Delft University of Technology Review of the recent progress on GaN-based vertical power Schottky barrier diodes (SBDs)

Gallium nitride (GaN)-based vertical power Schottky barrier diode (SBD) has demonstrated outstanding features in high-frequency and high-power applications. This paper reviews recent progress on GaN-based vertical power SBDs, including the following sections. First, the benchmark for GaN vertical SBDs with different substrates (Si, sapphire, and GaN) are presented. Then, the latest progress in the edge terminal techniques are discussed. Finally, a typical fabrication flow of vertical GaN SBDs is also illustrated briefly.


Introduction
Today silicon devices have reached their physical limits either in terms of scaling down or in terms of their physical properties [1,2]. To further optimize device performance, new materials must be explored. Wide band gap materials (e.g., silicon carbide (SiC), gallium nitride (GaN), and diamond) have recently attracted a lot of interest for high power and high temperature applications [3][4][5][6][7][8]. SiC-based power devices have been already commercialized for high-voltage and high-power application [9,10]; diamond is another promising candidate [11,12]. Among all of these wide bandgap semiconductor material, GaN has a higher electron mobility than SiC and higher critical electric field than Si [13]. GaN-based devices are expected to meet the requirements of the future advanced power systems in the field of radio frequency and power conversion application.
The field of power electronics is concerned with the processing of electrical power using electronic devices. Power diodes are essential component in power converters and inverters in power transmission. With the superior physical and chemical properties of GaN, GaN-based power diodes can significantly increase the efficiency and reduce the energy loss [6,14]. Since the year 2000, GaN-based rectifiers (including Schottky barrier diode (SBD) and PN junction diode) have attracted considerable interest from researchers. With the absence of minority carrier accumulation and low barrier height, SBD can operate at higher frequencies with a lower turn-on voltage (V ON ) than the PN junction diode [5]. Yoshimoto et al. [15] demonstrated the operation of GaN SBD at high frequency with low power loss in power converter, comparing them with commercial Si fast recovery diode (FRD) and SiC SBDs via a typical E resonant rectifying tested at a frequency of 30 MHz.

GaN-on-Si
The main objective in the design of power devices is to obtain a high breakdown voltage (BV) while keeping the R ON,sp as low as possible [39]. However, a large number of dislocations in the GaN drift layer can cause leakage current when device is reverse biased [40,41]. To study the substrates impact on the performance of GaN-based power device, Figure 2 summarizes the data from literatures of GaN vertical SBDs with different substrates. The R ON,sp versus BV characteristics of GaN vertical SBD devices is still far from the ideal GaN limit [30]. The GaN homoepitaxial has low dislocations and high crystal quality, thus SBD shows much better performance than heteroepitaxial SBD. Quasi-vertical SBDs have several drawbacks [20,22]: (1) nonuniform distribution of current, (2) the current crowding problems, (3) large total device area, and (4) the deep etched sidewall process. All these issues greatly promote the development of fully-vertical SBD, where the electrodes (anode and cathode) are located at two sides of the wafer separately, with current flowing from anode to cathode through the drift layer in a vertical direction, as shown in Figure 1b. The fully-vertical SBDs have the advantages of effective device size and good thermal performance by cooling from both sides of the wafer. The GaN epitaxy layer grown on GaN substrate has lower dislocation densities than foreign substrates (e.g., Si, sapphire, or SiC), because of low lattice mismatch and low thermal expansion coefficient mismatch [20][21][22][23][24][25][26][27]. However, the dislocation density of GaN-on-GaN is limited by the defect density in the GaN substrate [28][29][30]. Before 2010, most GaN SBD devices were fabricated on a foreign substrate, as a result of the poor availability of bulk GaN (or free-standing) substrates. Most studies of vertical GaN-on-GaN SBD power devices have appeared in the last decade [31,32]. Moreover, with the appearance of the quasi-vertical device and novel device structures [33,34], sapphire and Si substrates have been investigated in recent years [35][36][37][38]. NIT, [35] MIT, [37] IIT, [36] RWTH, [33] U. Florida, [3] U. Texas, [86] U. Texas, [86] U. Texas, [86] U. Florida, [88] U. Shenzhen, [26] U. Zhejiang, [81] ASU, [87] U. Shenzhen, [82] MIT, [76] MIT, [76] HRL, [17] HRL, [19] TOYODA, [67] Sumitomo, [15] Sumitomo, [83] Sumitomo, [68] Sumitomo, [32] U. Auburn, [31] U. Seoul, [73] U. Florida, [54] U. Florida, [5] U. Auburn, [29] MIT, [75] U. Florida, [34] Avogy, [52] Cornell, [80] NCSU, [71] GaN Limit The main objective in the design of power devices is to obtain a high breakdown voltage (BV) while keeping the RON,sp as low as possible [39]. However, a large number of dislocations in the GaN drift layer can cause leakage current when device is reverse biased [40,41]. To study the substrates impact on the performance of GaN-based power device, Figure 2 summarizes the data from  This paper reviews the recent progress of vertical GaN SBD from literatures. The following aspects are covered; the device characteristics of GaN-based SBD in Section 2, terminal edge techniques in Section 3, the typical fabrication flow of vertical GaN SBD in Section 4, and the conclusion in Section 5.

