Compact modeling of BJT self-heating in SPICE

Self-heating effects in bipolar junction transistors have been incorporated into PSpice dc and ac analyses. The effects are intrinsic to the operation of the transistor, and are treated within the device model, avoiding the need for thermal subcircuits. A physical thermal impedance model is provided, which allows prediction of thermal impedance for devices with rectangular emitters from device geometry. A simple approximation is used to predict thermal frequency response. The predictive model can be overridden by measured thermal model parameters. The modifications made to the PSpice code are presented, along with some discussion of implementation alternatives. An example simulation is presented demonstrating the significance of thermal effects in a typical circuit. Run-time comparisons show the modified code is about half the speed of unmodified PSpice, mostly because of slower convergence. It is likely that this performance can be improved with suggested implementation changes.