A large-signal modeling of power heterojunction bipolar transistor (HBT) is demonstrated for an accurate simulation of self-heating and ambient temperature effects and nonlinear behaviors such as output power, gain expansion, intermodulation distortion (IMD), and adjacent channel power ration (ACPR). The physical relationship between the device current and the rate of change in the built-in potential with respect to the device temperature has been utilized for a fully electrothermal modeling. To enable an immediate use for a circuit design, the model extraction was done for insitu output-stage device from two-stage power amplifier (PA) circuit. In each parameter extraction step, measurement data obtained under a consistent environment, which are current-voltages (I-Vs) at various temperatures and small-signal S-parameters under various bias conditions, have been carefully examined and utilized to relate the meaning of each parameter to the physical principle of the device. Measurements and simulations are compared for the verification of the model under dc condition at various temperatures. The gain expansion and sweet spot under, large-signal two-tone condition have been characterized at the various harmonic load conditions to assess the accuracy of the model. Finally, the spectral regrowth with the device being driven by the code division multiple access (CDMA) reverse link signal has been simulated under the various bias conditions to demonstrate the practicality of the model.