Profile design considerations for minimizing base transit time in SiGe HBT's

A unified closed form analytical model for base transit time of SiGe HBT's for uniform and exponential base dopant distributions with different Ge profiles in the base (e.g., box, trapezoidal, triangular) is reported. The model is subsequently used to study the design of Ge profile for different base doping profiles, including that of epitaxial base transistors. Consistent with the reported results, our unified model predicts that beyond a certain total Ge content, there is very little reduction in ia.sice It is further demonstrated that the trapezoidal Ge profile with X-T similar to 0.8W(B) gives near optimal base transit time for all doping profiles considered. Our analysis shows that 1) for a given base width and intrinsic base resistance, the exponential base doping profile with Ge yields the least value of tau(b,SiGe) and 2) for a given peak base doping concentration and the intrinsic base resistance, the uniform base doping with Ge gives minimum tau(b,SiGe). Also, the need for keeping the total base Ge content constant while optimizing the Ge profile in the base is emphasized by showing that a false minimum for tau(b.SiGe) may appear if the total Ge content is not kept constant.