The nonlinear DC and RF properties of HBTs were modeled using a customized Gummel-Poon-based large signal model. The characterization and modeling techniques presented here were validated using PNP InP-based HBTs, which manifest highly nonlinear features. On-wafer pulsed DC measurements were performed on these HBTs in order to extract various second-order effects. These pulsed measurements indicated the presence of distributed thermal resistances and thermal capacitances. An analytical parameter extraction procedure was then used to extract model data from measured small-signal S-parameters and from the DC characteristics. Pulsed characteristics under different pulse widths and different substrate temperatures were employed to complete the model extraction. Good agreement was obtained between the modeled and measured DC, pulsed, small-signal, and large-signal microwave characteristics. The approach developed in this work is useful for modeling of highly nonlinear HBT characteristics, and it is directly applicable to the HBT power amplifiers often encountered in wireless communication circuits and systems.