Polysilicon-emitter SiGe-base bipolar transistors-what happens when Ge gets into the emitter?

An analytical model is developed for evaluating the effect of emitter depth variation on base and collector currents in a polysilicon-emitter SiGe-base bipolar transistor. It is shown that the base current is relatively insensitive to the Ge distribution in the single-crystalline emitter region, consistent with reported experimental results. On the other hand, the collector current and Early voltage are functions of the Ge distribution, Ge-free cap thickness, and final emitter depth. In particular, the current gain and Early voltage could have a strong dependence on emitter depth when there is a residual Ge-free layer left in the final quasi-neutral base. However, if the emitter-base junction is located in a constant-Ge region, then large current gains can be achieved that are relatively insensitive to emitter-depth variation, consistent with reported results. The inverse relationship between current gain and Early voltage is noted and contrasted with that of a wide-gap-emitter HBT.