The effect of surface passivation using (NH4)(2)S and (NH4)(2)S/P2S5 on a AlGaN/GaN-based metal-semiconductor-metal diode above a two-dimensional electron gas (MSM-2DEG) varactor was investigated. The surface property, capacitance ratio (C-max/C-min), and leakage current of the prepared samples were studied before and after treatments using X-ray photoelectron spectroscopy and capacitance-voltage and current-voltage analyses. It showed that the (NH4)(2)S/P2S5-treated sample had the most excellent surface state and C-max/C-min and the least leakage current because of either reduced native oxide or deposited phosphorus compounds only provided by (NH4)(2)S/P2S5 and sulfide upon the surface, also validated by having the highest sheet carrier density. Hence, these promising results promote further potential for varactor applications. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3473728] All rights reserved.