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High-quality SiO2 is deposited on GaN by radio frequency (RF) magnetron sputtering at room temperature. Adding oxygen to the sputtering gas effectively compensated for the oxygen vacancies and resulted in a breakdown field of 9.6 MV/cm for the sputtered- SiO2 film on GaN. The reduced electron concentration and mobility of the 2-D electron gas due to the sputtering-induced surface damage were effectively removed by an optimized postannealing treatment. A sputtered-SiO2/ AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistor (HEMT) (MOSHEMT) was demonstrated with the lowest thermal energy requirement among all the dielectric deposition techniques, which exhibited a saturation drain current of 621 mA/mm and a breakdown voltage of 205 V at the gate-drain distance of 2 μm. More than four orders of magnitude lower gate leakage current than conventional HEMT of the same dimension was achieved. These characteristics demonstrate excellent potential of using RF magnetron sputtering to produce gate insulators for GaN-based MOSHEMTs.