Gate Stability and Reliability of Fully Depleted p-GaN Gate HEMTs

Abstract

Gallium Nitride high-electron-mobility transistors (GaN HEMTs) offer superior efficiency, and higher power density compared with traditional silicon counterparts and show great potential in emerging applications such as data centers and microinverters. Among various enhancement-mode (E-mode) GaN techniques, the p-GaN gate HEMTs has become the dominant approach in commercial devices. Conventional devices typically use a partially depleted p-GaN structure, while the fully depleted (FD) p-GaN design has recently emerged as a promising approach to suppress gate leakage and has already been adopted by some manufacturers. However, studies on the gate stability, reliability, and degradation mechanisms of FD p-GaN gate devices remain limited, highlighting the need for systematical research.

Degradation mechanisms of FD p-GaN gate HEMTs are comprehensively investigated in this thesis. First, the gate stress induced threshold voltage (V_th) instability is explored. Experimental results show positive gate stress induces positive V_th shift. By identifying electron thermionic emission as the dominant gate leakage mechanism under forward gate bias, the presence and dominance of electron spill-over process in the gate stack is confirmed. A physical model based on charge dynamics is developed, which attributes V_th shift to charge trapping and releasing in the p-GaN layer.

Gate reliability of FD p-GaN gate HEMTs is studied through accelerated testing in the second part of this thesis. Post-breakdown electrical analysis reveals a unique gate failure mechanism initiated by impact ionization within the p-GaN region. Electron trapping in p-GaN under high positive gate stress intensifies the local electric field, thereby promoting impact ionization. As the trapped charge accumulates, the local electric field continues to intensify until it reaches a critical strength, initiating gate breakdown. The negative V^th shift and increased OFF-state leakage observed after gate failure serve as strong evidence for the substantial generation and injection of holes during the avalanche process.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Kevin Jing CHEN (Supervisor)