A Review of β-Ga₂O₃ Power Diodes

Abstract

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.

1. 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, Ga₂O₃ 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 doping-controllable β-Ga₂O₃ wafers. Furthermore, several pivotal electrical properties of β-Ga₂O₃ exhibit superiority over those inherent to SiC and GaN [13,14,15,16,17,18,19]. Currently, 6-inch β-Ga₂O₃ wafers have been successfully fabricated using the edge-defined film-fed growth (EFG) technique. The cost of β-Ga₂O₃ 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-Ga₂O₃ substantially lower than that of SiC [20,21]. Moreover, during the production of β-Ga₂O₃ 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-Ga₂O₃ 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-Ga₂O₃ devices [22]. Electrically, β-Ga₂O₃ possesses a wider bandgap of 4.7–4.9 eV 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-Ga₂O₃, which is 10 times that of SiC and 4 times that of GaN; this indicates that β-Ga₂O₃ possesses greater potential for application in power devices [23,24,25,26].

Although β-Ga₂O₃ offers the advantages of low cost and high performance, similar to other wide-bandgap semiconductor materials, it is challenging to obtain p-type Ga₂O₃ with a high conductivity [27,28,29,30]. Calculations suggest the absence of shallow acceptors in Ga₂O₃, and holes exhibit self-trapping effects within the material [31,32]. Consequently, current research is primarily focused on unipolar Ga₂O₃ 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-Ga₂O₃ SBDs in 2012, β-Ga₂O₃ 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 Ga₂O₃ SBDs, leading to significant breakthroughs in device performance. Lateral β-Ga₂O₃ SBDs have achieved a BV exceeding 10 kV [37,38], and vertical NiO/β-Ga₂O₃ Schottky heterojunction diodes devices have reached a power figure of merit (PFOM = BV²/R_on) of 13.2 GW/cm² [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-Ga₂O₃ trench SBD device [40]. Therefore, it is essential to compile a summary of the current research progress on β-Ga₂O₃ 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-Ga₂O₃ power SBDs up to the year 2024. Section 2 elucidates the material properties of β-Ga₂O₃, substrate preparation, and epitaxial processes; Section 3 discusses the metal/β-Ga₂O₃ contacts, including ohmic and Schottky contacts; Section 4 presents the progress in research on differently structured β-Ga₂O₃ 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-Ga₂O₃ SBD devices; Section 6 summarizes the circuit applications and reliability of β-Ga₂O₃ SBDs; and Section 7 concludes with a summary and prospects.

2. β-Ga₂O₃ Materials

Ga₂O₃ material can exist in five crystal phases (α, β, γ, δ, and ε), among which the monoclinic β-Ga₂O₃ phase prevails instability. The remaining four phases are metastable and can be converted to β-Ga₂O₃ under certain conditions, as depicted in Figure 1 [13,14]. Notably, β-Ga₂O₃ exhibits commendable thermal and physicochemical stability, making it the most extensively studied phase to date. The α-Ga₂O₃ phase possesses a corundum structure and is typically grown heteroepitaxial on sapphire substrates [41,42,43,44], making it the second-most researched phase. The FLOSFIA company (Kyoto, Japan) predominantly employs Mist-epitaxy to prepare α-Ga₂O₃ wafers, and in March 2023, they unveiled an amperage-class 1700 V α-Ga₂O₃ trench SBD device [45,46,47,48,49]. The cubic spinel structure of 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 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-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-Ga₂O₃ 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-Ga₂O₃ single-crystal substrates using the EFG method. The company has also industrialized the production of 4-inch β-Ga₂O₃ single-crystal substrates, currently holding an international leading position, thereby laying the foundation for the large-scale commercialization of gallium oxide devices.

