The intrinsic ZnO (i-ZnO) film was deposited by a vapor cooling condensation system and used as the dielectric layer of the AlGaN metal-oxide-semiconductor (MOS) diodes. Before the deposition of the i-ZnO dielectric layer, the AlGaN surface was treated using (NH4)(2)S-x solution. In view of the lattice match of the i-ZnO film and the reduced surface state density of the (NH4)(2)S-x-treated surface, the quality of the i-ZnO/AlGaN interface was improved. According to the experimental results, the i-ZnO/(NH4)(2)S-x-treated MOS diodes revealed the lower leakage current, the lower interface state density, and the high electrical performances compared with the i-ZnO/untreated ones. Furthermore, the X-ray photoelectron spectroscopy and the charge neutrality level model were used to analyze the conduction band offset and the valence band offset of the i-ZnO/AlGaN interface. The valence band offset of the i-ZnO film contacted with the untreated and the (NH4)(2)S-x-treated AlGaN layer was 1.53 eV and 1.96 eV, respectively. The conduction band offset of the i-ZnO film contacted with untreated and (NH4)(2)S-x-treated AlGaN layers was 0.77 eV and 1.20 eV, respectively. The mechanisms of the enhanced conduction band offset were attributed to the effective reduce of interface states by using the (NH4)(2)S-x surface treatment. (C) 2012 Elsevier B.V. All rights reserved.