The temperature-dependent characteristics of (NH4)(2)S-x-passivated AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistors were studied and demonstrated. Due to the use of sulfur passivation, remarkable improvements in device performance, including higher forward turn-on voltage, higher reverse breakdown voltage, lower reverse leakage current, higher transconductance, lower on-resistance, more linear operating regime, and superior microwave performance, were obtained. In addition, the sulfur-passivated devices also show good properties in the higher operating temperature regime and relatively thermally stable performance over the operation temperature range 300-510 K. Therefore, the studied device with (NH4)(2)S-x treatment provides promise for high-performance digital and microwave device applications. (c) 2006 The Electrochemical Society.