Advancements in substrate materials have also facilitated the development of β-Ga₂O₃ homoepitaxial film technologies. The epitaxial techniques widely employed today include molecular beam epitaxy (MBE) [78,79,80], halide vapor phase epitaxy (HVPE) [81,82,83,84], metal–organic chemical vapor deposition (MOCVD) [85,86,87], pulsed laser deposition (PLD) [88,89,90], low-pressure chemical vapor deposition (LPCVD) [91,92,93], and mist chemical vapor deposition (Mist-CVD) [43,94,95]. Among these, HVPE has been distinguished for achieving high growth rates of approximately 28 µm/h for epitaxial films. It also allows for precise doping concentration control within the range of 10¹⁵ to 10¹⁹ cm⁻³. At present, the majority of the commercially produced β-Ga₂O₃ single-crystal epitaxial layers are fabricated utilizing HVPE technology.

As compared to the already implemented SiC and GaN, the β-Ga₂O₃ material exhibits superior properties, as shown in

Table 1. Comparative analysis of performance parameters of different power semiconductor materials [19,96].

Material Parameters	Si	GaAs	4H-SiC	GaN	β-Ga2O3	Diamond
Bandgap, Eg (eV)	1.1	1.43	3.25	3.4	4.7–4.9	5.5
Electron mobility, µ (cm2/V·s)	1480	8400	1000	1250	300	2000
Breakdown field, Ec (MV/cm)	0.3	0.4	2.5	3.3	8	10
Saturation velocity, vs (107 cm/s)	1	1.2	2	2.5	1.8–2	1
Dielectric constant, ε	11.8	12.9	9.7	9	10	5.5
Thermal conductivity, λ (W/cm·K)	1.5	0.5	4.9	2.3	0.1–0.3	20
Baliga, εµEc3	1	14.7	317	846	3444	24,660

[19,96]. The bandgap of β-Ga₂O₃ 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-Ga₂O₃ a rare and ideally suited material for solar-blind UV detection. Consequently, Ga₂O₃-based solar-blind UV photodetectors have been extensively investigated [97,98,99,100]. Furthermore, β-Ga₂O₃ features a maximum critical electric field strength of up to 8 MV/cm and a BFOM of 3444. This implies that β-Ga₂O₃ 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-Ga₂O₃ can give full play to the advantages of both. A diamond/β-Ga₂O₃ pn heterojunction diode with on/off ratios of greater than 10⁸ 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, Ga₂O₃ materials still lack effective p-type doping, and research applications are basically n-Ga₂O₃, typically doping with elements like Si and Sn within a range of 10¹⁵ to 10¹⁹ cm⁻³ in concentration. In recent years, researchers have begun to address the shortfall of p-type Ga₂O₃ by using p-type NiO to form a p-n heterojunction with Ga₂O₃, presenting a novel approach for the advancement of Ga₂O₃ devices [39,105]. Besides, the primary challenges associated with β-Ga₂O₃ materials are their relatively low electron mobility and thermal conductivity. β-Ga₂O₃ exhibits an electron mobility of 300 cm²/V·s compared to GaN, rendering it less ideal for the fabrication of high-frequency devices. However, its saturation electron velocity of 2 × 10⁷ cm/s offsets this disadvantage [106,107]. In response to potential heat dissipation issues due to the limited thermal conductivity of Ga₂O₃ materials, numerous studies have proposed solutions, such as hetero-bonding, substrate thinning, and junction-side cooling, which will be elaborated in Section 5. Overall, Ga₂O₃ devices possess considerable practical application value and hold broad prospects for utilization.

3. Metal/β-Ga₂O₃ Contact

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

3.1. 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-Ga₂O₃, including Pt, Ir, Pd, Au, Ni, Co, Ru, Cu, Mo, W, Cr, Ti, Ag, etc.; graphite also forms Schottky contacts with β-Ga₂O₃. Among these, Ni is widely used, due to its low cost and ability to form excellent Schottky contacts with β-Ga₂O₃ [110,111,112,113].

