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GaN-on-Si technology, AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrates, shows the promising characteristics and cost-competitiveness of power switching applications. In spite of the extraordinary performance and cost advantages, AlGaN/GaN HEMTs are still limited by their instabilities. For power-switching applications, GaN power devices operate at a high drain voltage during an OFF-state and at a high gate voltage during an ON-state, where good reliability is essential for these operating conditions. This dissertation focuses on the physical degradation mechanisms in the gate region that play a role in the long-term stability of Au-free enhancement-mode GaN power devices, especially for the two most important architectures: recessed gate Metal- Insulator-Semiconductor (MIS)-HEMTs/-FETs (Field-Effect Transistors) and p-GaN gate AlGaN/GaN HEMTs. Forward gate bias time-dependent dielectric breakdown (TDDB) and positive bias temperature instability (PBTI) are observed on depletion mode MIS-HEMTs and enhancement-mode MIS-FETs. The percolation model and Weibull distribution are used to understand the degradation mechanisms of forward gate bias TDDB, further calculating the lifetime. Regarding the PBTI, different techniques, i.e. a forward-reverse IDVG sweep, a frequency-dependent conductance method, and an AC-transconductance, are used to characterize the threshold voltage (VTH) hysteresis, interface states density (Dit), and the amount of border traps in the devices with different gate dielectrics. Furthermore, an eMSM (extend-Measure-Stress-Measure) method is used to study the stress-recovery phenomena in fully recessed gate MIS-FETs. A physical model, which can nicely reproduce the experimental data, is proposed to explain the origin of PBTI. Regarding p-GaN gate AlGaN/GaN HEMTs, temperature dependency of the forward bias gate breakdown is observed and characterized. Then, a physical model is proposed to explain the phenomenon. Furthermore, forward gate bias time-dependent p-GaN gate breakdown and positive bias temperature instability (PBTI) are also studied in p-GaN gate AlGaN/GaN HEMTs. The possible mechanisms are proposed to explain the time-dependent p-GaN gate breakdown phenomenon and a negative VTH shift under a positive gate bias.

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