The function of the (NH4)(2)S-x surface treatment on the AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) was investigated by using the pulsed output characteristics and the low frequency noise measurements. The low carrier concentration and high resistivity 30-nm-thick ZnO film was deposited using the designed vapor cooling condensation system and utilized as the gate dielectric layer of the AlGaN/GaN MOS-HEMTs. The significant improvement of the pulsed output performance and low frequency noise behavior of the (NH4)(2)S-x-treated AlGaN/GaN MOS-HEMTs showed that the (NH4)(2)S-x surface treatment was an effective technique to reduce the surface state density and to obtain high quality interface between the ZnO gate dielectric layer and the AlGaN layer. The decrease of the surface states is attributed to the reduction of Ga dangling bonds and passivation of N vacancies by the formation of Ga-S bond on the AlGaN surface. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3524282] All rights reserved.