High-performance GaN metal-oxide-semiconductor high-electron-mobility transistors (MOSHEMTs) using Al2O3 gate dielectric deposited by atomic layer deposition (ALD) and damage-free neutral beam etch (NBE) gate recess process for millimeter-wave power applications are demonstrated. The high-quality ALD Al2O3 reduces the gate leakage current of the device and the NBE method eliminates the plasmainduced defects in the nitride materials. The MOSHEMT device fabricated exhibits a high maximum drain current density (IDS,max) of 1.65 A/mm and a high peak extrinsic transconductance (gm.ext) of 653 mS/mm. The MOSHEMT device also demonstrates excellent RF performances, including fT/fMAX = 183/191 GHz, NFmin = 2.56 dB with GAS = 5.61 dB at 54 GHz, and an output power density of 2.7 W/mm associated with a power-added efficiency of 20.9% and a linear power gain of 9.4 dB at 38 GHz. To the best of our knowledge, the noise performance at 54 GHz is the best reported so far for the AlGaN/GaN HEMTs.