However, experimental results indicate that the SBH of β-Ga₂O₃ 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 Φ₀, 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-Ga₂O₃ 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-Ga₂O₃, 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-Ga₂O₃ SBDs [122,123,124]. Furthermore, given that metal oxides have a better high-temperature stability and higher SBH compared to metals, β-Ga₂O₃ 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-Ga₂O₃ Schottky contacts achieved a leakage current of only 2.3 × 10⁻⁹ A/cm² (@-3 V) at a high temperature of 350 °C and just 7.5 × 10⁻⁸ A/cm² at −100 V, with the rectification ratio exceeding 10¹⁰ at all temperatures. Dela Cruz et al. [129] showed the outstanding high-temperature performance of Pt_xIr_(1−x)O_y on ($\overline{2}$01) β-Ga₂O₃, with a rectification ratio of 10⁹ (±3 V) at 300 °C and 10⁶ at 500 °C, demonstrating the application potential of metal oxide Schottky contacts in high-temperature devices.

3.2. 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-Ga₂O₃, which has a low thermal conductivity. Hence, superior β-Ga₂O₃ ohmic contacts are a prerequisite for achieving high-performance devices. Currently, almost all Ga₂O₃ 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-Ga₂O₃, 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-Ga₂O₃ was improved in β-Ga₂O₃ 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 × 10²⁰ cm⁻³) β-Ga₂O₃, with a specific contact resistivity of 8.3 × 10⁻⁷ Ω·cm². Currently, the formation of high-quality Ti/β-Ga₂O₃ ohmic contacts is commonly achieved using post-annealing treatment.

4. β-Ga₂O₃ Power Diodes

In the last decade, β-Ga₂O₃ 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.

4.1. 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-Ga₂O₃ 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-Ga₂O₃ SBDs.

4.1.1. 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 R_on of 2 mΩ·cm², and an I_F of 200 A/cm² 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 × 10¹⁶ cm⁻³) epitaxially grown using the HVPE technique, featuring a R_on of 2.4 mΩ·cm², 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 R_on also reduced. At an electrode diameter of 20 µm, the BV was 1600 V and the R_on was 25 mΩ·cm², whereas at 0.53 mm diameter, the BV fell to only 250 V and the R_on to 1.6 mΩ·cm².

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, ($\overline{2}$01) and (010). Findings indicated that the (010) plane had more negative charges and defects on its surface, leading to a more pronounced upward bending of the conduction band. As a result, the R_on, V_on, n, and SBH for devices on the (010) plane were higher compared to those on the ($\overline{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 experimental process, they fabricated a device with a R_on of 2.25 mΩ·cm² and a BV of 1720 V. The resulting PFOM reached 1.32 GW/cm², 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.

4.1.2. 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 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·cm². 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 mm², the I_F 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/SiN_x 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 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 mm², 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·cm² [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 (SiO₂/SiN_x, Al₂O₃/SiN_x, HfO₂/SiN_x) on β-Ga₂O₃ FP-SBDs, finding that their forward performance was similar. The Al₂O₃/SiNx bilayer field plate exhibited the smallest reverse leakage current, with a BV of 730 V, exceeding that of the SiO₂/SiN_x bilayer field plate at 562 V and the HfO₂/SiN_x bilayer field plate at 401 V [154]. In 2022, Guo et al. [155] fabricated β-Ga₂O₃ FP-SBDs with a 1 mm diameter using an Al₂O₃/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 Ga₂O₃ 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-Ga₂O₃ wafer with an epitaxial layer of only 1.7 µm to deposit an ultra-high-k dielectric BaTiO₃ (BTO) and BaTiO₃/SrTiO₃ (BTO/STO) stack as the field plate structure to fabricate β-Ga₂O₃ FP-SBDs. The devices achieved a low R_on of 0.32 mΩ·cm², 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/cm². Subsequently, BTO was used as the field plate dielectric on a β-Ga₂O₃ wafer with an epitaxial layer of 11 µm, resulting in devices with a R_on of 6.9 mΩ·cm² 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·cm² to 2.8 mΩ·cm². Kumar et al. [158] fabricated a stair-shaped field plate through PECVD deposition of SiO₂ 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-Ga₂O₃ 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 high-k 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.