Device Characteristics of Vertical GaN SBDs
This section focuses on the forward and reverse characteristics of vertical SBD. A typical SBD structure and its electric field distribution under reverse bias are shown in Figure 3a,b. It reveals that the maximum electric field is located at the interface between anode and semiconductor [42,43]. literatures of GaN vertical SBDs with different substrates. The RON,sp versus BV characteristics of GaN vertical SBD devices is still far from the ideal GaN limit [30]. The GaN homoepitaxial has low dislocations and high crystal quality, thus SBD shows much better performance than heteroepitaxial SBD. This paper reviews the recent progress of vertical GaN SBD from literatures. The following aspects are covered; the device characteristics of GaN-based SBD in Section 2, terminal edge techniques in Section 3, the typical fabrication flow of vertical GaN SBD in Section 4, and the conclusion in Section 5.

Device Characteristics of Vertical GaN SBDs
This section focuses on the forward and reverse characteristics of vertical SBD. A typical SBD structure and its electric field distribution under reverse bias are shown in Figure 3a, b. It reveals that the maximum electric field is located at the interface between anode and semiconductor [42,43].

Forward Conduction Characteristics
A conventional SBD structure consists of anode metal, drift layer, substrate, and cathode metal. When the diode is working in on-state, the turn-on voltage (VON), and RON are the important parameters [44]. For a vertical power Schottky diode, from the theory of the thermionic emission, the VON of an SBD can be expressed as [45] = * * where Jon is the forward current at VON, n is the ideality factor of the Schottky contact, A** is the effective Richardson's constant, ФB is the Schottky barrier height (SBH), and RON is the on-resistance. The ambipolar diffusion coefficient Richardson's constant is expressed as * * = 4 * ℎ (2) where mn* is the effective electron quality and h is Planck's constant. The theoretically calculated value of A** for GaN is 26.4 A·cm −2 ·K −2 . Combining Equations (1) and (2), VON is determined mainly by the Schottky barrier height and on-state resistance.
The total specific series resistance (RS,sp) consists of three parts: the ohmic contact resistance of the cathode (RCont), the substrate resistance (RSub), and the specific on-state resistance of the drift layer (RD,sp). The ideal RON,sp is equal to RD,sp in vertical SBD devices and given by [46] ,

Forward Conduction Characteristics
A conventional SBD structure consists of anode metal, drift layer, substrate, and cathode metal. When the diode is working in on-state, the turn-on voltage (V ON ), and R ON are the important parameters [44]. For a vertical power Schottky diode, from the theory of the thermionic emission, the V ON of an SBD can be expressed as [45] where J on is the forward current at V ON , n is the ideality factor of the Schottky contact, A** is the effective Richardson's constant, Φ B is the Schottky barrier height (SBH), and R ON is the on-resistance. The ambipolar diffusion coefficient Richardson's constant is expressed as where m n * is the effective electron quality and h is Planck's constant. The theoretically calculated value of A** for GaN is 26.4 A·cm −2 ·K −2 . Combining Equations (1) and (2), V ON is determined mainly by the Schottky barrier height and on-state resistance. The total specific series resistance (R S,sp ) consists of three parts: the ohmic contact resistance of the cathode (R Cont ), the substrate resistance (R Sub ), and the specific on-state resistance of the drift layer (R D,sp ). The ideal R ON,sp is equal to R D,sp in vertical SBD devices and given by [46] where W D is thickness of the depletion region, q is the electronic charge, µ n is the electron mobility, and N D is the doping concentration of the drift layer. The depletion width W D under a certain BV is given by The N D can be expressed as where Ec is the critical electric field of the material and ε s is the dielectric constant of the drift layer. The specific capacitance (capacitance per unit area) associated with this depletion region is given by

