Abstract Al₂O₃ is a broadly employed dielectric material in GaN high electron mobility transistors. Nevertheless, at the Al₂O₃/GaN interface, numerous traps induced by nonidealities of the native GaO _x layer on the surface of GaN can lead to threshold voltage instability and other reliability issues. In this study, after removing the native GaO _x layer, a stoichiometric Ga₂O₃ layer was sandwiched between Al₂O₃ and GaN. The interfacial state density of Al₂O₃/GaN can be reduced by more than two orders of magnitude to an extremely low level of 2.4 × 10¹⁰ eV⁻¹ cm⁻² at the energy level of 0.36 eV.