Comparison of methods to calculate capacitances and cutoff frequencies from DC and AC simulations on bipolar devices

The question of whether or not the convenient stored-charge (SC) method is appropriate to calculate small-signal characteristics of bipolar devices is considered. A general definition of the total SC capacitance of a semiconductor device, and an expression for the difference with the AC capacitance are presented. The SC method is found to yield correct capacitance for practical p-n junction diodes at moderate and high forward bias. The discrepancy is attributed to the diffusive delay of minorities and is seen to be enhanced by ohmic fields. Nevertheless, the SC method accurately predicts the common-emitter input capacitance of a bipolar transistor with a high current gain and a low base resistance, in spite of its forward-biased emitter-base junction. The various methods proposed in the literature to derive the cutoff frequency from AC small-signal parameters are found to disagree with the SC method as well as with each other if the current gain is lower than about five. Especially at low current gain, Gummel´s cutoff frequency is found to be preferable, both for simulations and for measurements