Power electronics represent an ever-changing domain of vibrant research that covers a wide spectrum of materials, physics, devices, and applications concerning power conversion and management^[1]. Of relevance to optoelectronic and display technologies are boost converters used to drive display backlights in modern portable electronic devices. Visible-light communication systems have been proposed to make use of high-brightness light-emitting diodes not only for lighting but also for transmitting information by rapidly modulating the intensity of the emitted light using fast drivers based on switched-mode converters. As the critical electric field (E_cr) of semiconductors increases super-linearly with increasing bandgap energy^[2], ultrawide-bandgap (UWBG) semiconductors can tolerate high fields to enable high-power electronic devices. Apart from applications in power management, UWBG materials emit deep ultraviolet (UV) light and are therefore attractive for extending the working wavelengths of photonic devices beyond the UV–visible (UV–vis) spectrum to enable potential applications in deep-UV optoelectronics, quantum information science, and bio-chemical sensing.

The UWBG binary oxide, β-Ga₂O₃, has attracted significant attention because of its large 4.8-eV bandgap, controllable n-type doping with Si/Sn/Ge, and relatively high electron mobility of ~200 cm²/(V∙s)^[3−6]. With an E_cr approximately three times that of SiC and GaN, β-Ga₂O₃ offers greater intrinsic power conversion efficiencies and further expansion of the operating-voltage–switching-frequency power electronics application space. Melt-grown native substrates are available for β-Ga₂O₃, indicating a path to commercially viable β-Ga₂O₃ devices^{[7, 8]}. Due to its wide bandgap, broadband transparency, low cost, and high thermal/chemical stability, β-Ga₂O₃ has also emerged as a new platform for UV–vis nonlinear optics and integrated photonics such as waveguides and solar-blind photodetectors^{[9, 10]}.

As an integral part of power converters and inverters, β-Ga₂O₃ Schottky barrier diodes (SBDs) have been intensively studied and undergone rapid progress. The majority of β-Ga₂O₃ SBDs reported in the literature are vertical devices. Thanks to the availability of native β-Ga₂O₃ substrates with low dislocation density, thick β-Ga₂O₃ drift layers with low background carrier concentration can be grown homoepitaxially^[11−16]. Compared with a lateral configuration, the breakdown voltage (V_br) of vertical SBDs scales with the drift layer thickness as opposed to a lateral anode–to–cathode separation, which translates into a higher power density or a higher blocking voltage without sacrificing chip area. In addition, heat distribution is more uniform in vertical devices because of current spreading. Realization of the full potential of these devices requires proper edge termination designs that are suitable for implementation in β-Ga₂O₃ to limit the maximum electric field around the periphery of the rectifying contact.

β-Ga₂O₃ lacks p-type doping capability. This issue is fundamental to the material’s electronic band structure^[17−19]. The absence of definitive demonstrations of p-type doping in β-Ga₂O₃ renders bipolar homojunction diodes that are prevalent in other power device technologies impractical for this oxide semiconductor. To circumvent the challenge of developing p-type doping in β-Ga₂O₃ for edge termination, heterogeneous integration of β-Ga₂O₃ with other p-type oxide semiconductors has been proposed. Reactive-sputtered nickel oxide (NiO) has been widely adopted for forming p–n heterojunctions with β-Ga₂O₃ because of its relatively large bandgap (3.7 eV), relative ease of deposition with readily available toolsets, and propensity to exhibit native p-type conductivity tunable over several orders of magnitude through controlling deposition conditions.

This paper reviews a broad portfolio of β-Ga₂O₃ SBDs, the vast majority of which employ one or more edge-termination techniques to mitigate edge breakdown and enhance their power figure-of-merit (PFOM). The design and operating principles of advanced rectifier structures are then examined. An assessment of β-Ga₂O₃ rectifier performance at elevated temperatures is provided toward the end.

Field plating is a relatively simple edge-termination technique that makes use of a metal–insulator–semiconductor (MIS) capacitor structure located at the edge of the main device junction to modulate the local electric field^[20]. Apart from a single-step field plate, which is the simplest field plate geometry, various other designs of field plates such as multi-step and slanted geometries have been developed for high-voltage β-Ga₂O₃ SBDs. As a consequence of the simplicity and versatility of field plates, the literature contains an abundant documentation of field-plated β-Ga₂O₃ SBDs with a V_br higher than 1 kV (Fig. 1)^[21−25]. Large-area field-plated devices have also been realized^[26−31], with the highest reported forward current exceeding 100 A. However, the reverse leakage and V_br of large-area SBDs are typically worse than small-area devices of equivalent construction because of a higher probability of incorporating defects such as voids, dislocations, or polycrystalline inclusions within the active region^[32−35].

