⎯ A two-dimensional theoretical analysis of bipolar transistor operation under reversal base current conditions is presented. This model describes the current crowding effect occurring when the device is biased above the open-base breakdown voltage BV CEO , also known as the "pinch-in" effect. In addition, the model clarifies, for the first time, the physical origin of instability phenomena occurring under common-base operating conditions. Closed form analytical relations are derived for the conditions which define the onset of instability under forced-V BE and forced-I E conditions. Finally, we present a simple analytical model for the base current-and geometry-dependence of the base resistance. This model is suitable for being incorporated into BJT compact models to properly describe device operation above BV CEO .Index Terms ⎯ SiGe HBT, safe operating area, base resistance, impact ionization, pinch-in effect.

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