I. INTRODUCTION
GaAs heterojunction bipolar transistors (HBTs) are widely used in power amplifiers for portable wireless applications because of their excellent linearity, efficiency, and power density. Because of the potential for improved electrical and thermal characteristics, InP HBTs are currently being considered for similar applications. The thermal conductivity of InP is 73% higher than that of GaAs, so that superior thermal resistance is frequently asserted as an important advantage of InP-based technology. Unfortunately, HBTs cannot be fabricated from InP alone. The subcollector layer in InP based HBTs, and usually the collector layer as well, are composed of InGaAs, which has a thermal conductivity which is only 1/10th that of GaAs. Because most heat is dissipated in the collector, these InGaAs layers may compromise the thermal advantage of the InP substrate. Therefore, simulations are required to determine the actual advantage of InP technology in terms of thermal resistance, and are useful for guiding the design of thermally-efficient devices in this technology.