Effect of drift layer doping and NiO parameters in achieving 8.9 kV breakdown in 100 μm diameter and 4 kV/4 A in 1 mm diameter NiO/β-Ga₂O₃ rectifiers

The effect of doping in the drift layer and the thickness and extent of extension beyond the cathode contact of a NiO bilayer in vertical NiO/β-Ga₂O₃ rectifiers is reported. Decreasing the drift layer doping from 8 × 10¹⁵ to 6.7 × 10¹⁵ cm⁻³ produced an increase in reverse breakdown voltage (V_B) from 7.7 to 8.9 kV, the highest reported to date for small diameter devices (100 μm). Increasing the bottom NiO layer from 10 to 20 nm did not affect the forward current–voltage characteristics but did reduce reverse leakage current for wider guard rings and reduced the reverse recovery switching time. The NiO extension beyond the cathode metal to form guard rings had only a slight effect (∼5%) in reverse breakdown voltage. The use of NiO to form a pn heterojunction made a huge improvement in V_B compared to conventional Schottky rectifiers, where the breakdown voltage was ∼1 kV. The on-state resistance (R_ON) was increased from 7.1 m Ω cm² in Schottky rectifiers fabricated on the same wafer to 7.9 m Ω cm² in heterojunctions. The maximum power figure of merit (V_B)²/R_ON was 10.2 GW cm⁻² for the 100 μm NiO/Ga₂O₃ devices. We also fabricated large area (1 mm²) devices on the same wafer, achieving V_B of 4 kV and 4.1 A forward current. The figure-of-merit was 9 GW  cm⁻² for these devices. These parameters are the highest reported for large area Ga₂O₃ rectifiers. Both the small area and large area devices have performance exceeding the unipolar power device performance of both SiC and GaN.