4.1.3. 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 groove-filling 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-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 ion-implanted high-resistance edge terminations. The devices had a R_on of 4 mΩ·cm² and an ideality factor of 1.02; compared to traditional structures, the leakage current was reduced by a factor of 10³, with an on/off ratio reaching 10¹³, 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-Ga₂O₃ SBDs in fast-switching circuits. Zhou et al. [165] achieved β-Ga₂O₃ SBD devices with a R_on of 5.1 mΩ·cm², an on/off ratio of 10⁸–10⁹, a BV of 1.55 kV, and a PFOM of 0.47 GW/cm² 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].

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 × 10¹⁶ 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·cm² and a PFOM of 295 MW/cm². 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 × 10¹⁶ cm⁻³), whereas a pronounced depletion occurred in the exposed region (3.0 × 10¹⁴ 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·cm², a BV of 1800 V, and a PFOM up to 0.78 GW/cm², 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 SiO₂ layer to efficiently block current conduction. Since the bandgap of SiO₂ (8~9 eV) is much larger than that of β-Ga₂O₃, it can withstand higher voltages and critical fields, and the relative permittivity of SiO₂ (~4) is smaller; according to Poisson’s equation, the electric field of the β-Ga₂O₃ layer at the interface is less than 1/3 of that of the SiO₂ layer. Test results showed that the device reached a maximum BV of 6 kV and a minimum R_on of 3.4 mΩ·cm², achieving a PFOM of 10.6 GW/cm², surpassing the unipolar limit of SiC and GaN devices and confirming the tremendous potential of edge termination-structured β-Ga₂O₃ 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-Ga₂O₃ 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.

4.1.4. 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-Ga₂O₃ trench MOS SBDs for the first time on β-Ga₂O₃ substrates with a 7 µm drift layer (6 × 10¹⁶ 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·cm² to 2.9 mΩ·cm². 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-Ga₂O₃ trench MOS SBDs on a β-Ga₂O₃ substrate with a 10 µm drift layer (1–2 × 10¹⁵ 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 × 10¹⁶ 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-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/cm² up to the BV, which consistently occurred at the trench corners, where electric field crowding is prone to happen [186].

Figure 7. Device schematic of β-Ga₂O₃ trench MOS SBDs with (a) 7 µm drift layer [181]; (b) 10 µm drift layer [183]; (c) trench bottom corner [187]; (d) edge field-plated structure [188]; (e) edge stair-shaped field-plated structure [189]; (f) BaTiO₃ dielectric [190].

Figure 7. Device schematic of β-Ga₂O₃ trench MOS SBDs with (a) 7 µm drift layer [181]; (b) 10 µm drift layer [183]; (c) trench bottom corner [187]; (d) edge field-plated structure [188]; (e) edge stair-shaped field-plated structure [189]; (f) BaTiO₃ dielectric [190].

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/cm². 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/cm², 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 mm²) 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 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 10⁹. Additionally, Roy et al. [190] used the ultra-high-k dielectric material BaTiO₃ 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 µm²) exhibited a BV exceeding 3 kV, and the reverse leakage current remained below 1 µA/cm² at 3 kV. For large-area devices, devices measuring 1 × 1 mm² and 2 × 2 mm² 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 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 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 × 10¹¹ 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/cm² at 3 V and an on/off ratio surpassing 10¹⁰ [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 R_on of 1.20 mΩ·cm², and a PFOM exceeding 2 GW/cm², 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-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.

4.1.5. 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 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/cm². Moreover, this device maintained a leakage current density of less than 10 µA/cm² up to −2000 V, thereby manifestly showcasing the application potential of PtO_x 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 SiO₂ passivation layer to prepare β-Ga₂O₃ SBDs with a Schottky contact size of 2 × 2 mm². 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 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 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·cm², with a PFOM of 0.8 GW/cm². 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.

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-Ga₂O₃ power devices.