Reverse Breakdown Characteristics
From Equations (5) and (6), the relationship of W D , N D and BV can be expressed as According to the Equation (8), the reverse breakdown voltage is inversely dependent on the doping level in the drift layer and positively dependent on the depletion width. For example, in the quasi-vertical SBDs, increasing the depth of the mesa region, further leading to increase of the effective thickness in the drift layer, can help improve the BV characteristics. Low N D or high W D of the drift layer results in a high BV, however, with an increase of R ON characteristics.
According to Equations (3)-(6), the relationship between R ON,sp and BV can be expressed as [14] R ON,sp = 4BV 2 ε s µ n E C 3 (9) where ε s µ n E c is the intrinsic properties of semiconductor materials, commonly referred to as Baliga's figure of merit (BFOM). Table 1 lists the physical properties of Si, GaAs, SiC, GaN, and Diamond [8,42,47,48]. The critical electric field of GaN is 11 times greater than Si and the saturation velocity is 2.5 times greater than Si. According to the equations above, the ideal R ON,sp for the drift region of a vertical power device is given by [14,30] R on,sp (Si) = 5.93 × 10 −9 BV 2.5 (10) R on,sp (GaN) = 3.12 × 10 −12 BV 2.5 (12) A large number of traps and threading dislocations exist in GaN materials in reality. The presence of unintentional surface defect donors (e.g., nitrogen vacancy) can cause a reduction of the effective width of Schottky barrier, resulting in high leakage current under reverse bias, explained by the thin surface barrier (TSB) model [49]. Trap-assisted tunneling (TAT) is another trap-related leakage conduction mechanism [50]. Besides of the leakage induced by traps, threading dislocation is also a main cause of the devices leakage when device is reverse biased.
Temperature has a strong impact on leakage current under high reverse bias. The thermionic emission is positive correlated with temperature, thus, the Frenkel-Poole Emission (PFE) is a domination conduction mechanism in vertical GaN Schottky diodes at high electronic field and high temperature [11,51]. Additionally, trap-assisted tunneling (TAT) is also a temperature-dependent mechanism that causes increase in leakage current [50].
In a vertical power device, the peak electric field is crowded at the interface between the Schottky contact metal and the n-GaN drift layer, as shown in Figure 4, causing early device breakdown at the edge under reverse bias. Thus, novel terminal techniques are need to improve the vertical device breakdown.
A large number of traps and threading dislocations exist in GaN materials in reality. The presence of unintentional surface defect donors (e.g., nitrogen vacancy) can cause a reduction of the effective width of Schottky barrier, resulting in high leakage current under reverse bias, explained by the thin surface barrier (TSB) model [49]. Trap-assisted tunneling (TAT) is another trap-related leakage conduction mechanism [50]. Besides of the leakage induced by traps, threading dislocation is also a main cause of the devices leakage when device is reverse biased.
Temperature has a strong impact on leakage current under high reverse bias. The thermionic emission is positive correlated with temperature, thus, the Frenkel-Poole Emission (PFE) is a domination conduction mechanism in vertical GaN Schottky diodes at high electronic field and high temperature [11,51]. Additionally, trap-assisted tunneling (TAT) is also a temperature-dependent mechanism that causes increase in leakage current [50].
In a vertical power device, the peak electric field is crowded at the interface between the Schottky contact metal and the n-GaN drift layer, as shown in Figure 4, causing early device breakdown at the edge under reverse bias. Thus, novel terminal techniques are need to improve the vertical device breakdown.

