Simulation Study on Electrical Characteristics of NiO/β-Ga₂O₃ Heterojunction Enhancement Mode HJ-FinFET

Abstract

In this paper, a novel enhancement-mode β-Ga₂O₃-based FinFET structure with a gate formed by the NiO/β-Ga₂O₃ heterojunction named HJ-FinFET has been proposed, and the excellent performance of the device has also been demonstrated. The primary operational mechanism of this structure involves integrating p-type NiO on both sides of the fin-shaped channel, which forms p-n junctions with β-Ga₂O₃. The depletion regions thus generated are utilized to establish electron channels, enabling enhancement-mode operation. The reverse p-NiO/n-Ga₂O₃ heterojunction diode is integrated to reduce the reverse free-wheeling loss. Compared with the conventional devices, the threshold voltage of the HJ-FinFET is greatly improved, and normally off operation is realized, showing a positive threshold voltage of 2.14 V. Meanwhile, the simulated breakdown voltage of the HJ-FinFET reaches 2.65 kV with specific on-resistance (R_on,sp) of 2.48 mΩ·cm² and the power figure of merit (PFOM = BV²/R_on,sp) reaches 2840 MW/cm², respectively. In addition, the influence of the doping concentration of the heterojunction layer constituting the gate, the doping concentration of the drift layer, and the channel width on the electrical characteristics of the devices were focused on. This structure provides a feasible idea for high-performance β-Ga₂O₃-based FinFET.

1. Introduction

Power semiconductor devices play a critical role in the power systems, drive systems, and control systems across various fields such as aerospace, smart grids and other fields. They serve as core components for efficient energy conversion and precise control.

The new generation of power systems aims to achieve higher power capacity, lower power consumption, smaller size, higher efficiency, and higher reliability. Si-based power devices have reached their theoretical limits due to limitations such as operating frequency and critical breakdown electric field. Wide bandgap semiconductors SiC and GaN in the form of materials and devices have gradually developed and been applied, but are limited by the high production cost of high-quality single crystals [1,2,3,4]. β-Ga₂O₃ with an ultra-wide bandgap (E_g = 4.5~4.9 eV) and high critical breakdown field strength (E_br = 8 MV/cm) can be grown into high-quality, large-sized single crystals through various low-cost melt methods [5,6,7,8]. The Baliga figure of merit (BFOM) of the devices are 3444 times, 10 times, and 4 times higher than those of Si, SiC, and GaN devices, which indicates that at the same breakdown voltage (BV) level, the drift region width of β-Ga₂O₃ can be reduced to obtain lower specific on-resistance (R_{on, sp}), thereby achieving lower losses and smaller device size [9,10,11,12]. Therefore, various crystal orientation materials based on galium oxide have different properties and are widely used in various fields such as power electronic devices and photodetectors. Theoretically, it has significant advantages in high power, high efficiency, low loss, small size, and high reliability, which is conducive to improving the power density and conversion efficiency of the system, meeting the needs of its high efficiency, integration, and miniaturization development [13].

Previous research on gallium oxide power devices has focused on diodes and field-effect transistors, fully utilizing the material properties of β-Ga₂O₃ with a wide bandgap and high breakdown field strength to achieve high voltage resistance and low power consumption. Field plates, heterojunctions, ion implantation, and trench termination technologies are commonly used techniques in gallium oxide power devices to improve voltage resistance and reduce power consumption [14,15,16,17]. In 2021, S.H. Abhi et al. [18]. from Rajshahi University of Engineering in Bangladesh reported an optimization analysis model for the transconductance of β-Ga₂O₃ MESFET by analyzing the characteristic curves of MESFET designed for different channel heights, electron concentrations, gate source voltages, and drain source voltages. In 2022, A. Bhattacharyya et al. [19] from the University of Utah reported a transverse gallium oxide field-effect transistor with high merit. They used metalorganic vapor phase epitaxy-regrown (MOVPE) technology to achieve a high performance and high breakdown voltage on β-Ga₂O₃ lateral MOSFET. PFOM of 355 MW/cm² was achieved at a breakdown voltage of 2.44 kV. It also demonstrated a ρ_c value of 8.3 × 10⁻⁷ Ω·cm² for extremely low metal/regenerated β-Ga₂O₃ and an R_C value of 80 mΩ·mm. In 2024, X.Q. Chen et al. [20] from the University of Idaho in the United States designed an enhanced-mode β-Ga₂O₃ vertical current aperture MOSFET with a trench gate. The transistor with a trench gate groove (Gr) of 130 nm achieved a high current switching ratio of 10⁸, a high conduction current density of 65 A/cm², a low specific on-resistance of 42.5 mΩ·cm², and a breakdown voltage of 488 V in enhanced-mode operation. In 2025, Jinxiu Zhao et al. [21] from Shanghai Jiao Tong University reported an enhancement-mode Schottky barrier TFT fabricated through the Ga₂O₃/lGzO heterojunction and Ni/lGZO Schottky junction. A high-density two-dimensional electron gas with a sheet carrier density over 6 × 10¹³/cm² was achieved, by introducing an oxygen-vacancy-rich Ga₂O₃/lGZO interface. The sheet carrier density is beyond the gate control limitation, leading to a low channel resistance.

