Part I: Physical Insights Into Dynamic RON Behavior and a Unique Time-Dependent Critical Stress Voltage in AlGaN/GaN HEMTs

We report a unique OFF-state drain-to-source critical stress voltage above which the dynamic performance of AlGaN/GaN HEMTs is significantly deteriorated. Physical insights are developed through detailed experiments conducted for different design variants, increased substrate bias, varying temperature, varying stress time, and experimenting buffers with lower C-doping. The surface leakage measurements on dedicated test structures and experimentation of devices realized over GaN buffer with different carbon doping reveal that the observed phenomenon is solely caused by traps present in carbon-doped GaN buffer. Surface traps have no role to play as far as they do not modulate the electric field in the GaN buffer (discussed in Part-II). Stress time, substrate temperature, substrate bias, electric-field shape, and depletion regions formed under the gate as well as field plate region were found to play a key role in governing trap-assisted transport and carrier trapping in the GaN buffer. A deeper physical insight to explain the observed phenomena and its dependency on various conditions are developed through detailed measurements, TCAD simulations, electroluminescence, and photoluminescence spectroscopy. With the help of the physical insight gained into the underlying mechanism, a novel surface passivation scheme using a p-type oxide is reported in Part-II, which helps relaxing the field distribution across the access region and buffer, and in turn improves the dynamic performance of the HEMTs. Besides, the Part-II also unifies the mechanism across different gate-stack designs and reveals the interplay between the surface and buffer, which governs the dynamic performance of these devices.