Estimated effect of germanium and carbon on the Early voltage of a Si1-x-yGexCy heterojunction bipolar transistor

We have presented a generalized method of calculating the Early voltage VA of a Si/Si1-x-yGexCy/Si heterojunction bipolar transistor (HBT) and successfully explained the published measured values of VA with and without incorporation of C. The investigation was carried out on the dependence of V-A of an N-p-N HBT on different compositions, namely Ge and C, incorporated into the base and on their various distribution profiles such as box, trapezoidal and triangular, for an exponential base doping. It is found that the triangular compositional profile yields the largest value of V-A as compared to box-type and trapezoidal profiles for fixed amounts of individual composition in the base. The highest grading, which corresponds to a triangular profile, generates the highest value of effective intrinsic-carrier concentration local to the CB junction causing very little change in minority carrier concentration gradient in the base with V-BC. The small change in minority carrier concentration is responsible for the increased value of V-A. Furthermore, a large value of V-A is obtained for rising and falling exponential base doping profiles relative to the box doping profile with the same amount of base doping. The value of V-A is found to increase with the incorporation of small amounts of C in the SiGe base due to its ability to prevent boron out-diffusion as reported for a fabricated device. Further improvement in the value of V-A is accounted for due to the sharp fall-off of the C profile at the collector-base junction as observed from the SIMS profile of a fabricated device. For the same amount of strain between Si and SiGe, and between Si and SiGeC, the latter yields a larger value of V-A.