Temperature-dependent characteristics of high electron mobility transistors (HEMTs) with sulfur and SiNx passivations are comprehensively studied and demonstrated. Experimentally, for the studied device with formal passivations, better dc and microwave characteristics are obtained over a wide operating temperature range. In particular, as compared with the device only with sulfur passivation, the slight degradations of device performance are caused by the temperature stress during the deposition of SiNx layer and presence of surface traps at the SiNx/AlGaAs interface. However, under an accelerated stress test, this formal-passivated device shows improved reliability performance. Based on these good results, the formal-passivated HEMT is expected to exhibit relatively better long-term operation stability and reliable device characteristics. (c) 2007 The Electrochemical Society.