A RESURF Structure for Normally-Off High-Voltage Hydrogen-Terminated Diamond FETs

kV-class high-voltage power devices play a critical role in modern power grids, high-voltage electronic platforms and renewable energy applications. As an ultra-wide bandgap material, diamond has attracted increasing attention due to its excellent properties, especially in high-voltage and energy-efficient systems. However, conventional H-diamond FETs face two intrinsic challenges: surface conduction induced by two-dimensional hole gas (2DHG) causes surface electric field crowding and results in inherent normally-on operation, limiting their applications. In this work, a reduced surface field (RESURF) structure is demonstrated in normally-off H-diamond FETs. By ultraviolet ozone treatment, the RESURF structure is realized in the gate-to-drain region. Devices with different gate-excluded RESURF lengths (L_E) are fabricated and systematically investigated. The proposed normally-off device (L_E = 4 μm) achieves over 1.5 kV breakdown voltage due to effective surface field modulation and reduced field crowding. Meanwhile, a low specific on-resistance of 172 mΩ·cm² is achieved, demonstrating an excellent tradeoff between on-state conduction and off-state performance. Silvaco TCAD simulations further confirm the role of the RESURF structure in electric field modulation.