Prediction of fTand hfeat high collector currents

A simple model has been used to calculate the fTand hfefalloff in bipolar transistors operating under high-level injection conditions in the collector. The calculated values are in close agreement with the experimental data. Conventional calculations of fTfalloff ignore the effect of the collector capacitance charging time in the calculation of the total emitter-collector delay time [3]. However, it has been shown that, in Kirk´s base-widening model [1], the collector capacitance increases significantly in the quasi-saturation region due to a large increase in the stored charge in the collector [4], [11]. It will be shown here that earlier calculations of the fTbehavior have been inadequate and that an accurate prediction of the fTfalloff is possible by calculating the delay time (τ) associated with the total stored charge (Q) in the base and the collector regions using τ = dQ/dIc. It will be shown that the Kirk effect is indeed the dominant physical mechanism in the quasi-saturation region in our devices. This conclusion is consistent with Bowler and Lindholms analysis [5] and with the study of the collector capacitance behavior [4], [11].