We report the fabrication and DC and microwave characteristics of 0.5 mu m AlGaN/GaN high electron mobility transistors using double plasma treatment process. Silicon nitride layers 700 and 150 angstrom thick were deposited by plasma-enhanced chemical vapor deposition at 260 degrees 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 0.5 mu m 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 0.5 mu m AlGaN/GaN high electron mobility transistor devices fabricated by dry etching exhibited a peak transconductance, gm, of 330 mS/mm, a breakdown voltage of 115 V, a current-gain cutoff frequency (f(T)) of 18 GHz, and a maximum oscillation frequency (f(max)) of 66 GHz. (C) 2013 Elsevier B.V. All rights reserved.