The choice of dielectrics used for field plate termination structures profoundly affects the reverse V_br and leakage current of field-plated SBDs. To avoid accelerated dielectric degradation, the maximum field sustained by the dielectric should be limited to a value well below the threshold of catastrophic breakdown. Ultrahigh-permittivity dielectrics such as barium titanate and strontium titanate provide a new approach to edge termination for β-Ga₂O₃ power devices^[36−40], whose high breakdown field calls for new methods of field management owing to a lack of dielectrics that can be stressed beyond the theoretical breakdown field of β-Ga₂O₃. These dielectrics are strongly polarizable by an external electric field, which induces internal dipoles and fixed surface charges in the dielectric material. When a reverse bias is applied to an ultrahigh-permittivity MIS capacitor associated with a field plate structure, the fringing electric field through the dielectric causes a lateral expansion of the depletion region in the semiconductor, thereby reducing the peak electric fields at the semiconductor surface. Furthermore, the induced polarization charges in the dielectric effectively screen both the positive donor charges in the semiconductor and the negative electron charges imaged in the anode metal. Thanks to this screening effect, the electric field in the ultrahigh-permittivity dielectric can be significantly lower than in the semiconductor, which leads to an increase in the effective barrier width for electron tunneling from the metal to the semiconductor. Achievements of a parallel-plane electric field of 5.7 MV/cm and a PFOM of 1.47 GW/cm²—the first >1 GW/cm² demonstration for a unipolar vertical β-Ga₂O₃ device—highlight the efficacy of ultrahigh-permittivity dielectrics in enabling efficient edge termination and pushing the performance of β-Ga₂O₃ power devices toward intrinsic critical limits (Fig. 2)^{[39, 40]}.

The field-crowding effect at the edge of an SBD can be relaxed by removing materials around the device edge to form a mesa structure such that the depletion region can be terminated on the exposed sidewall of the mesa. Junction curvature at the device edge is inherently relieved or eliminated, allowing more uniform distribution of electric field within the device. A deep etch is most effective when etching is past the maximum depletion layer thickness for non-punch-through breakdown or the drift layer thickness for punch-through breakdown; in both cases, electric field is prevented from developing at the mesa foot, hence avoiding premature breakdown even with steep (~90°) mesa profiles^{[41, 42]}. Alternatively, beveled mesas characterized by an inclined sidewall and a bevel angle at the mesa foot can be implemented. With sufficiently small bevel angles, these structures enable effective edge termination without a need for deep etching beyond the depletion boundary at the rated V_br^[43]. A bevel angle as small as ~1° has been fabricated (Fig. 3)^[44]. In the forward conduction mode, the beveled geometry is advantageous for facilitating current spreading and enhancing current density. A dielectric is often deposited over the entire mesa structure after etching to passivate surface trap states caused by etch damage so as to mitigate parasitic leakage currents and ensure long-term device reliability.

The concept of resistive edge termination is based upon the introduction of a nominally insulating or intrinsic layer in contact with the anode periphery to spread the electric field by fringing. Resistive layers in β-Ga₂O₃ SBDs are commonly formed by ion implantation, which creates intrinsic defects via high-dose ion bombardment of either an inert species (e.g. He^[45] or Ar^{[46, 47]}) or a deep-level impurity (e.g. Mg^{[45, 48]} or N^{[49, 50]}) to form trap levels in the bandgap (Fig. 4). Resistive termination structures have also been implemented by etching a deep trench under the periphery of the Schottky contact and backfilling it with an insulating dielectric material like SiO₂^{[25, 51]}. Thermal annealing of n-type β-Ga₂O₃ in oxygen is known to deplete or reduce carriers over a finite volume underneath the surface^{[16, 52, 53]}, an effect exploited to fabricate resistive edge-termination structures in β-Ga₂O₃ SBDs^[54−56]. Through a combination of large V_br due to efficient termination and low specific on-resistance (R_ON,sp) due to high-quality epitaxy, a PFOM of up to 10.6 GW/cm² that exceeds the 1-D unipolar limits of SiC and GaN has been realized in SBDs employing resistive termination^[51].