Edge Termination Techniques
Edge termination techniques are proposed and utilized to improve the crowded electric field at the periphery of active region. The most commonly used termination techniques are discussed in this section, including field rings, junction termination extension (JTE), field plates, trench termination, reduced surface field (RESURF), and N-based termination.

Field Rings
A planar junction termination structure adopting the p-GaN region can provide the redistribution of high electric field at the edge of Schottky contact metal [30], as shown in Figure 5a.

Edge Termination Techniques
Edge termination techniques are proposed and utilized to improve the crowded electric field at the periphery of active region. The most commonly used termination techniques are discussed in this section, including field rings, junction termination extension (JTE), field plates, trench termination, reduced surface field (RESURF), and N-based termination.

Field Rings
A planar junction termination structure adopting the p-GaN region can provide the redistribution of high electric field at the edge of Schottky contact metal [30], as shown in Figure 5a. The p-GaN termination regions are formed by Mg+ ion implantation under anode edges in the n-GaN drift layer.
A large number of traps and threading dislocations exist in GaN materials in reality. The presence of unintentional surface defect donors (e.g., nitrogen vacancy) can cause a reduction of the effective width of Schottky barrier, resulting in high leakage current under reverse bias, explained by the thin surface barrier (TSB) model [49]. Trap-assisted tunneling (TAT) is another trap-related leakage conduction mechanism [50]. Besides of the leakage induced by traps, threading dislocation is also a main cause of the devices leakage when device is reverse biased.
Temperature has a strong impact on leakage current under high reverse bias. The thermionic emission is positive correlated with temperature, thus, the Frenkel-Poole Emission (PFE) is a domination conduction mechanism in vertical GaN Schottky diodes at high electronic field and high temperature [11,51]. Additionally, trap-assisted tunneling (TAT) is also a temperature-dependent mechanism that causes increase in leakage current [50].
In a vertical power device, the peak electric field is crowded at the interface between the Schottky contact metal and the n-GaN drift layer, as shown in Figure 4, causing early device breakdown at the edge under reverse bias. Thus, novel terminal techniques are need to improve the vertical device breakdown.

Edge Termination Techniques
Edge termination techniques are proposed and utilized to improve the crowded electric field at the periphery of active region. The most commonly used termination techniques are discussed in this section, including field rings, junction termination extension (JTE), field plates, trench termination, reduced surface field (RESURF), and N-based termination.

Field Rings
A planar junction termination structure adopting the p-GaN region can provide the redistribution of high electric field at the edge of Schottky contact metal [30], as shown in Figure 5a.   In early 2002, a research group from the University of Florida [50,51] reported a vertical GaN-on-GaN SBD structure with p-guard ring junction termination. They found that a GaN SBD with p-guard ring termination structure has a BV of 160 V. As shown in Figure 5b, the superior characteristics are obtained by "moving" the peak electric field at edge away from the surface into bulk with the help of depletion region in PN junction. Furthermore, the peak electric field at the edges can be reduced by extension of PN junction in depletion layer. However, the p + -GaN implantation method with high-temperature annealing conditions can increase the significant risk and complexity of the p-guard ring fabrication [52][53][54][55][56][57][58].
Comparing with the n-type GaN dopant process (Si implantation, activation temperature of 1250 • C [59]), p-GaN dopants require a higher annealing temperature of 1340 • C to activate Mg ions [60]. A low activation ration of Mg + for GaN results in low hole concentration, which can affect the GaN crystal quality causing low mobility and carrier concentration. The poor activation of dopants (Mg + atoms) has only produced 10 15 -10 17 cm −3 orders of magnitude of the carrier concentration [21]. Besides, the extremely high temperatures required to activate the implanted Mg during the annealing process also damage the GaN surface [61,62]. Greenlee, J.D., et al. [63] reported that the second annealing process followed by multicycle rapid thermal annealing (MRTA) process can compensate for the defects of the GaN surface and improve the crystalline quality of implanted p-GaN.

