The high power operation of the heterojunction bipolar transistor (HBT) has been analyzed by experimentally determining the junction temperature and separating temperature effects from other high power effects. In addition, an HBT large signal model has been developed that is valid for the linear, saturation, and cutoff regions, with low frequency temperature effects included. This model has been implemented in a commercial harmonic balance simulator, LIBRA from EEsof, making it particularly suitable for the design and simulation of HBT microwave power integrated circuits. In addition, an analysis of the most temperature-sensitive microwave elements for the HBT has been performed using measured s-parameter data at five elevated temperatures from 23 deg. C to 226 deg. C. The element values were compared to a physical model showing excellent agreement in magnitude and direction of change with temperature and bias. The transistor cutoff frequencies were also measured and calculated, showing a monotonic decrease with temperature of approximately 50% over the 200 deg. C range. Heterojunction Bipolar Transistor, Large Signal Modeling, Thermal Effects.