Analytical modeling of base transit time for a Si1−yGey heterojunction bipolar transistor

An analytical model for base transit time of an exponentially doped base npn Si
_1-y
Ge
_y
HBT has been developed. The model is valid in all levels of injection before the onset of Kirk effect. In this analysis, bandgap-narrowing effect, high-injection effect, carrier velocity saturation at the base edge of the base collector junction and doping dependent mobility are incorporated. It is found that base transit time depends on the Ge profiles in the base. The increase of Ge content for the same profile results in a decrease of transit time. Results of this work are compared with results available in literature.