However, due to the lack of effective P-type doping and low electron mobility, the voltage resistance of gallium oxide power devices is still far below the theoretical value [22,23] and current research mainly focuses on exploring methods to alleviate the phenomenon of electric field concentration through small-sized devices, but there is relatively little research on large-sized high-power devices and their thermal stability. In addition, the low thermal conductivity of gallium oxide materials, coupled with complex intrinsic defect energy-level distribution, process defects, interface states, and oxide layer traps, lead to reliability issues such as electrical characteristic drift and accelerated performance degradation in gallium oxide devices.

In this paper, a novel Ga₂O₃ FinFET with NiO/n-Ga₂O₃ heterojunction (named Heterojunction FinFET: HJ-FinFET) for E-mode-type operation has been proposed. The p-NiO layer dramatically improves V_br; meanwhile, p-NiO/n-Ga₂O₃ forms an HJ and hetero-reduces the reverse conducting loss. It is expected that the proposed structure can cause the threshold voltage to shift to the right and realize the design of E-mode devices, and this device can be fabricated through ALD, MOCVD and lithography techniques.

2. Device Structure and Simulation Description

Based on the traditional structure of β-Ga₂O₃-based Fin-FET, Figure 1a,b illustrate the the β-Ga₂O₃-based Fin-FET with a p-NiO heterojunction structure (HJ-FinFET) proposed in this paper. The simulated structure comprises a 2 μm β-Ga₂O₃ substrate with a 2 μm thick β-Ga₂O₃ epitaxial layer on top. This epitaxial layer is doped with silicon at a concentration of 1.45 × 10¹⁶ cm⁻³. Additionally, a 20 nm Fin-shaped channel is formed above the expitaxial layer. A thin layer of the NiO channel directly bonds to the top surface of the β-Ga₂O₃ and along both sides of the Fin-shaped channel. To ensure reliable device operation, the p-NiO is heavily doped with boron at a concentration of 1 × 10²⁰ cm⁻³, which facilitates the formation of a heterojunction diode (HJD) with β-Ga₂O₃. As wide-bandgap semiconductors, β-Ga₂O₃ materials can be driven into an inversion state under appropriate conditions. The HJD is formed by p-NiO/Ga₂O₃ with a low electron barrier of 2.24 eV as illustrated in Figure 1c. The work function of the gate electrode (Ti) is set to 4.6 eV, while the gate, source and drain metal contacts maintain ohmic behavior.

The β-Ga₂O₃ based HJ-FinFET model was constructed using the commercial Sentaurus2022 TCAD software and the electrical properties of the device were simulated. The physical models adopted in this paper include Fermi level, effective intrinsic density, SRH, Algon effect, mobility (Doping-Dependent Arora Model), High-field Saturation Enormal (Unibo Interface Charge) and trap-assisted tunneling, etc. These models were used for calibration of the p-NiO/Ga₂O₃ interface [24,25,26]. The key parameters related to β-Ga₂O₃ and NiO are shown in

Table 1. The key parameters of the β-Ga₂O₃.

Parameter	Value
Band gap, Eg/eV	4.8
Electron mobility, μ (cm2·V−1·s−1)	300
Dielectric constant, κs	10.2
Electron affinity/eV, χ	4.0
Valence band state density/cm−3, NC	3.72 × 1018
Conduction band state density/cm−3, Nv	3.72 × 1018
Effective electronic mass, m0	0.28
Breakdown field strength/(MV·cm−1), Ec	8

[24,27] and

Table 2. The key parameters of the NiO.

Parameter	Value
Band gap, Eg/eV	3.68
Hole mobility, μ (cm2·V−1·s−1)	0.5
Dielectric constant, κs	11.8
Electron affinity/eV, χ	1.8
Effective hole mass, m0	6
Breakdown field strength/(MV·cm−1), Ec	4.8–6.2

[28].

