The effect of carrier storage in the emitter on transistor input admittance

A variation with frequency of Cb¿e and gb¿e in the hybrid-¿ common-emitter equivalent circuit, which differs from that predicted by Giacoletto, is reported. It is found that the effect is present in a significant proportion of, but not in all, junction transistors. The phenomenon is explained in terms of minority-carrier storage in the emitter region or in terms of extra minority-carrier storage in the base, and experimental measurements which correlate well with theory are presented. It is pointed out, first, that Cb¿e at low frequencies may be more than double its value at the cut-off frequency, so that calculation of Cb¿e from the value of the cut-off frequency may be seriously in error, and secondly that, since emitter storage may be the dominant effect in determining cut-off frequency, it is expected that, in general, the cut-off frequency is less than 2D/w2.