Prediction of fTand hfeat high collector currents

A simple model has been used to calculate the f
T
and h
fe
falloff 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 f
T
falloff 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 f
T
behavior have been inadequate and that an accurate prediction of the f
T
falloff 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/dI
c
. 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].