In this work, we present the terrace etching technique to obtain excessive thickness series of atomic layer deposition (ALD) grown Al2O3 and HfO2 on GaN-cap/AlGaN/GaN heterostructures allowing for the detailed study of oxide charge distribution and its impact of the metal-insulator-semiconductor high electron mobility transistor (MISHEMT) threshold voltage. By modeling the experimental plot of threshold voltage versus oxide thickness on the basis of experimentally determined two-dimensional electron gas charge density in AlGaN/GaN MISHEMTs, we separated the interface and bulk charge components and determined the oxide-metal barrier height for the investigated gate dielectrics. In both Al2O3 and HfO2 gate dielectrics, the oxide charges are mainly located at the oxide/GaN interface. Determining the interface trap charges from comparison of the pulsed capacitance-voltage (CV) technique with very fast voltage sweep to the modulation type CV method with slow DC voltage ramp, we extracted positive fixed charges of NOx=2.7×1012 cm−2 for Al2O3 and NOx=7.8×1012 cm−2 for HfO2. We found a strong Vth shift of opposite direction for both high-k materials, corresponding to negatively charged up trap states at the HfO2/GaN interface and positively charged up trap states at the Al2O3/GaN interface. The evaluation of the metal-oxide barrier height in dependence of the metal work function followed the trend of the Schottky model, whereas HfO2 showed less Fermi level pinning compared to Al2O3 indicating the presence of an increased number of interface states in Al2O3 on GaN.