In this work, a low-power plasma oxidation surface treatment followed by Al₂O₃ gate dielectric deposition technique is adopted to improve device performance of the enhancement-mode (E-mode) AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOSHEMTs) intended for applications at millimeter-wave frequencies. The fabricated device exhibited a threshold voltage (V_th) of 0.13 V and a maximum transconductance (g_m) of 484 (mS/mm). At 38 GHz, an output power density of 3.22 W/mm with a power-added efficiency (PAE) of 34.83% were achieved. Such superior performance was mainly attributed to the high-quality Al₂O₃ layer with a smooth surface which also suppressed the current collapse phenomenon.