrates to significantly reduce costs and improve thermal dissipation, mitigating the self-heating effects due to the low thermal conductivity of β-Ga2O3 materials.This section predominantly summarizes lateral β-Ga2O3 SBDs with a BV exceeding 1 kV.
In 2018, Hu et al. [248] transferred a β-Ga2O3 nano-membrane onto a sapphire substrate via mechanical exfoliation, producing simple-structured lateral SBDs as depicted in Figure 15a.With an anode-cathode spacing (LAC) of 15 µm, these devices achieved a BV of 1.7 kV, although the breakdown field was only 1.13 MV/cm and the Ron was relatively high at 190 mΩ•cm 2 .Even at 150 °C, the on/off ratio remained over 10 7 , demonstrating the cooling advantage of the sapphire substrate.In the same year, by integrating a field plate structure as shown in Figure 15b   Roy et al. [250] epitaxially grew a Si-doped β-Ga 2 O 3 thin film on Fe-doped β-Ga 2 O 3 insulating substrates by MOCVD.They etched the film into rectangular trenches using RIE and utilized the ultra-high-k material BaTiO 3 as the dielectric to construct lateral super junction structures, as depicted in Figure 15d.The devices achieved a maximum BV of 2359 V, with a L AC = 5 µm and fin width of 2 µm resulting in a BV of 1487 V and a R on of only 1.65 mΩ•cm 2 , demonstrating a PFOM reaching 2.7 GW/cm 2 and effectively showcasing the superiority of the lateral super junction structure.
The introduction of p-NiO also significantly enhanced the performance of lateral structure devices.Liu et al. [251] proposed a β-Ga 2 O 3 field-effect rectifier (FER) featuring a p-NiO x gate, as illustrated in Figure 15e.Compared to the lateral HJDs and SBDs on the same substrate, the p-NiO x provided an additional conductive path under high forward bias, resulting in the highest I F and the lowest R on , with the V on being only 41% of the HJDs.Under reverse bias, the p-NiO x effectively depleted the channel layer, achieving a leakage current four orders of magnitude lower than that of the SBDs.With a L AC = 12 µm, the BV reached 1.55 kV.Hence, FERs amalgamate the advantages of HJDs and SBDs.Qin et al. [37] deposited p-NiO on the surface of β-Ga 2 O 3 to form a RESURF structure to deplete the charge in the channel, thereby minimizing surface electric field concentration, as shown in Figure 15f.Charge balance was achieved with a p-NiO thickness of 75 nm, resulting in a BV exceeding 10 kV for a L AC > 30 µm, a R on of 270 mΩ•cm 2 , and a persistent performance above 10 kV even at 200 • C, thus fully demonstrating the potential of β-Ga 2 O 3 power devices.
Furthermore, the development of lateral flexible SBDs based on β-Ga 2 O 3 has been explored.Due to the crystal structure of β-Ga 2 O 3 , strain induces microcracks in the nanofilm, leading to device performance degradation, which will not be recounted here in detail [252,253].

Summary
The preceding sections have delineated the performance characteristics of various β-Ga 2 O 3 power SBD structures.In the simple structure, the high-voltage potential intrinsic to β-Ga 2 O 3 material was unveiled by eliminating unreliable surface layers.Field plate structures were employed to mitigate electric field concentration at the electrode edges, while edge termination further reduced geometric catastrophe in devices.The trench structure, through the side MOS structure's RESURF effect, significantly improved the reverse performance of the devices, albeit at the expense of forward performance due to the reduced conduction area.The mesa structure shifted the geometric discontinuities between the electrodes and β-Ga 2 O 3 inwards into the β-Ga 2 O 3 material itself, and device performance was further enhanced through the introduction of the MOS structure.Heterojunction diodes exhibited the most exceptional comprehensive performance and have been the subject of the broadest research.The lateral structure achieved the highest BV but has a sizable R on .Table 2 comprehensively compares devices with high performance across the various structures.

