Enhanced Breakdown Voltage in GaN HEMTs Using Step-Graded AlGaN Buffer Layers

In this work, step-graded AlGaN buffer layers were introduced into GaN high-electron-mobility transistors to enhance their off-state voltage-blocking capability without intentional deep-level buffer doping. Hall measurements showed increased two-dimensional electron gas sheet densities, while the electron mobilities remained above 1550 cm²/V·s. The fabricated devices exhibited similar threshold voltages near -18 V and comparable pulsed output current densities. At approximately 1000 V, sample B showed an average drain leakage-current reduction of approximately 27.5% relative to sample A. The average breakdown voltage increased from 1540 V for the conventional GaN-buffer device to 1814 V for the optimized step-graded AlGaN-buffer device, corresponding to an enhancement of approximately 17.8%. TCAD simulations show that the step-graded buffer introduces a multi-step electron-blocking barrier and enhances vertical carrier confinement. Under high-voltage off-state conditions, it also redistributes the drain-side electrostatic potential and suppresses electron penetration into the buffer. In addition, dislocation-related trapping may also contribute to the improved off-state blocking performance. These results demonstrate that step-graded AlGaN buffers offer an effective approach for improving the breakdown performance of GaN HEMTs while largely preserving their conduction capability.