4.2. 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-Ga₂O₃ 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 Ga₂O₃ has hindered the development of β-Ga₂O₃ p-n homojunction diodes. Researchers have thus turned their attention to constructing β-Ga₂O₃ p-n heterojunction diodes (HJDs), using other p-type materials to achieve Ga₂O₃ bipolar power devices. These include Cu₂O [202], NiO [203,204], GaN [205,206], SnO [207], CuAlO₂ [208], and others. Notably, NiO has a bandgap of 3.8–4 eV, controllable doping characteristics, and a hole mobility of 0.5–5 cm²/V·s, which can form a type II band alignment with β-Ga₂O₃ [209,210]. Currently, the majority of β-Ga₂O₃ p-n heterojunction devices utilize NiO and have achieved significant breakthroughs in performance. This section primarily summarizes the NiO/β-Ga₂O₃ p-n HJDs, heterojunction barrier Schottky (HJBS) diodes, and junction termination extension (JTE) structures.

4.2.1. 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-Ga₂O₃ HJDs by sputtering a 200 nm NiO layer with a net doping concentration of 4 × 10¹⁶ cm⁻³ onto β-Ga₂O₃, as shown in Figure 10a, exhibiting a R_on of 3.5 mΩ·cm², lower than that of the Ni/β-Ga₂O₃ SBDs (4.2 mΩ·cm²). The BV was improved from 500 V to 1059 V, with a leakage current below 1 µA/cm² 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-Ga₂O₃ 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/cm² 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 mm²) small-angle (~11°) beveled mesa NiO/β-Ga₂O₃ 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·cm². 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·cm², with a PFOM of 1.84 (2.87) GW/cm². 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].

Figure 10. Device schematic of (a) NiO/β-Ga₂O₃ HJDs and Ni/β-Ga₂O₃ SBDs [201]; (b) NiO/β-Ga₂O₃ HJDs with bilayer p-NiO [215]; (c) beveled mesa NiO/β-Ga₂O₃ HJDs [219]; (d) NiO/β-Ga₂O₃ HJDs with JTE [221].

Figure 10. Device schematic of (a) NiO/β-Ga₂O₃ HJDs and Ni/β-Ga₂O₃ SBDs [201]; (b) NiO/β-Ga₂O₃ HJDs with bilayer p-NiO [215]; (c) beveled mesa NiO/β-Ga₂O₃ HJDs [219]; (d) NiO/β-Ga₂O₃ HJDs with JTE [221].

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 × 10¹⁶ cm⁻³). The device achieved a BV of 4.7 kV and a R_on of 11.3 mΩ·cm², with a PFOM reaching 2 GW/cm². Subsequently a large-area HJD, with a 1 mm diameter, demonstrated a R_on of 11.86 mΩ·cm² 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-Ga₂O₃ with a doping concentration of the drift layer less than 10¹⁶ 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 R_on of 7.9 mΩ·cm² and a PFOM surpassing 10 GW/cm². Devices with an area of 1 mm² 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/cm² [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 mm²) NiO/β-Ga₂O₃ HJD on β-Ga₂O₃ with a 15 µm drift layer, simultaneously introducing a SiO₂/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·cm², enabling a PFOM up to 5.18 GW/cm². 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 R_on of 5.24 mΩ·cm², and a PFOM as high as 13.2 GW/cm². This surpasses the unipolar 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 [230,231,232,233,234].

4.2.2. 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 μm² achieved a BV of 1715 V and a Ron of 3.45 mΩ∙cm², yielding a PFOM of up to 0.85 GW/cm². Additionally, large-scale HJBS diodes with an area of 1 × 1 mm² 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-Ga₂O₃ groove, as illustrated in Figure 12b. With a fin width of 3 µm, the device exhibited a R_on of only 1.94 mΩ∙cm² and a BV of 1.34 kV, leading to a PFOM of 0.93 GW/cm². Furthermore, the sidewall depletion effect of the p-NiO was enhanced with a reduction in fin width, resulting in a decreased reverse leakage current.