Surge Current Ruggedness and Thermal Management
In practical circuit applications, anomalies such as short circuits, overloads, and arcing can induce surge currents.Under such conditions, β-Ga 2 O 3 SBDs generate substantial heat instantaneously.Given the inherently low thermal conductivity of the β-Ga 2 O 3 material, this leads to rapid temperature increases, profoundly affecting device performance and even causing thermal breakdown, resulting in irreversible circuit damage.Thus, surge current robustness and thermal management for devices are paramount in real applications.
Experiments have demonstrated that integrating β-Ga 2 O 3 heterostructures onto highthermal-conductivity substrates can effectively enhance device heat dissipation.One of the most effective solutions is to utilize diamond substrates to grow Ga 2 O 3 thin films, due to the excellent thermal conductivity of diamond, which can effectively dissipate heat [254][255][256][257][258].However, due to lattice mismatch, the quality of β-Ga 2 O 3 films grown by heteroepitaxy is often poor, limiting device performance [259][260][261].In 2019, Xu et al. [262] first reported the heterogeneous integration of 2-inch β-Ga 2 O 3 films onto 4H-SiC and Si (001) substrates using an ion-cutting process.Due to the high thermal conductivity of the substrate, β-Ga 2 O 3 MOSFETs fabricated on the heterogeneous integration wafers exhibited excellent thermal stability at 500 K. Infrared thermal imaging analysis revealed that, under the same power, the temperature rise of SBDs on the β-Ga 2 O 3 /SiC heteroepitaxial wafer was only a quarter of that on the β-Ga 2 O 3 wafer, demonstrating that the combination of β-Ga 2 O 3 thin film with a high-thermal-conductivity SiC substrate effectively promoted the heat dissipation of β-Ga 2 O 3 -based devices [263].
However, for most β-Ga 2 O 3 SBDs that have vertical structures, this approach is not applicable.Researchers have discovered through simulation and experimentation that substrate thinning and junction-side cooling can effectively lower the thermal resistance of β-Ga 2 O 3 SBDs, thereby enhancing their surge current robustness [264][265][266][267]. Zhou et al. [268] fabricated a field plate heterojunction diode and reduced the substrate from 650 µm to 150 µm, enabling the device to withstand a surge current of 50 A. Xiao et al. [269] applied bottom cooling and double-side cooling for device packaging for the first time, as shown in Figure 16a,b, achieving peak surge currents of 37.5 A and 68 A, respectively, demonstrating the feasibility of double-side cooling.Due to the small temperature dependence of the R on , the thermal runaway is significantly reduced, resulting in a peak surge current to rated current ratio higher than that of commercial SiC SBDs of the same level.Later, by using a transient dual-interface method, they measured junction-case thermal resistances of 1.43 K/W and 0.5 K/W for bottom cooling and junction-side cooling, respectively, with the latter being lower than that of commercial SiC SBDs of the same level, proving the effectiveness of junction-side cooling for β-Ga 2 O 3 devices [270].Gong et al. [271] thinned the substrate to 70 µm and employed junction-side cooling packaging, as shown in Figure 16c.Compared to non-thinned devices, the junction-case thermal resistance was reduced from 2.71 K/W to 1.48 K/W, and the surge current resistance improved from 47A to 58A.When the device was applied to a 150 W system-level power conversion circuit, a record-breaking conversion efficiency of 98.9% was achieved, revealing the prospect of device-level thermal management for high-power β-Ga 2 O 3 SBDs.Despite the low thermal conductivity of the β-Ga 2 O 3 material, with appropriate thermal management methods, β-Ga 2 O 3 diodes can achieve temperature rises equivalent to or even lower than those of SiC diodes, effectively enhancing the robustness of their surge current.
applied to a 150 W system-level power conversion circuit, a record-breaking conversion efficiency of 98.9% was achieved, revealing the prospect of device-level thermal management for high-power β-Ga2O3 SBDs.Despite the low thermal conductivity of the β-Ga2O3 material, with appropriate thermal management methods, β-Ga2O3 diodes can achieve temperature rises equivalent to or even lower than those of SiC diodes, effectively enhancing the robustness of their surge current.

Circuit Application and Reliability
In the past decade, the rapid advancement of β-Ga2O3 SBDs has led to significant improvements in their performance, prompting researchers to explore their potential in power circuit applications.Oishi et al.Power semiconductor devices are often expected to operate under high-load conditions and sustain prolonged durations, thus rendering their long-term operational

Circuit Application and Reliability
In the past decade, the rapid advancement of β-Ga 2 O 3 SBDs has led to significant improvements in their performance, prompting researchers to explore their potential in power circuit applications.Oishi et al. efficiency of 98.9% was achieved, revealing the prospect of device-level thermal management for high-power β-Ga2O3 SBDs.Despite the low thermal conductivity of the β-Ga2O3 material, with appropriate thermal management methods, β-Ga2O3 diodes can achieve temperature rises equivalent to or even lower than those of SiC diodes, effectively enhancing the robustness of their surge current.

