GaN-based high electron mobility transistors (HEMTs) are emerging as one of the most promising candidates for high efficiency power switching applications owing to their capability to deliver low on-resistance, high switching frequency, and high breakdown voltage. However, some key technical challenges are still present in state-of-the-art GaN HEMTs, preventing them from being widely adopted to the level the industry is expecting. One of the challenges is the dynamic on-resistance issue, also known as current collapse. Another challenge is the long-term reliability concern, for which the most significant liability is the gate stack. Mitigating these two major challenges to advance the GaN HEMT technology for wide utilization motivates the research work in this thesis.

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GaN-based high electron mobility transistors (HEMTs) are emerging as one of the most promising candidates for high efficiency power switching applications owing to their capability to deliver low on-resistance, high switching frequency, and high breakdown voltage. However, some key technical challenges are still present in state-of-the-art GaN HEMTs, preventing them from being widely adopted to the level the industry is expecting. One of the challenges is the dynamic on-resistance issue, also known as current collapse. Another challenge is the long-term reliability concern, for which the most significant liability is the gate stack. Mitigating these two major challenges to advance the GaN HEMT technology for wide utilization motivates the research work in this thesis.

This thesis is dedicated to the development of advanced gate stacks and passivation techniques to achieve stable device operation with minimal threshold voltage (V_th) shift and low current collapse in GaN HEMTs and metal-insulator-semiconductor HEMTs (MISHEMTs). First, a surface engineering process combining a pre-gate surface treatment and a post-gate thermal annealing was developed for Schottky gate HEMTs. High breakdown voltage (V_BR) and low current collapse were achieved in the devices. Second, in situ SiN was investigated systematically as the gate dielectric and surface passivation for GaN MISHEMTs. Minimal V_th shift was realized under long gate stress and high temperature operation. Taking the advantages of in situ SiN, high power MISHEMTs with a 20-mm gate width were demonstrated using a passivation-first process and a bilayer SiN passivation scheme, realizing low dynamic on-resistance and high V_BR. Third, atomic layer deposited high-k ZrO₂ was developed as the gate dielectric for GaN MISHEMTs to enhance the gate control. A high on/off current ratio, a nearly ideal subthreshold slope, a high V_BR, and suppressed current collapse were achieved simultaneously in the device. Power MISHEMTs with a 20 mm gate width were also demonstrated using the ZrO₂ gate dielectric, exhibiting excellent switching characteristics. Finally, a novel enhancement-mode GaN MISHEMT was demonstrated using an ultrathin-barrier AlGaN/GaN heterostructure, selective area barrier regrowth, and a high-k ZrO₂ gate dielectric. A uniform and large positive V_th was achieved as a result of the high quality gate stack in the MISHEMTs.