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 R_on of 7.7 mΩ∙cm², and a PFOM of 0.46 GW/cm², 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 mm² and 4 × 4 mm² through thermal oxidation of p-NiO, incorporating SiO₂ field plates, as shown in Figure 12d. The BV and specific R_on were measured at 550 V/500 V and 11 mΩ·cm²/15 mΩ·cm², 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°) 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 mm²) diode from 1895 V to 2395 V, achieving a PFOM of 0.72 GW/cm². For the large-scale (3 × 3 mm²) 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-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 mm² NiO_x/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 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.

4.2.3. 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∙cm², a BV of 2.11 kV, and a PFOM reaching 1.54 GW/cm². Additionally, they fabricated large-area SBDs with an area of 0.78 mm², achieving a forward current density of 180 A/cm² 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 stair-shaped JTE structures comprising multiple layers of p-NiO, as depicted in Figure 14b; the device showed a R_on of 5.9 mΩ∙cm², a BV of greater than 2.5 kV, and a PFOM surpassing 1 GW/cm². 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/cm². 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-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 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 R_on of 3.12 mΩ∙cm², and a PFOM of 1.11 GW/cm². 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/cm².

Qin et al. [247] successfully fabricated vertical super junction Schottky barrier diodes (SJ-SBDs) by integrating p-NiO within the trenches of β-Ga₂O₃, as illustrated in Figure 14f. They employed bilayer β-Ga₂O₃ epitaxial growth to realize a low R_on, used a SiO₂ 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·cm², 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.

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.

4.3. 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 (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·cm². Even at 150 °C, the on/off ratio remained over 10⁷, demonstrating the cooling advantage of the sapphire substrate. In the same year, by integrating a field plate structure as shown in Figure 15b, they developed lateral β-Ga₂O₃ SBDs with a reduced R_on of 10.2 mΩ·cm², an enhanced BV of 2.25 kV, and a PFOM of 500 MW/cm² 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-Ga₂O₃ interface, as shown in Figure 15c. With a L_AC = 90 μm, they achieved lateral β-Ga₂O₃ SBDs with a BV surpassing 10 kV and a R_on of 485 mΩ·cm².

Roy et al. [250] epitaxially grew a Si-doped β-Ga₂O₃ thin film on Fe-doped β-Ga₂O₃ insulating substrates by MOCVD. They etched the film into rectangular trenches using RIE and utilized the ultra-high-k material BaTiO₃ 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·cm², demonstrating a PFOM reaching 2.7 GW/cm² 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-Ga₂O₃ 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-Ga₂O₃ 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·cm², and a persistent performance above 10 kV even at 200 °C, thus fully demonstrating the potential of β-Ga₂O₃ power devices.

Furthermore, the development of lateral flexible SBDs based on β-Ga₂O₃ has been explored. Due to the crystal structure of β-Ga₂O₃, strain induces microcracks in the nanofilm, leading to device performance degradation, which will not be recounted here in detail [252,253].

4.4. Summary

The preceding sections have delineated the performance characteristics of various β-Ga₂O₃ power SBD structures. In the simple structure, the high-voltage potential intrinsic to β-Ga₂O₃ 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-Ga₂O₃ inwards into the β-Ga₂O₃ 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. Comparison of electrical performances for different-structure β-Ga₂O₃ SBDs.

