TCAD Investigation of Thick Bottom Oxide in GaN Trench-Gate MOSFETs for Electric Field Mitigation and Dynamic Performance Enhancement

This work investigates the impact of introducing a thick bottom oxide (TBO) beneath the trench gate on the electrical performance of GaN trench-gate MOSFETs using physics-based TCAD simulations. The added oxide layer redistributes the internal electric field and suppresses the peak field at the trench bottom, thereby improving breakdown voltage and gate oxide reliability under high off-state bias. The effects of varying TBO thickness are evaluated in terms of transfer and output characteristics, electric field distribution, gate-to-drain capacitance (Cgd), and gate charge (Qgd). Simulation results show that a thicker TBO significantly reduces both Cgd and Qgd, enhancing switching performance and minimizing dynamic losses. The Qgd reduction of up to 48% is observed. Although a slight increase in on-resistance is introduced, the device retains favorable conduction performance. These findings demonstrate that the TBO structure serves as an effective strategy for improving both breakdown robustness and switching efficiency in GaN trench-gate MOSFETs.