Junction Termination Extension (JTE)
The p-type region formed by Mg+ implantation can redistribute the surface electric field at the edge of the PN junction. This p-type region has been named JTE [64]. Koehler et al. [61] reported a GaN junction barrier-controlled Schottky (JBS) device with a JTE structure in 2016. Their study shows an improved reverse characteristic in JBS with JTE, achieving a higher BV of 610 V than conventional SBD (BV of 200 V). In the JBS rectifier, the forward current is designed to flow in the undepleted gaps between the P + regions when the diode is forward biased to keep in unipolar operation mode [65]. The pn junction below Schottky metal creates a potential barrier to shield the Schottky contact under reverse bias [13,30].

Field Plates
The field plate is another technique to redistribute the electric field at the edge of the Schottky contact metal under reverse bias. Currently, three types of field plates are used for SBDs, including a metal field plate (Figure 6a), a resistive field plate (Figure 6b), and a floating field plate ( Figure 6c). As shown in Figure 6d, the field plate located at the edge of an electrode can extend the depletion boundary and reduce electric field crowding under the reverse bias [66].
The metal field plate is formed by extending the contact metal over the field oxide/dielectrics at the edge of the junction [67], as shown in Figure 6a. In 2009, Horii et al. [68] demonstrated improved reverse characteristics in vertical GaN SBDs with a metal field plate (FP) for the first time, achieving 680 V of BV (400 V of BV without FP). Zhang et al. [37,69] was the first to report a quasi-vertical GaN-on-Si SBD where the destructive BV of the SBD without and with a FP structure is 90 V and 205 V, respectively.
Resistive field plate is an alternative field plate technique to smooth the electric field around the surface. This high-resistivity region can help spread the electric field at the edge of the anode metal. As shown in Figure 6b, Ozbek and Baliga [70,71] reported two types of GaN SBDs with a resistive field plate formed by Ar ion implantation on GaN and sapphire substrates, respectively. The vertical GaN-on-GaN SBD shows a higher breakdown voltage of 1650 V compared with the diodes without termination (BV of 300 V) [70]. Another GaN-on-Sapphire SBD show a BV of 1700 V with a resistive field plate, which is four times higher than that of the conventional SBD [71].
The structure of GaN SBD with floating field plate is shown in Figure 6c. When a negative bias is applied to the floating metal plate (or biased field plate) on a n-GaN drift layer it repels electrons away from the device surfaces. This will result in an expansion of the depletion region and then the peak electric field can be reduced around the edges. Thus, vertical SBDs with a biased field plate termination can achieve higher BV than devices without it. However, the introduction of an additional package terminal to provide a separate bias to the biased field plate increases the additional bias circuit cost [72]. Seung-Chul et al. [73] demonstrated that vertical GaN SBDs with a metal floating field plate termination has a higher BV of 353V than conventional structure (159 V).

Field Plates
The field plate is another technique to redistribute the electric field at the edge of the Schottky contact metal under reverse bias. Currently, three types of field plates are used for SBDs, including a metal field plate (Figure 6a), a resistive field plate (Figure 6b), and a floating field plate ( Figure 6c). As shown in Figure 6d, the field plate located at the edge of an electrode can extend the depletion boundary and reduce electric field crowding under the reverse bias [66]. The metal field plate is formed by extending the contact metal over the field oxide/dielectrics at the edge of the junction [67], as shown in Figure 6a. In 2009, Horii et al. [68] demonstrated improved reverse characteristics in vertical GaN SBDs with a metal field plate (FP) for the first time, achieving 680 V of BV (400 V of BV without FP). Zhang et al. [37,69] was the first to report a quasi-vertical GaNon-Si SBD where the destructive BV of the SBD without and with a FP structure is 90 V and 205 V, respectively.
Resistive field plate is an alternative field plate technique to smooth the electric field around the surface. This high-resistivity region can help spread the electric field at the edge of the anode metal. As shown in Figure 6b, Ozbek and Baliga [70,71] reported two types of GaN SBDs with a resistive field plate formed by Ar ion implantation on GaN and sapphire substrates, respectively. The vertical GaN-on-GaN SBD shows a higher breakdown voltage of 1650 V compared with the diodes without termination (BV of 300 V) [70]. Another GaN-on-Sapphire SBD show a BV of 1700 V with a resistive field plate, which is four times higher than that of the conventional SBD [71].
The structure of GaN SBD with floating field plate is shown in Figure 6c. When a negative bias is applied to the floating metal plate (or biased field plate) on a n-GaN drift layer it repels electrons away from the device surfaces. This will result in an expansion of the depletion region and then the peak electric field can be reduced around the edges. Thus, vertical SBDs with a biased field plate termination can achieve higher BV than devices without it. However, the introduction of an additional package terminal to provide a separate bias to the biased field plate increases the additional bias circuit cost [72]. Seung-Chul et al. [73] demonstrated that vertical GaN SBDs with a metal floating field plate termination has a higher BV of 353V than conventional structure (159 V).

