Impact of Buffer Capacitance-Induced Trap Charging on Electric Field Distribution and Breakdown Voltage of AlGaN/GaN HEMTs on Carbon-Doped GaN-on-Si

In this work, we reveal a buffer capacitive action-induced channel electric field modulation in AlGaN/GaN HEMTs fabricated on a carbon-doped GaN-on-Si buffer. The field modulation is probed by analyzing a slew rate dependence of the OFF-state breakdown voltage of the device. The role of capacitive action is established with the help of quasistatic (emulating low slew rate condition) and pulsed stressing (emulating high slew rate condition) and well-calibrated computations. Experimental measurements showed devices to have a higher breakdown voltage when the drain was subjected to faster slew rates. Leakage current and electric field-based analysis showed a slew rate-dependent field rearrangement in the GaN channel to be responsible for the observed breakdown behavior. A field redistribution between the field plate (FP) edge and drain edge at higher slew rates (when capacitive action is stronger) results in the improvement in the breakdown voltage. The electric field magnitude near the drain edge is shown to be determined by the buffer capacitive action induced electron trapping near the drain edge. Based on these observations, a mechanism involving dV/dt-dependent electron current injection and subsequent trapping in the GaN buffer is proposed to explain the slew rate dependence of the breakdown voltage. Simulations with varying dielectric constant of the C-doped GaN buffer, and experimental semi-ON state electroluminescence (EL) measurements are used to validate the proposed mechanism.