Stability Study of >13.5 kV Field Plated NiO/ β -Ga 2 O 3 Heterojunction Rectifiers Fabricated on 4-Inch Wafers

100 μm diameter vertical NiO/ β -Ga 2 O 3 heterojunction diodes with dielectric-assisted field-plate edge termination were fabricated on a 4-inch β -Ga 2 O 3 wafer with ∼10–14 μm thick Si-doped 8.5 × 10 15 cm −3 drift layers demonstrating breakdown voltages (V B ) exceeding 13.5 kV, limited by the measurement equipment, on-voltage (V ON ) of ∼2.2 V and on-state resistance R ON of 5.5 mΩ·cm 2 . The average critical breakdown field in heterojunctions was 9.6–13.5 MV cm −1 , within the reported theoretical value range from 8–15 MV cm −1 for β -Ga 2 O 3 . The associated maximum power figure-of-merit, V B 2 /R ON of >33.1 GW cm −2 was achieved. Scanning transmission electron microscopy (STEM) reveals a polycrystalline NiO layer with multiple crystallographic orientations and a well-defined device stack. Forward current–voltage measurements performed 10 days and 150 days after NiO deposition show stable turn-on behavior with only minor changes in low-bias current. Cross-sectional STEM and energy-dispersive X-ray spectroscopy (EDS) analysis reveal localized oxygen enrichment near the outer edge (∼500 nm) of the Ni contact on NiO, likely associated with fabrication-related oxidation, while the NiO/ β -Ga 2 O 3 heterojunction remains intact. These results demonstrate stable ultra-high-voltage operation of NiO/ β -Ga 2 O 3 heterojunction rectifiers fabricated on wafer-scale substrates, highlighting their potential for next-generation extreme-voltage power electronics.