Fixed negative charges can be introduced, via plasma exposure or otherwise, to the surface of an n-type drift layer at the junction periphery to expand the depletion region along the surface from the main blocking junction, thus modifying the surface electric field and alleviating potential crowding at the junction periphery. Surface doping of β-Ga₂O₃ with fluoride ions for edge termination has been demonstrated via high-power CF₄ plasma exposure (Fig. 5)^[57]. In addition to the negative fixed charge model, the effect of fluorine has also been conjectured to arise from compensation of positively ionized Si donors via the formation of neutral complexes^[58], and may thus be regarded as a type of resistive termination. Plasma treatment inflicts much lower damage than mesa etching or ion implantation. Determination of the optimal charge profile for a given device structure requires systematic simulations that consider the thickness and doping of the drift layer, the lateral extension of the termination region, as well as the rated voltage of the device.

Schottky power rectifiers can be edge terminated by incorporating a highly doped guard ring to create a p–n junction that overlaps the edge of the Schottky metal, thereby completely screening the metal edge from high electric fields. Typical implementations of this approach in β-Ga₂O₃ make use of p-NiO to form the guard ring^[59]. Another p-type material option is the group-Ⅲ nitride semiconductor in view of the prospects of high-quality Ⅲ-nitride epitaxy on β-Ga₂O₃. Simulations of hybrid p-AlGaN/β-Ga₂O₃ devices have predicted very promising performance that awaits experimental validation^[60].

The efficiency of highly doped, single-layer termination such as the guard ring is known to be sensitive to the charge density of the p-type layer. To realize broad design window and process latitude, advanced junction termination structures make use of multiple p-NiO layers with various lengths and acceptor densities to allow for a graded decrease in effective charge density away from the device active region while forming an extended depletion region at the device periphery^{[61, 62]}. At breakdown, the p-NiO is fully depleted to balance the charge in the β-Ga₂O₃ drift layer.

The concept of field-limiting rings (FLRs) for terminating a Schottky rectifier involves surrounding the main blocking junction by one or more isolated concentric ring junctions that are either Schottky or p–n (Fig. 6)^[63]. When the rectifier is under the blocking condition, this ring system reduces the amount of field crowding at the main junction by spreading the depletion layer consecutively past the ring junctions that are typically left floating. FLR termination of β-Ga₂O₃ SBDs is first demonstrated experimentally using mechanically exfoliated β-Ga₂O₃ flakes^[64]. For higher voltage ratings, multiple rings are required to provide a V_br close to the parallel-plane limit, whereby the electric field developed at the edge of a floating ring can be reduced by the addition of another floating ring. Judicious biasing of the FLRs can improve the termination efficiency owing to reduced sensitivity of the biased structure to the inter-ring distance and the additional control over breakdown ^[65].

SBDs typically display soft breakdown characteristics. At high reverse bias, Schottky barrier lowering due to the image-force effect results in large tunneling and thermionic emission currents that limit the breakdown of SBDs to voltages much smaller than the 1-D parallel-plane value even if edge effects are eliminated^[66]. Consequently, the rated V_br is determined by the maximum allowable leakage current instead of avalanche breakdown. The constraint imposed by high leakage current renders the effective PFOM of regular SBDs much lower than the projected theoretical limit of the semiconductor. Such discrepancy exacerbates for ultrawide-bandgap semiconductors like β-Ga₂O₃. While it is possible to approach the intrinsic E_cr of 6–8 MV/cm in β-Ga₂O₃ SBDs with a sufficiently large Schottky barrier (2.2–3 eV) (Fig. 7)^[67], reducing the leakage current by increasing the Schottky barrier height comes with the penalty of a higher turn-on voltage (V_ON). To obtain a large V_br without increasing the barrier height, the leakage needs to be decoupled from the reverse-bias voltage such that the maximum electric field is transferred from the Schottky interface to within the device body.

The aforementioned electric-field profile can be achieved by embedding p–n junctions under the Schottky contact as a screen to lower the surface electric field during reverse bias^[68−70], in a device structure known as the junction barrier Schottky diode (JBSD). The reduced-surface-field (RESURF) effect due to charge coupling between the p⁺ regions and the n^– drift layer in JBSDs effectively minimizes Schottky barrier lowering and enables the JBSD structure to exhibit the low reverse leakage current of a p–n diode. Furthermore, one can capitalize on the RESURF effect to reduce the R_ON,sp and thus the power loss of a JBSD since the surface electric field at breakdown can be lower than the maximum electric field in the drift layer, which allows for higher doping in the drift layer for a given voltage rating. For UWBG semiconductors like β-Ga₂O₃, it is even more important to utilize the RESURF effect for medium- to high-voltage rectifiers owing to the large difference between E_cr and the leakage-limited practical maximum surface field. With effective native p-type doping remaining elusive in β-Ga₂O₃, researchers have sought to realize β-Ga₂O₃ JBSD structures based on p-NiO/n-Ga₂O₃ heterojunctions. JBSDs showing several orders of magnitude smaller leakage current with no or insignificant penalty on V_ON and R_ON,sp compared with a cofabricated regular SBD serve as a clear proof of concept for the RESURF effect (Fig. 8)^{[63, 71−74]}.