3. Results and Discussion

To visually analyze the operating mechanism of the β-Ga₂O₃-based HJ-FinFET proposed in this study, simulation investigations were conducted to examine the variations in electron density distribution within the device’s channel under different gate voltages (V_GS). The corresponding simulation results are presented in Figure 2. When the gate voltage is zero (V_GS = 0 V), the channel region of the HJ-FinFET is completely depleted. At the interface of the NiO/Ga₂O₃ heterojunction, due to the different carrier concentrations and conductivity types on both sides, a p-n junction space charge region (often referred to as the depletion region) is formed, which is located within the conductive channel of the HJ-FinFET and results in a significantly lower electron density along the conductive channel region compared to the doping concentration (N_D = 1.5 × 10¹⁶ cm⁻³), as illustrated in Figure 2b. It can also be observed from the figure that the p-n depletion region has extended into the drift layer (the white line in the figure represents the boundary line of the depletion region). When a negative bias voltage is applied to the gate, the P-N junction constituting the gate control is in a reverse bias state, and the depletion region further expands, with the depletion line extending deeper into the β-Ga₂O₃ epitaxial layer of the drift layer, resulting in a lower conductive electron concentration in the channel [29], as shown in Figure 2a. On the contrary, as shown in Figure 2c, when a positive bias voltage is applied to the gate, the P-N junction constituting the gate control is in a forward bias state, the width of the space charge region narrows; at this time, the conductive channel is not completely occupied by the depletion region, and the conductive charge density of the channel increases, forming a conductive channel. At this point, the β-Ga₂O₃-based HJ-FET is officially turned on. However, the electron density at this stage remains lower than the doping concentration N_D, causing the device to exhibit a small current during turn-on. With a further increase in the forward bias voltage, the channel charge continues to rise, as depicted in Figure 2d. The above results fully prove that the β-Ga₂O₃-based HJ-FET designed in this paper belongs to an enhancement-type structure device. In order to further compare the advantages of the structure designed in this paper, a comparative analysis of the threshold voltage and on-resistance of the two structures was conducted. Figure 2e presents a comparative analysis of the threshold voltage (V_TH) between the FinFET and HJ-FinFET structures, which are measured as 0.93 V and 2.11 V, respectively. Furthermore, the specific on-resistance (R_on,sp) of the HJ-FinFET is significantly reduced to 2.48 mΩ·cm², representing a 79% improvement compared to that of the conventional FinFET, as illustrated in Figure 2f.

In order further analyze the internal mechanism of the device breakdown, the 2-D surface diagram of the electric field, the ionization integral diagram, and the electron distribution diagrams at V_DS = 50 V, 1000 V and 3200 V (or V_br) were analyzed in detail, as shown in Figure 3. To accurately analyze the breakdown characteristics, an appropriate collision ionization model is required. Most impact ionization models from the literature are based on Selberherr’s model (or its variants), and this model describes the collision ionization coefficients for electrons (holes), α_n and (α_p), through Equation (1) [30].

$$\alpha =A_n\exp {\left(-{\displaystyle \frac{B_n}{E}}\right)}^{C_n}$$

(1)

Here, E represents the local electric field strength experienced by charge carriers during transport and A_n (A_p), B_n (B_p), and C_n (C_p) denote distinct fitting parameters for electrons (holes). Ghosh and Singisetti recently reported a set of parameters for the electron ionization coefficient (see

Table 3. Impact parameters of β-Ga₂O₃.

Parameter	Value
AN1/cm−1	2.5 × 108
AN2/cm−1	2.5 × 108
BN1/(V·cm−1)	2.26 × 107
BN2/(V∙cm−1)	2.26 × 107
AP1/cm−1	2.23 × 108
AP2/cm−1	2.23 × 108
BP1/(V·cm−1)	2.7 × 108
BP2/(V·cm−1)	2.7 × 108
βTAN	1
βTAP	1

) [31,32]; however, similar parameters for holes remain unavailable in the literature.

Figure 3a demonstrates that the channel was pinched off at V_DS = 50 V, as evidenced by the formation of a depletion region beneath the gate contact. As the V_DS increased, the depletion region extended toward the drain terminal. Upon approaching V_br, a sharp increase in both electron and hole densities within the depletion region was observed. The electrons generated through impact ionization were swept toward the drain, while the generated holes followed a vertical trajectory and exited via the gate contact. As illustrated in Figure 3c,d, the peak electric field during the breakdown was concentrated near the heterojunction interface. This concentration is further corroborated by the ionization integral distribution diagram, as shown in Figure 3b, where the ionization integral exceeds unity at the heterojunction interface. The device achieves a peak electric field of 9.7 MV/cm, and with a breakdown voltage of 2.65 kV. On the other hand, in the design of power device structures, efforts are made to enhance the performance and PFOM value of the device. The PFOM value of the device designed in this paper reaches 2840 MW/cm² according to Equation (2) [33].

$$PFOM={\displaystyle \frac{V_{br}^2}{R_{on,sp}}}$$

(2)

where R_on,sp is the specific on-state resistance, and V_br is the breakdown voltage.