Device Structure	Anode Area	Von (V)	IF	Ron	BV	PFOM (GW/cm2)	References
				(mΩ·cm2)	(kV)
w/o unreliable surface	Φ100 µm	—	—	2.25	1.72	1.32	[138]
BTO FP	Φ50–300 µm	—	—	6.9–8.7	2.1	0.51–0.64	[157]
Mg ion implantation ET	Φ180 µm	—	200 A/cm2@2 V	5.1	1.55	0.47	[165]
Oxygen-annealing ET	Φ110 µm	—	—	4.1	1.8	0.78	[172]
SiO2 ET	Φ180 µm	1.45	400 A/cm2@3 V	3.4	6	10.6	[173]
Trench with FP	100 × 150 µm2	—	—	8.8 (pulsed)	2.89	0.95 (pulsed)	[188]
Trench with BTO dielectric	200 × 200 µm2	—	—	6.8	>3	>1.32	[190]
	1 × 1 mm2	0.9	3.7 A@5 V (pulsed)	7.1	1.8	0.46
	2 × 2 mm2	—	15 A@5 V (pulsed)	10.8	1.4	0.18
MT with PtOx	Φ100 µm	1.45	—	7.33	2.74	1.02	[196]
MT with PBT	Φ100 µm	1.48	—	3.6	1.71	0.8	[199]
AHJD	Φ100 µm	1.72	—	2.5	2.66	2.83	[218]
Beveled-mesa HJD	1 × 1 mm2	1.82	20 A@ < 12 V	1.9	1.95	2	[219]
Double-layered NiO HJD	Φ100 µm	1.9–2.1	>100 A/cm2@3 V	7.9	8.9	10.2	[226]
	1 × 1 mm2	—	4.1 A@10 V	1.8	>4	9
HJD with SiOx/SiOx FP	1 × 1 mm2	~2	1 A@3 V	5.4	7	9.2	[228]
HJD with BFP	Φ60 µm	1.6	—	1.12	2.41	5.18	[229]
HJD with FP and ET	Φ150 µm	1.8	~400 A/cm2@5 V	5.24	8.32	13.2	[39]
HJD with JTE	Φ100 µm	—	—	3.9	>3.2	2.5–2.7	[244]
Vertical SJ	—	1	—	0.7	2	5.71	[247]
Lateral with dual FP and PAAT	LAC=90 µm	1.02	7.43 mA/mm@5 V	485	>10	>0.21	[38]
Lateral SJ with BTO	LAC/WFin = 5 µm/2 µm	—	—	1.65	1.49	1.34	[250]
Lateral with charge balance	LAC>30 µm	1	—	270	>10	>0.37	[37]

comprehensively compares devices with high performance across the various structures.

5. 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-Ga₂O₃ SBDs generate substantial heat instantaneously. Given the inherently low thermal conductivity of the β-Ga₂O₃ 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-Ga₂O₃ heterostructures onto high-thermal-conductivity substrates can effectively enhance device heat dissipation. One of the most effective solutions is to utilize diamond substrates to grow Ga₂O₃ 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-Ga₂O₃ 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-Ga₂O₃ films onto 4H-SiC and Si (001) substrates using an ion-cutting process. Due to the high thermal conductivity of the substrate, β-Ga₂O₃ 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-Ga₂O₃/SiC heteroepitaxial wafer was only a quarter of that on the β-Ga₂O₃ wafer, demonstrating that the combination of β-Ga₂O₃ thin film with a high-thermal-conductivity SiC substrate effectively promoted the heat dissipation of β-Ga₂O₃-based devices [263].

However, for most β-Ga₂O₃ 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-Ga₂O₃ 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-Ga₂O₃ 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-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.

6. 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. [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.

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-Ga₂O₃ SBDs using N ion-implanted terminations and SiO₂ 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-Ga₂O₃ 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-Ga₂O₃ SBD devices remain sparse, necessitating extensive research to further their practical implementation.

7. Summary and Prospect

This article provides a summary of the research progress on β-Ga₂O₃ power diode devices. Due to the low cost and high performance of β-Ga₂O₃ 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 Ga₂O₃ 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-Ga₂O₃ power devices, the following points will have a considerable impact on the practical applications of β-Ga₂O₃ power diode devices.

(a)

The development of iridium-free β-Ga₂O₃ single-crystal manufacturing technologies and the improvement of crystal quality will greatly facilitate the practical application of β-Ga₂O₃.

(b)

The mobility of NiO layers prepared by sputtering is significantly lower compared to β-Ga₂O₃, which limits the performance of HJDs to some extent. The realization of effective p-type doped Ga₂O₃, or developing alternative high-performance p-type materials, will further enhance the performance of β-Ga₂O₃ HJDs.

(c)

Due to the very low thermal conductivity of β-Ga₂O₃ materials, there is still a need for better β-Ga₂O₃ thermal management methods and further research on device reliability.