A study of forward second-breakdown in silicon bipolar power transistors using the unit-cell concept

We present a unit-cell model that accurately delineates the forward-biased safe operating area (SOA) of power transistors containing hundreds of integrated unit cells. All model parameters are temperature dependent. The slope of the upper boundary of the SOA is not "constant in power", due to the effect of high-temperature leakage currents. For certain combinations of emitter and base ballast resistors, stable operation occurs above BVceo(αM = 1). We discuss electrical and thermal stability criteria. Regions of stable hot-spot formation and thermal degeneracy are predicted. The effects on SOA of cell parameter variations are shown.