Normally-off hydrogen-terminated diamond MESFET with Baliga's figure of merit of 37.5 MW/cm²

Diamond exhibits excellent material properties for high-power applications. However, hydrogen-terminated diamond (H-diamond) FETs still suffer from the surface two-dimensional hole gas (2DHG), which inherently leads to normally-on operation and limits their application in energy-efficient and high-voltage systems. In this work, a normally-off H-diamond FET with Schottky gate and oxygen extension region formed by oxygen annealing is demonstrated. AFM and hall measurements reveal reduced surface roughness and carrier density after annealing. Besides, the optimized oxygen annealing process improves the trade-off between on-state and off-state performances. The fabricated normally-off device exhibits a drain current density of 36 mA/mm, a specific on-resistance of 120.7 mΩ·cm², a subthreshold swing of 200 mV/dec, and a breakdown voltage of 2128 V, yielding a BFOM of 37.5 MW/cm².