The origin of gate degradation under HTRB operation: Buffer engineering to suppress impact ionization in GaN HEMT

Abstract

This study investigates the origin of gate degradation in AlGaN/GaN HEMTs under high-temperature reverse bias (HTRB) conditions and proposes a buffer engineering strategy to mitigate this degradation. Transmission electron microscopy (TEM) analysis reveals the presence of a thin oxide layer between the gate and AlGaN contact. It is found that the holes generated through impact ionization (under high electric fields of HTRB operation), are directed toward the gate due to the intense local electric field under the gate and the field plate overhang, and are accumulated under this energetic barrier. Consequently, these trapped holes cause gate degradation and increased gate leakage. To address this issue, impact ionization, as the initial forcing mechanism of the degradation, is suppressed via thinning the channel GaN (C-GaN) layer. This improvement is attributed to the suppression of the local electric field near the gate region in thin C-GaN HEMTs. Additionally, the impact of C-GaN thinning on breakdown characteristics and RF performance is discussed. Overall, the findings provide insights into the root cause of gate degradation and offer a buffer engineering strategy to minimize gate degradation under deep off-state stress. This approach enhances the reliability of future high-power and high-frequency GaN HEMTs, contributing to their long-term performance in demanding applications.