The temperature-dependent direct current (dc) characteristics and radio frequency performance of an InGaP/GaAs heterojunction bipolar transistor with the composite passivations on base surface are studied and demonstrated. For comparison, the characteristics of other samples with different treatments on the base surfaces are also included in this work. The device with composite passivations, i.e., the ledge structure and sulfur treatment, shows the best performance and related thermal stabilities on dc current gain beta(F), base surface recombination current density J(SR), and base current ideality factor n(B). Experimentally, the device with composite passivations can be operated in the extremely low collector current (I-C <= 10(-9) A) regime with a useful current gain (beta(F) > 10). Moreover, this device exhibits improved microwave characteristics and hence is promising for low-power electronic and communication applications. (c) 2006 The Electrochemical Society.