Avalanche multiplication and pinch-in models for simulating electrical instability effects in SiGe HBTs

The onset of impact ionization-induced instabilities limits the operative range of SiGe hetero-junction bipolar transistors. Based on referential Monte Carlo simulation results, a critical review of major models for the avalanche multiplication factor (M) is presented, and a new analytical model is proposed and successfully verified by measurements. The novel M formulation has been incorporated in a two-dimensional theoretical model describing bipolar transistor operation under pinch-in conditions/above the open-base breakdown voltage BV CEO. The physical mechanisms leading to electrical instability are addressed, and closed form analytical relations defining the onset of instability under forced-I E conditions are derived. The proposed model defines the limits of the Safe Operating Area (SOA) related to impact ionization, enabling the reliable usage of HBTs above BV CEO.