A large-signal model of a heterojunction bipolar transistor (HBT) is demonstrated for accurate simulations of the self-heating and the ambient temperature effects and of the nonlinear behaviors of the output power, gain expansion, and intermodulation distortion (IMD). This procedure initially extracts isothermal parameters from pulsed measurements, and then thermal parameters from thermal measurements. The separated extractions remove the ambiguity that is caused by the simultaneous effects of voltage drops due to series resistances and self-heating, which are usually encountered when obtaining Gummel plots under a high current density. After the self-heating effects are eliminated from the measured characteristics, the Gummel-Poon model equations are used to fit the measurement data to extract the isothermal parameters. A comparison of the model parameters between unballasted and base-ballasted HBTs with the same geometry is given in detail. To incorporate ambient temperature effects, we utilize the physical relationship between the device current and the rate of change in the built-in potential with respect to the device temperature. Measurements and simulations are compared to verify the model under DC conditions at various temperatures. Also, the gain expansion and the sweet spot under a large-signal two-tone condition are characterized to assess the accuracy of the model.