Base transit time of a Si/SiGe HBT considering gaussian doped base

Abstract

Base transit time of an HBT is more than 70% of its total transit time. Therefore it plays a very crucial role in determining the transistor’s high frequency performance. In the present work, base transit time of a Gaussian doped base Si1-yGey HBT with different type of Ge profiles (triangular, trapezoidal and box shape) for low and moderate levels of injection is studied. First of all, a model is developed for low injection only and then using perturbation theory, it is extended to cover moderate level of injection. In doing so, electric field and doping dependent mobility, bandgap narrowing effect due to heavy doping, presence of Ge and change in the density of states, velocity saturation at collector-base junction etc. are considered to study the injected minority carrier, collector current density and stored base charge. Base transit time is calculated numerically using MATLAB as the integrations are quite cumbersome to follow analytical derivations. The results show that base transit time increases with base-emitter voltage, minority carrier injection ratio, base width, peak base doping concentration and if velocity saturation and field dependent mobility is considered. On the other hand base transit time decreases with slope of base doping and gradient in Ge profiles. The results are compared with some available data from literature and published results.