In this paper, we adopted thermally stable HfOx Ny as gate dielectric for TiN/HfOx Ny/AlGaN/GaN heterostructure field-effect transistors (HFETs) application. It demonstrated that the surface morphologies, composition, and optical prop-erties of the HfOx Ny films were dependent on oxygen flow rate in the O2/N2/Ar mixture sputtering ambient. The obtained metal–oxide–semiconductor heterostructure field-effect transistors by depositing HfO2 and HfOx Ny dielectric at different oxygen flow rates possessed a small hysteresis and a low leakage current. After post deposition annealing at 900 ℃, the device using HfOx Ny dielectric operated normally with good pinch-off characteristics, while obvious degradation are ob-served for the HfO2 gated one at 600 ℃. This result shows that the HfOxNy dielectric is a promising candidate for the self-aligned gate process.