Effects of the field dependent mobility on the high frequency performance of BJT under base pushout condition

Abstract

Base pushout due to current-induced perturbation is one of the dominant factors for the degraded speed performance of the modern bipolar junction transistors (BJT).With the scaling down of the feature size of the modern BJT's, the degrading effects due to base pushout becomes increasingly prominent. Therefore, an accurate modeling of base transit time, which is the most significant component in determining the speed performance of BJT's under base pushout condition, is required. However, the present day BJT's use heavy base doping in order to improve its performance. But use of such high doping introduces various non-ideal effects such as bandgap narrowing effects, doping and field dependency of carrier mobility etc. Inclusion of these effects makes the analytical modeling of base transit time, especially under base pushout condition, a formidable task. Therefore, conventional models ignore the field-dependency of the carrier mobility. In this work the complexity due to the inclusion of this field-dependency is addressed and an analytical model has been developed by resolving this problem. The energy-bandgap-narrowing effects due to heavy doping is also considered in the proposed model. The developed model shows that the field-dependent mobility has significant effects on the base transit time of a heavily doped base under base pushout condition.