GaN has raised wide attention owing to its high breakdown voltage and capability of performing efficiently even at high temperature. However, there are several challenges to overcome for achieving the superior performance of GaN HEMTs, including management and minimization of gate leakage and interface states located at the insulator-semiconductor interface. In this work, I studied several gate insulators readily available at CNF and their insulator-semiconductor interface in the metal-insulator-semiconductor capacitor structure. I found that the Al2O3/HfO2 bilayer is helpful in reducing gate leakage by one order of magnitude and improving the breakdown field from 4.5 MV/cm to 6.5 MV/cm compared to the single HfO2 layer with a similar equivalent oxide thickness. The observed improvement can be attributed to the large bandgap and excellent conduction band offset of Al2O3. Moreover, I found that a UV/ozone oxidation surface treatment prior to dielectric deposition helps to decrease the interface density of states at 0.39 eV below the conduction band to 1.2 × 10^12 eV^−1 cm^−2 characterized by the frequency-dependent conductance method. I also studied PECVD SiNx passivation of AlN/GaN/AlN quantum well HEMTs with several surface treatment methods. With NH3 plasma pretreatment, I observed an improvement in the maximum drain current by 30 % and peak transconductance by 60 % in these HEMTs. These are positive indicators for the effectiveness of NH3 plasma pretreatment in managing dispersion in HEMTs.