Circuit Application and Reliability
In the past decade, the rapid advancement of β-Ga2O3 SBDs has led to significant improvements in their performance, prompting researchers to explore their potential in power circuit applications.Oishi et al.Power semiconductor devices are often expected to operate under high-load conditions and sustain prolonged durations, thus rendering their long-term operational Power semiconductor devices are often expected to operate under high-load conditions and sustain prolonged durations, thus rendering their long-term operational reliability as an inevitable challenge.Wilhelmi et al. [275] fabricated large-area β-Ga 2 O 3 SBDs using N ion-implanted terminations and SiO 2 field plates, achieving a BV greater than 1.1 kV.When deployed in a 400 V to 200 V step-down converter, the devices were able to operate stably for several hours at switching frequencies up to 350 kHz, with an efficiency markedly superior to fast recovery silicon diodes, particularly under high-frequency and high-power conditions.Zhou et al. [207] ascertained the reliability of beveled mesa NiO/β-Ga 2 O 3 HJDs following more than one million dynamic breakdown events at a peak overvoltage of 1.2 kV, with no significant performance degradation observed.Additionally, the devices demonstrated an up to 98.5% conversion efficiency and a stable operational capability of 100 min in a 500 W power conversion circuit.
In conclusion, current studies on the circuit application and reliability verification of β-Ga 2 O 3 SBD devices remain sparse, necessitating extensive research to further their practical implementation.

Summary and Prospect
This article provides a summary of the research progress on β-Ga 2 O 3 power diode devices.Due to the low cost and high performance of β-Ga 2 O 3 materials, they have significant potential for surpassing GaN and SiC as one of the leading materials in the high-power electronics market.Presently, research on Ga 2 O 3 devices is in its infancy, yet the future holds promising prospects and opportunities.At the same time as optimizing materials and structures to enhance the performance of β-Ga 2 O 3 power devices, the following points will have a considerable impact on the practical applications of β-Ga 2 O 3 power diode devices.

Figure 1 .
Figure 1.Transformation relationships between different crystal phases of β-Ga2O3 [13].Monoclinic-phase β-Ga2O3 belongs to the C2/m space group with a densely stacked anion structure.Its crystal structure and unit cell schematic are shown in Figure 2 [19,62].The unit cell is comprised of two GaO4 tetrahedra and two GaO6 octahedra, containing two different Ga atomic positions, as well as three distinct O atomic sites.The lattice constants of β-Ga2O3 are as follows: a = 12.21 Å, b = 3.03 Å, c = 5.79 Å, with α = γ = 90° and β = 103.8°.The length of the a-axis is four times that of the b-axis, and the length of the caxis is 1.9 times that of the b-axis, which facilitates cleavage along the (100) and (001) directions.Utilizing these crystallographic properties of β-Ga2O3, it is feasible to perform mechanical exfoliation to create quasi-two-dimensional thin-layer materials.This attribute endows β-Ga2O3 with significant potential in the domain of two-dimensional materials and devices[63][64][65][66].