Trench Termiantion
Baliga [66] investigated a typical trench metal oxide semiconductor (MOS) barrier Schottky (TMBS) rectifier structure consisting of an MOS structure at the trench region, as shown in Figure 7a. A trench region containing an MOS structure produces a potential barrier that can shield the Schottky contact at the reverse bias. The potential barrier is against the high electric field in the bulk of the GaN drift region. The reduction of the electric field around Schottky metal enables the reduction of the leakage current under reverse bias [74]. However, it has oxide layer reliability issues due to the high electric field at the corner of the trench.
Zhang et al. [75] reported a novel GaN trench metal-insulator-semiconductor (MIS) barrier Schottky diode with trench field rings, as shown in Figure 7b. They demonstrated that the GaN vertical FR-TMBS has a higher BV (700 V) than conventional SBDs (400 V). The FR regions were formed by Ar ion implantation, which could avoid a peak electric field at the trench corner and premature breakdown in the oxide layer. The electric field distribution at -1000V along the vertical cutline in the trench is shown in Figure 7c. Zhang et al. [76] also reported a vertical GaN-JBS with Arimplanted trench termination, achieving a BV of 500-600 V.

Trench Termiantion
Baliga [66] investigated a typical trench metal oxide semiconductor (MOS) barrier Schottky (TMBS) rectifier structure consisting of an MOS structure at the trench region, as shown in Figure 7a. A trench region containing an MOS structure produces a potential barrier that can shield the Schottky contact at the reverse bias. The potential barrier is against the high electric field in the bulk of the GaN drift region. The reduction of the electric field around Schottky metal enables the reduction of the leakage current under reverse bias [74]. However, it has oxide layer reliability issues due to the high electric field at the corner of the trench.

Reduced Surface Field (RESURF)
Planar junction termination is an effective technique to improve breakdown voltage. RESURF is the most popular termination techniques in the design of high-voltage power devices. RESURF vertical SBDs can achieve a high BV since the depletion of the Schottky contact in the vertical direction is reinforced by the adjacent PN junction [77][78][79]. Li et al. [80] studied the reduced surface field (RESURF) impact on JBS diodes, and reported that the vertical surface field within the trench is much reduced compared to conventional SBD, as shown in Figure 8b.  Zhang et al. [75] reported a novel GaN trench metal-insulator-semiconductor (MIS) barrier Schottky diode with trench field rings, as shown in Figure 7b. They demonstrated that the GaN vertical FR-TMBS has a higher BV (700 V) than conventional SBDs (400 V). The FR regions were formed by Ar ion implantation, which could avoid a peak electric field at the trench corner and premature breakdown in the oxide layer. The electric field distribution at -1000V along the vertical cutline in the trench is shown in Figure 7c. Zhang et al. [76] also reported a vertical GaN-JBS with Ar-implanted trench termination, achieving a BV of 500-600 V.

Reduced Surface Field (RESURF)
Planar junction termination is an effective technique to improve breakdown voltage. RESURF is the most popular termination techniques in the design of high-voltage power devices. RESURF vertical SBDs can achieve a high BV since the depletion of the Schottky contact in the vertical direction is reinforced by the adjacent PN junction [77][78][79]. Li et al. [80] studied the reduced surface field (RESURF) impact on JBS diodes, and reported that the vertical surface field within the trench is much reduced compared to conventional SBD, as shown in Figure 8b.

Reduced Surface Field (RESURF)
Planar junction termination is an effective technique to improve breakdown voltage. RESURF is the most popular termination techniques in the design of high-voltage power devices. RESURF vertical SBDs can achieve a high BV since the depletion of the Schottky contact in the vertical direction is reinforced by the adjacent PN junction [77][78][79]. Li et al. [80] studied the reduced surface field (RESURF) impact on JBS diodes, and reported that the vertical surface field within the trench is much reduced compared to conventional SBD, as shown in Figure 8b.

