Analytical modeling of current gain-Earth voltage products in Si/Si_1-xGe_x/Si heterojunction bipolar transistors

The tradeoff between common emitter current gain beta and Early voltage V_A in heterojunction bipolar transistors (HBTs), where both bandgap and doping can vary in the base, has been studied. The Early voltage depends exponentially on the difference between the bandgap at the collector side of the base and the largest bandgap in the base, allowing very high Early voltages with only very thin narrow bandgap regions. Using Si/Si_1-xGe_x/Si narrow gap base HBTs with a two layer stepped base, beta V_A products of over 100000 V have been achieved for devices with a base width of only 400 AA. However, the presence of parasitic potential barriers for minority carriers at the base-collector junction of a HBT degrades V_A because changing the base-collector voltage affects not only the width but also the height of these barriers. They can be caused, e.g., by base dopant outdiffusion into the collector due to high-temperature processing. An analytical model of the effect of such barriers on the Early voltage is presented.