Quantitative modelling of DC and transient characteristics at high injection level at 77 and 300K in silicon bipolar transistor

Abstract

The current gain of a silicon bipolar transistor has been quantitatively modelled by the Early effect, the conductivity modulation effects in the base and in emitter, the effective base widening effect and the emitter current crowding effect at 77 and 300K involved in the carrier recombination currents in the emitter-base space charge region and in the base. On this basis, the transit times, the total transit time from the emitter to collector and the cutoff frequency are modelled as functions of the collector current at 77 and 300K. The results obtained are in agreement with the experimental data. Besides this, the influence of the maximum base doping concentration on the maximum current gain and the maximum cutoff frequency has been also analysed. The main results show that the current gain is mainly determined by the conductivity modulation effect and the emitter current crowding effect at high injection level at 77K, but at 300K it is mainly determined by the effective base widening effect. The cutoff frequency is mainly determined by the minority-carrier base transit time at 300K, and at 77K the minority-carrier emitter transit time may be dominant for the large bandgap narrowing effect in emitter. The bandgap narrowing effect is the main reason for the degradation of the cutoff frequency with lowering temperature, instead of the low-temperature trapping effect by the compensated impurities with the shallow energy levels.