Gallium oxide (Ga₂O₃) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga₂O₃, namely, the monoclinic (β-Ga₂O₃), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (ε) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO₃, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga₂O₃ relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga₂O₃, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga₂O₃. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga₂O₃ are identified.

AZO interlayers between n-Ga₂O₃ and Ti/Au metallization significantly enhance Ohmic contact formation after annealing at ≥ $300^{°}$ C. Without the presence of the AZO, similar anneals produce only rectifying current-voltage characteristics. Transmission Line Measurements of the Au/Ti/AZO/Ga₂O₃ stacks showed the specific contact resistance and transfer resistance decreased sharply from as-deposited values with annealing. The minimum contact resistance and specific contact resistance of 0.42 Ω-mm and 2.82 × 10^-5 Ω-cm² were achieved after a relatively low temperature $400^{°}$ C annealing. The conduction band offset between AZO and Ga₂O₃ is 0.79 eV and provides a favorable pathway for improved electron transport across this interface.

Films of β-Ga₂O₃ grown by halide vapor phase epitaxy on native substrates were subjected to Ar inductively coupled plasma treatment. As a result, the built-in voltage of Ni Schottky diodes deposited on the plasma treated surfaces decreased from 1 V to −0.02 V due to the buildup of deep trap concentration in the near surface region. Deep level spectra measurements indicate a strong increase in the top ∼200 nm of the plasma treated layer of the concentration of E2* (Ec − 0.8 eV) and especially E3 (Ec − 1.05 eV) deep electron traps. Capacitance-voltage profiling with monochromatic illumination also indicated a large increase in the upper ∼100 nm of the film in the concentration of deep acceptors with optical threshold for an ionization of ∼2.3 eV and 3.1 eV. Such defects at the surface led to a significant increase in reverse current, an increase in the ideality factor in forward current, and a dramatic decrease in the diffusion length of nonequilibrium charge carriers from 450 nm to 150 nm.

The effects of hydrogen plasma treatment of β-Ga₂O₃ grown by halide vapor phase epitaxy and doped with Si are reported. Samples subjected to H plasma exposure at 330 °C developed a wide (∼2.5 μm-thick) region near the surface, depleted of electrons at room temperature. The thickness of the layer is in reasonable agreement with the estimated hydrogen penetration depth in β-Ga₂O₃ based on previous deuterium profiling experiments. Admittance spectroscopy and photoinduced current transient spectroscopy measurements place the Fermi level pinning position in the H treated film near E_c-1.05 eV. Annealing at 450 °C decreased the thickness of the depletion layer to 1.3 μm at room temperature and moved the Fermi level pinning position to E_c-0.8 eV. Further annealing at 550 °C almost restored the starting shallow donor concentration and the spectra of deep traps dominated by E_c-0.8 eV and E_c-1.05 eV observed before hydrogen treatment. It is suggested that hydrogen plasma exposure produces surface damage in the near-surface region and passivates or compensates shallow donors.

The performance of arrays consisting of 21 β-Ga₂O₃ field-plated rectifiers fabricated on thick epitaxial layers (n-type carrier concentration ∼1.6 × 10¹⁶ cm⁻³) grown on conducting substrates (carrier concentration 3 × 10¹⁹ cm⁻³) is reported. We show that by interconnecting the output of 21 smaller (0.4 × 0.4 mm² to 1 × 1 mm², total area 0.09 cm²) individual rectifiers using e-beam deposited Au, we can achieve a high total forward output current of 33.2 A, at 4.25 V in the single-sweep voltage mode, and a low forward turn-on voltage of 2.9 V (defined at 100 A cm⁻²) and maintain a reverse breakdown voltage of 240 V (defined at 1 μA cm⁻²). The current density was 376 A cm⁻², and the on-state resistance was 0.012 Ω cm². The total forward current was 10 A at 1.9 V and 22 A at 3 V. The power figure-of-merit for the array, V_B²/R_ON, was 4.8 MW cm⁻², with a reverse recovery time of individual rectifiers of 32 ns. The on/off ratio of the rectifier array was in the range of 10⁵–10¹⁰ for +1 V/−1 to −100 V.

The effects of bilayer field plates with various dielectric (SiO₂/SiN_x, Al₂O₃/ SiN_x, HfO₂/ SiN_x) on Ga₂O₃ Schottky rectifier performance were investigated. The rectifiers were fabricated on 10 μm thick, Si doped (n = 2.8 × 10¹⁶ cm⁻³) β-Ga₂O₃ epitaxial layers grown by hydride vapor phase epitaxy on Ga₂O₃ Sn-doped substrates (n = 4.8 × 10¹⁸ cm⁻³) grown by edge-defined, film-fed growth. Temperature-dependent forward current-voltage characteristics were used to extract the average Schottky barrier height of 1.14 eV ± 0.03 eV for Ni, average ideality factor of 1.02 ± 0.02, and the Richardson's constant of 48.1 A/cm²K². The reverse breakdown and leakage current were the two characteristics predominantly affected by the field plate dielectrics. The highest reverse breakdown reached was 730 V for rectifiers with Al₂O₃/ SiN_x, which was significantly higher than 562 V and 401 V for rectifiers with SiO₂/ SiN_x and HfO₂/ SiN_x, respectively. The on-resistance ranged from 3.8-5.0 × 10⁻³ Ω-cm², which was dependent on diode size, with diameters from 50 to 200 μm. This led to a power figure-of-merit (V_B²/R_ON) of 140 MW-cm². Design of the field plate is crucial in determining where reverse breakdown occurs.