We report the fabrication and DC and microwave characteristics of 0.5μm AlGaN/GaN high electron mobility transistors using double plasma treatment process. Silicon nitride layers 700 and 150Å thick were deposited by plasma-enhanced chemical vapor deposition at 260°C to protect the device and to define the gate footprint. The double plasma process was carried out by two different etching techniques to obtain enhancement-mode AlGaN/GaN high electron mobility transistors with 0.5μm gate lengths. The enhancement-mode AlGaN/GaN high electron mobility transistor was prepared in parallel to the depletion-mode AlGaN/GaN high electron mobility transistor device on one wafer. Completed double plasma treated 0.5μm AlGaN/GaN high electron mobility transistor devices fabricated by dry etching exhibited a peak transconductance, gm, of 330mS/mm, a breakdown voltage of 115V, a current-gain cutoff frequency (fT) of 18GHz, and a maximum oscillation frequency (fmax) of 66GHz. •The double plasma process was carried out by two different etching techniques.•Double plasma treated device exhibited a transconductance of 330mS/mm.•Completed 0.5μm gate device exhibited a current-gain cutoff frequency of 18GHz.•The off-state breakdown voltage of 115V for 0.5μm gate device was obtained.•Continuous-wave output power density of 4.3W/mm was obtained at 2.4GHz.