Modeling intermodulation distortion in SiGe HBTs

The accuracy of large-signal equivalent circuit models of silicon-germanium heterojunction bipolar transistors (HBTs) is examined. Nominal transistor model parameters are found to be insufficiently accurate to reliably predict intermodulation distortion (IMD) in individual heterojunction bipolar transistors. A method for optimizing the model parameters to give a more accurate simulation of IMD performance is outlined. For best results, some form of parameter optimization for IMD should be considered as a part of the parameter extraction procedure. Differences between transistor models are observed and analyzed by comparing measured IMD power levels with computer simulation results. The Gummel-Poon model is found to be simpler and easier to optimize than the Kull-Nagel model.