Here, we present the comparison of β-Ga₂O₃ MOSFET in this work and other reported devices (see

Table 4. Comparison of β-Ga₂O₃ MOSFET in this work and other reported devices.

Device	MODE	VTH	BV (V)	Ron,sp (mΩ·cm2)	PFOM (MW/cm2)	Ref.
β-Ga2O3/NiO HJFET	E-MODE	2.14	2650	2.48	2650	This work
DDLs β-Ga2O3 MOSFET	-	-	2310	11.7	582.0	25
β-Ga2O3/4H-SiC HC-MISFET	E-MODE	3.5	3580	7	183	33
β-Ga2O3/4H-SiC HJFET	E-MODE	0.82	1817	-	180	34
Ga2O3-on-SiC MOSFETs	D-MODE	-	1000	100	100	35
β-Ga2O3 Recess Channel MOSFETs	D-MODE	−20	8560	897 k	-	36
β-Ga2O3/NiO HJFET	E-MODE	1.5	5500	14.75	2050	37
FP β-Ga2O3 MOSFET	E-MODE	0.65	2443	-	-	38

) [25,34,35,36,37,38], demonstrating that our device performs well in terms of on-resistance.

Building on the investigation of the device’s operating mechanism, and to further analyze the influence of the channel on its electrical conductivity, the effect of varying channel width (W = 100–300 nm) on the device’s electrical characteristics was studied in detail. The corresponding results are presented in Figure 4. Figure 4a shows the transfer characteristics of the simulated device for various channels (W). A notable observation is that as the W increases, the sub-threshold voltage begins to appear. When the gate voltage V_GS < V_TH, the gate electric field is not enough to form a strong inversion layer (conductive channel), resulting in only a small number of induced charge carriers at the surface. At this state, the transistor is in a “weak inverse” state. Although the device is not fully turned on, a small drain current (sub-threshold current) can still be observed. Even in the “off” state, such sub-threshold currents contribute to static power dissipation. This effect becomes particularly significant in nanoscale technologies, where cumulative leakage currents can elevate overall chip power consumption. As W continues to increase, the channel width extends beyond the width of the space charge region (as indicated by the white lines), as shown in Figure 4b. Consequently, the heterojunction is no longer capable of fully depleting the channel under the zero gate bias, and the device eventually becomes a depletion mode type, characterized by a negative threshold voltage. The values of threshold voltage and breakdown voltage for different W are summarized in Figure 4c.

The output characteristic curves under different channel widths (W = 100–300 nm) are shown in Figure 4d. In the linear region (V_DS < 4), the current density increases approximately linearly with the V_DS. For the device with W = 300 nm, the specific on-resistance is as follows: R_on,sp = 2.12 mΩ·cm² (calculated from the slope ΔV_DS/ΔI_DS), which is 29.3% lower than that of the W = 100 nm device (R_on,sp = 3.0 mΩ·cm²). In the saturation region (V_DS > 10 V), the saturation current density of W = 300 nm devices reaches 1.37 kA/cm², which is 2.1 times higher than that of the devices where W = 100 nm (0.65 kA/cm²). Although the inversion effect is not clearly obvious in the W = 100 nm devices, its low saturation current and high on-resistance lead to increased conduction losses. The values of R_{on sp,} and PFOM for different L are summarized in Figure 4e.

As shown in Figure 4b, as the W continues to increase, the electron density within the channel also rises, with a certain number of electrons already present in the channel under zero gate bias. Therefore, when a small drain voltage is applied, the devices become more susceptible to break down. Figure 4f compares the electric field distribution for the devices with W = 100 nm and W = 300 nm. For W = 300 nm, the peak field strength is 2 MV, which is significantly lower than that for W = 200 nm.

