Temperature-dependent dc characteristics of InGaP/GaAs heterojunction bipolar transistors (HBTs) without any surface passivation, with sulfur passivation, and with emitter ledge structure were systematically studied and demonstrated. Experimentally, due to the effective suppression of the thermal leakage current and hot-carrier injection around the emitter mesa edge, the device with ledge structure revealed lower base-surface recombination current density as well as better thermal stability and electrical reliability than the devices with/without sulfur passivation. (c) 2007 The Electrochemical Society.