1.5 kV NiO/Ga ₂ O ₃ Vertical Heterojunction Diodes Based on Lightly Doped Single‐Crystal Substrate with In Situ Ohmic Contact Formation

A lightly doped drift layer with ultralow defect density, micrometer thickness, and appropriate interface barrier height is critical for boosting the performance of β‐Ga ₂ O ₃ ‐based power diodes. Compared to the significant advance of β‐Ga ₂ O ₃ power diodes using various thin film techniques for growing a high‐quality drift layer, constructing the drift layer directly from the single‐crystal substrate could be an alternative, cost‐effective approach, owing to the commercial availability of large‐scale, high‐quality β‐Ga ₂ O ₃ substrates. Here, vertical NiO/β‐Ga ₂ O ₃ heterojunction diodes are reported using a lightly doped β‐Ga ₂ O ₃ single‐crystal substrate as the drift layer, achieving a record‐high breakdown voltage of 1.51 kV among substrate‐based devices. To enhance forward characteristics, a heavily Si‐doped β‐Ga ₂ O ₃ thin film is in suit grown on the cathode as the Ohmic contact, which reduces the specific on‐resistance ( R _on,sp ) by two orders of magnitude, from 21.2 Ω·cm ² to 67 mΩ·cm ² . Numerical simulations reveal that over 95% of R _on,sp originates from the substrate's neutral region. Further reduction of R _on,sp is possible through substrate thinning without compromising its breakdown voltage ( V _br ) value. This study demonstrates the promising potential of the lightly doped β‐Ga ₂ O ₃ substrate as an alternative industry‐level approach for developing next‐generation power diodes.