Figure 5 presents the electrical characteristics of the simulated device for p-NiO with varying doping concentrations (N_A) ranging from 1 × 10¹⁸ cm⁻³ to 1 × 10²¹. Figure 5a illustrates the transfer characteristics of the β-Ga₂O₃-based HJ-FinFET. As the doping concentration decreases, the transfer characteristics shift to the left, resulting in a more negative threshold voltage V_TH. The threshold voltage and breakdown voltage values for different doping concentrations are summarized in Figure 5c. Figure 5b presents the output characteristics of the device under different doping concentrations at V_GS = 3 V. The drain current is significantly reduced for N_A = 1 × 10²¹ cm⁻³, with I_DS = 0.88 kA/cm². This reduction can be attributed to the significant energy band shift at the heterojunction interface induced by the increased doping concentration, which enhances the electric field intensity in the space charge region and further depletes electrons in the β-Ga₂O₃ channel. Additionally, the increase in the electron injection barrier due to the band migration hinders the transport of electrons from the source to the drain. Moreover, the increased impurities and defects at the heterojunction interface intensify the electron scattering in the channel, thereby reducing the carrier mobility.

With the increase in the doping concentration of p-NiO, the breakdown voltage exhibits only a minor change, which can be attributed to the influence of the P region doping. The space charge region primarily extends toward the light doped side. Therefore, an increase in doping concentration results in a narrowing of the space charge region within p-NiO, thereby reducing its impact on the breakdown voltage. However, when the dopant concentration is decreased to 10 × 10¹⁸ cm⁻³, the depletion region shifts toward the p-NiO side. Consequently, the breakdown location transitions from β-Ga₂O₃ to p-NiO, leading to a sharp decrease in the breakdown voltage, as illustrated in Figure 5e.

Figure 6 presents the electrical characteristics of β-Ga₂O at varying doping concentrations (N_D) ranging from 1.45 × 10¹⁵ cm⁻³ to 1.45 × 10¹⁷ cm⁻³. Figure 6a illustrates the transfer characteristics of the β-Ga₂O₃ based HJ-FinFET. As the doping concentration (N_D) increased from 10¹³ cm⁻³ to 10¹⁶ cm⁻³, the space charge region shifts toward the p-NiO layer, resulting in a narrowing of the space charge region within the β-Ga₂O₃ layer, as shown in Figure 6b. At a doping concentration of 8 × 10¹⁶ cm⁻³, the space charge region (indicated by the white lines) is insufficient to fully extend across the channel, thereby preventing complete depletion of the heterojunction channel. This results in electron accumulation in the channel under zero bias, causing the device to operate in depletion mode. The threshold voltage and breakdown voltage values corresponding to different doping concentrations are summarized in Figure 6d.

Figure 6c presents the output characteristics of the device under varied doping concentrations for V_GS = 3 V. A significant reduction in drain current is observed for N_D = 1.45 × 10¹⁵ cm⁻³, where the I_DS reaches only 0.7 kA/cm². This can be attributed to the reduced carrier concentration associated with the lower doping levels. The corresponding values of R_on,sp, and PFOM for different doping concentrations are summarized in Figure 6e. Although the PFOM reaches its maximum value at a doping concentration of 1.45 × 10¹⁵ cm⁻³, the processing technologies of current Ga₂O₃ are not yet capable of reliably achieving such low doping levels.

Figure 6d illustrates the effect of variation doping concentration on the breakdown voltage of the β-Ga₂O₃ based HJ-FinFETs. As doping concentration increases, the breakdown voltage decreases due to the avalanche breakdown mechanism, which causes a rapid increase in current at a relatively low drain voltage. Figure 6f presents the electric field distribution in the Ga₂O₃ channel for both cases at the breakdown voltage. The breakdown occurs at the heterojunction interface.

4. Conclusions

In this work, a novel E-mode NiO/β-Ga₂O₃ HJ-FinFET has been proposed and investigated to achieve high-performance E-mode β-Ga₂O₃-based FinFETs. The influence of the doping concentration of the heterojunction layer constituting the gate, the doping concentration of the drift layer, and the channel width on the electrical characteristics of the devices were focused on. The simulation results shown that the width of the channel and the doping concentration of the β-Ga₂O₃ layer are the dominant factors affecting the device characteristics. By optimizing the relevant parameters, the device can operate stably in E-mode and a higher breakdown voltage was obtained. Ultimately, the device achieved a turn-on voltage of 2.14 V, a breakdown voltage of 2.65 kV, a low specific on-resistance (R_on,sp) of 2.48 mΩ·cm², and a high PFOM of 2840 MW/cm². In summary, the novel architecture proposed in this study effectively overcomes the common performance limitations of traditional lateral β-Ga₂O₃ MOSFETs. This advancement enables the realization of E-mode-type β-Ga₂O₃-based FinFET with low specific on-state resistance (R_on,sp) and high breakdown voltage, thus providing a new method for achieving high-performance power devices.