Figure 13 .
Figure 13.(a) Schematic cross-sectional diagram of NiOx/β-Ga2O3 HJBS diodes; (b) schematic (top views) of anode layout of stripe HJBS diodes and (c) honeycomb HJBS diodes.(The green regions are p-NiOx and the white regions are Ga2O3.)[240].4.2.3.Junction Termination Extension Structures and Super Junction SBDsBy introducing p-NiO into various termination structures to replace conventional field plate dielectrics, devices can benefit from the conductance modulation effect to effectively reduce the Ron, while lateral expansion of the junction termination can alleviate electric field crowding and enhance device performance.In 2022, Hao et al.[241,242]
[247] successfully fabricated vertical super junction Schottky barrier diodes (SJ-SBDs) by integrating p-NiO within the trenches of β-Ga 2 O 3 , as illustrated in Figure 14f.They employed bilayer β-Ga 2 O 3 epitaxial growth to realize a low R on , used a SiO 2 sacrificial layer to prevent etching damage to the NiO, and controlled the parameters of the NiO layer for charge balance.The resulting devices exhibited a R on of only 0.7 mΩ•cm 2 , a BV reaching 2000 V, and maintained a BV in excess of 1.8 kV at 175 • C. Device robustness was confirmed under dynamic voltage conditions, withstanding breakdown at 2.2 kV in unclamped inductive switching (UIS) tests.Furthermore, the SJ-SBDs demonstrated no parameter changes after hundreds of cycles at a peak voltage of 1.7 kV during repetitive UIS tests, validating the device's robust functionality under continuous switching.
, they developed lateral β-Ga 2 O 3 SBDs with a reduced R on of 10.2 mΩ•cm 2 , an enhanced BV of 2.25 kV, and a PFOM of 500 MW/cm 2 when the L AC = 16 µm [249].Wang et al. [38] employed a bilayer field plate structure and introduced a post-anode annealing (PAA) technique to refine the metal/β-Ga 2 O 3 interface, as shown in Figure 15c.With a L AC = 90 µm, they achieved lateral β-Ga 2 O 3 SBDs with a BV surpassing 10 kV and a R on of 485 mΩ•cm 2 .
, they developed lateral β-Ga2O3 SBDs with a reduced Ron of 10.2 mΩ•cm 2 , an enhanced BV of 2.25 kV, and a PFOM of 500 MW/cm 2 when the LAC = 16 µm [249].Wang et al. [38] employed a bilayer field plate structure and introduced a post-anode annealing (PAA) technique to refine the metal/β-Ga2O3 interface, as shown in Figure 15c.With a LAC = 90 µm, they achieved lateral β-Ga2O3 SBDs with a BV surpassing 10 kV and a Ron of 485 mΩ•cm 2 .
[272] fabricated β-Ga2O3 FP SBDs and demonstrated their application in a single-ended parallel microwave power-rectifying circuit, successfully converting a 1.4 GHz microwave input signal at a power level of 23.7 dBm to a DC output of 43 mV.Guo et al. conducted extensive studies on the application of β-Ga2O3 SBDs in DC-DC converters with the circuit depicted in Figure 17a, achieving a peak conversion efficiency of 95.81% [155,273,274].Wu et al. [239] employed β-Ga2O3 HJBS diodes and commercial SiC SBDs to construct a hybrid half-wave (HW) Cockcroft-Walton (CW) voltage multiplier, as shown in Figure 17b.Compared with a four-stage hybrid voltage multiplier based on SiC SBDs, it exhibited a nearly equivalent multiplication factor of up to 3.81 and a circuit efficiency of approximately 86.07%.These findings underscore the substantial potential for the application of β-Ga2O3 devices in power circuits.
[272] fabricated β-Ga 2 O 3 FP SBDs and demonstrated their application in a single-ended parallel microwave power-rectifying circuit, successfully converting a 1.4 GHz microwave input signal at a power level of 23.7 dBm to a DC output of 43 mV.Guo et al. conducted extensive studies on the application of β-Ga 2 O 3 SBDs in DC-DC converters with the circuit depicted in Figure 17a, achieving a peak conversion efficiency of 95.81% [155,273,274].Wu et al. [239] employed β-Ga 2 O 3 HJBS diodes and commercial SiC SBDs to construct a hybrid half-wave (HW) Cockcroft-Walton (CW) voltage multiplier, as shown in Figure 17b.Compared with a four-stage hybrid voltage multiplier based on SiC SBDs, it exhibited a nearly equivalent multiplication factor of up to 3.81 and a circuit efficiency of approximately 86.07%.These findings underscore the substantial potential for the application of β-Ga 2 O 3 devices in power circuits.
[272] fabricated β-Ga2O3 FP SBDs and demonstrated their application in a single-ended parallel microwave power-rectifying circuit, successfully converting a 1.4 GHz microwave input signal at a power level of 23.7 dBm to a DC output of 43 mV.Guo et al. conducted extensive studies on the application of β-Ga2O3 SBDs in DC-DC converters with the circuit depicted in Figure 17a, achieving a peak conversion efficiency of 95.81% [155,273,274].Wu et al. [239] employed β-Ga2O3 HJBS diodes and commercial SiC SBDs to construct a hybrid half-wave (HW) Cockcroft-Walton (CW) voltage multiplier, as shown in Figure 17b.Compared with a four-stage hybrid voltage multiplier based on SiC SBDs, it exhibited a nearly equivalent multiplication factor of up to 3.81 and a circuit efficiency of approximately 86.07%.These findings underscore the substantial potential for the application of β-Ga2O3 devices in power circuits.
(a) The development of iridium-free β-Ga 2 O 3 single-crystal manufacturing technologies and the improvement of crystal quality will greatly facilitate the practical application of β-Ga 2 O 3 .(b) The mobility of NiO layers prepared by sputtering is significantly lower compared to β-Ga 2 O 3 , which limits the performance of HJDs to some extent.The realization of effective p-type doped Ga 2 O 3 , or developing alternative high-performance p-type materials, will further enhance the performance of β-Ga 2 O 3 HJDs.(c) Due to the very low thermal conductivity of β-Ga 2 O 3 materials, there is still a need for better β-Ga 2 O 3 thermal management methods and further research on device reliability.
[243]o form junction termination extension (JTE) structures, as shown in Figure14a.The resulting devices exhibited a Ron of 2.9 mΩ•cm 2 , a BV of 2.11 kV, and a PFOM reaching 1.54 GW/cm 2 .Additionally, they fabricated large-area SBDs with an area of 0.78 mm 2 , achieving a forward current density of 180 A/cm 2 at 2 V, a BV of up to 1.3 kV, and measuring a trr of 15.6 ns and Crr of 15.3 nC, on par with commercial SiC SBDs.Wang et al.[243]constructed stair-shaped JTE structures comprising multiple layers of p-NiO, as depicted in Figure