The intrinsic ZnO (i-ZnO) film deposited by a vapor cooling condensation system was used as the gate dielectric layer of the AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs). To reduce the surface states, the chlorine surface treatment was utilized to obtain a high quality i-ZnO/AlGaN interface. The resulting saturation drain-source current and the maximum extrinsic transconductance were 0.85 A/mm and 207 mS/mm, respectively. Comparing to the untreated AlGaN/GaN MOS-HEMTs, the chlorine-treated MOS-HEMTs revealed better direct-current output performances and pulsed output performances. The improved performances of the chlorine-treated MOS-HEMTs were attributed to the reduction of Ga dangling bonds and the passivation of N vacancies resided on the AlGaN surface by using the chlorine surface treatment. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3597648] All rights reserved.