N-Based Termination
N-based termination (NT) techniques was reported by Yang et al. [6,81], as shown in Figure 9. By utilizing the nitridation plasma to form an N-based termination region around the edge of the anode, the N vacancies can be compensated and the Ga dangling bonds on the GaN surface can also be passivated. The NT structure at the junction edge presents a higher energy barrier height and/or effective barrier thickness around vertical GaN SBDs. Therefore, the leakage current at the edge is decreased under reverse bias due to the suppression of electron transport via thermionic-field emission (TFE) or tunneling. The NT-SBD with a BV of 995 V and a RON,sp of 1.2 mΩ·cm 2 has shown excellent static (high JF density of 2000 A/cm 2 ) and switching characteristics (fast reverse recovery

N-Based Termination
N-based termination (NT) techniques was reported by Yang et al. [6,81], as shown in Figure 9. By utilizing the nitridation plasma to form an N-based termination region around the edge of the anode, the N vacancies can be compensated and the Ga dangling bonds on the GaN surface can also be passivated. The NT structure at the junction edge presents a higher energy barrier height and/or effective barrier thickness around vertical GaN SBDs. Therefore, the leakage current at the edge is decreased under reverse bias due to the suppression of electron transport via thermionic-field emission (TFE) or tunneling. The NT-SBD with a BV of 995 V and a R ON,sp of 1.2 mΩ·cm 2 has shown excellent static (high J F density of 2000 A/cm 2 ) and switching characteristics (fast reverse recovery time of~17 ns and small reverse recovery charge of~0.8 nC), which impart potential advantages in high-power and high-frequency applications.

Fabrication Steps of Vertical GaN SBDs
A proposed process flow of vertical GaN SBD has been shown in Figure 11, which is similar to the literature [84]. The GaN epilayer was subsequently grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD) [85]. In Figure 11a, the contact metal was deposited on GaN epilayer. After that, the samples were annealed at a certain condition to further improve the ohmic behavior. (Ti/Al/Ni/Au at 600-840 • C in N 2 for 20-30s [2,26,29,33,65,67,71,72]; Ti/Al/Pt/Au at 700 • C-850 • C in N 2 for 30s [5,53,54,86]; Ti/Al [19,31,37,38,68,75,76,82]; Ti/Al/Ti/Au [32,67,83,87]; Ti/Al/Au [18,81].) Then, the nickel layer is formed on ohmic contact metal by an electroplating process, as shown in Figure 11b. To remove the sapphire substrate, a laser power was used to cause local heating at the GaN/sapphire interface which in turn lead to the decomposition of the GaN, as shown in Figure 11c,d. Finally, the Schottky metal (Ni/Au) was deposited using e-beam evaporation, as shown in Figure 11e. A proposed process flow of vertical GaN SBD has been shown in Figure 11, which is similar to the literature [84]. The GaN epilayer was subsequently grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD) [85]. In Figure 11a, the contact metal was deposited on GaN epilayer. After that, the samples were annealed at a certain condition to further improve the ohmic behavior. (Ti/Al/Ni/Au at 600-840°C in N2 for 20-30s [2,26,29,33,65,67,71,72]; Ti/Al/Pt/Au at

Conclusions
This paper reviewed and summarized the reported GaN vertical power SBDs ( Table 2). The recent progress of GaN vertical power SBD on different substrates has been presented. The R ON,sp versus BV characteristics of GaN vertical SBD devices is still far from the ideal GaN limit [30]. The GaN homoepitaxial has low dislocations and high crystal quality, thus SBD shows much better performance than GaN heteroepitaxial SBD. Then, the performance of GaN vertical SBDs with different edge termination techniques are discussed, including field rings, JTE, field plates, trench termination, RESURF, and N-based termination. In the practical design, the breakdown voltage is effectively improved by redistribution of electric field at electrode edges with all these edge termination techniques.
In the future studies, the performance of GaN vertical SBDs should be improved by optimizing the device structure and quality of the epitaxial layers. We hope this review can be valuable for research on GaN vertical power SBDs.