The periodic array of p–n junctions in a JBSD can be replaced with nonplanar MOS structures constructed in an alternating fin/trench geometry (Fig. 9) ^[75−83]. By virtue of the RESURF effect, the reverse leakage current in trench SBDs is generally much lower than in regular SBDs at destructive breakdown. The RESURF effect can be controlled through optimizations of the fin/trench geometry (fin width, trench depth, pitch size) to satisfy design constraints on R_ON,sp, reverse leakage, and maximum electric fields (Fig. 10). However, the RESURF effect also leads to enhanced electric fields at the bottom corners and the center of the trenches, the former of which can be managed by increasing the corner radius^[84]. Orientation of the fin is found to have a strong influence on the forward conduction^[85], where sidewall interface trapping is found to cause current collapse and delayed turn-on due to sidewall depletion^[86]. To mitigate the sidewall trapping effects, wet acid treatments are performed to reduce the etch-induced damage and smooth the fin sidewalls.

β-Ga₂O₃ rectifiers are attractive for high-temperature applications by virtue of their significantly lower intrinsic carrier density than in Si, SiC, and GaN. High-temperature operation at 500 °C has been demonstrated^{[87, 88]}. Unique challenges arise from the device-level thermal resistance due to the low thermal conductivity of β-Ga₂O₃. It has been found that packaging strategies involving junction-side or flip-chip cooling as well as double-side cooling are the most effective. Thin-body rectifiers that are flip-chip packaged have achieved a high surge current of 59 A^[89]. Double-side-cooled β-Ga₂O₃ SBDs have been shown to be capable of sustaining a peak surge current over 60 A (Fig. 11), with a ratio between the peak surge current and the rated current superior to that of similarly rated commercial SiC SBDs^{[90, 91]}. Trench SBDs of β-Ga₂O₃ have also shown fascinating thermal stability^{[92, 93]}. The resilience of β-Ga₂O₃ SBDs against high surge current is attributed to two key mechanisms ^[91]. First, the power-law temperature dependence of the R_ON,sp in β-Ga₂O₃ devices shows a temperature coefficient 2 to 4 times smaller than the corresponding values of SiC SBDs. This is possibly due to a weak temperature dependence of electron mobility in β-Ga₂O₃ and an increase in donor ionization at high temperatures^[94], both reducing the risk of thermal runaway. Second, the double-side and flip-chip packaging strategies allow heat extraction directly from the Schottky junction rather than through the thermally resistive β-Ga₂O₃ substrate. It is noteworthy that impact ionization breakdown of β-Ga₂O₃ was recently demonstrated, albeit in a p-NiO/n-Ga₂O₃ heterojunction instead of an SBD^[95]. Avalanche capability imparts surge robustness to β-Ga₂O₃ devices by enabling effective dissipation of the surge energy.

β-Ga₂O₃ SBDs have demonstrated favorable attributes to be the next frontier in power electronic devices. These achievements benefit from the excellent quality of homoepitaxial growth on native substrates with low dislocation density. Various types of edge termination methods have been implemented for field management to effectively harness the high E_cr afforded by β-Ga₂O₃. To this end, there have been applications of p-type NiO for edge termination in β-Ga₂O₃ SBDs and for constructing β-Ga₂O₃ JBSD structures. The p-NiO/n-Ga₂O₃ heterojunction p–n diode is also attractive in its own right and has shown exemplary performance as a standalone power device ^[96−100]. By employing an optimized junction termination extension scheme, the PFOM for heterojunction diodes with a V_br of 3.3 kV exceeds 2.5 GW/cm², which surpasses the 1-D SiC limit and is among the highest in multi-kilovolt β-Ga₂O₃ rectifiers^[100]. In JBSD and trench SBD structures, the maximum electric field is transferred from the Schottky interface to within the bulk of the drift layer thanks to the RESURF effect, which allows the field to be decoupled from the reverse leakage current, thus reducing the optimal R_ON,sp and improving the PFOM. Along with field management, heat dissipation to limit the junction temperature and improve the component reliability is also critical for maximizing the performance of β-Ga₂O₃ power devices.