AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) on a silicon substrate were fabricated with silicon oxide as a gate dielectric by sputtering deposition and electron-beam (EB) evaporation. It was found that the oxide deposition method and conditions have great influences on the electrical properties of HEMTs. The low sputtering temperature or oxygen introduction at higher temperature results in a positive equivalent charge density at the oxide/AlGaN interface (N_equ), which induces a negative shift of threshold voltage and an increase in both sheet electron density (n_s) and drain current density (I_D). Contrarily, EB deposition makes a negative N_equ, resulting in reduced n_s and I_D. Besides, the maximum transconductance (g_m-max) decreases and the off-state gate current density (I_G-off) increases for oxides at lower sputtering temperature compared with that at higher temperature, possibly due to a more serious sputter-induced damage and much larger N_equ at lower sputtering temperature. At high sputtering temperature, I_G-off decreases by two orders of magnitude compared to that without oxygen, which indicates that oxygen introduction and partial pressure depression of argon decreases the sputter-induced damage significantly. I_G-off for EB-evaporated samples is lower by orders of magnitude than that of sputtered ones, possibly attributed to the lower damage of EB evaporation to the barrier layer surface.