Effects of doping and Ge fraction grading in the base on the base transit time and Early voltage of Si/SiGe HBTs

The base transit time /spl tau//sub b/ and the Early voltage V/sub A/ for Si/SiGe HBTs with gradients of Ge fraction and doping in the base are studied. It is found that the retarding field, caused by doping grading exhibits 40%-80% contribution to /spl tau//sub b/ depending on the doping level near the emitter, V/sub A/ increases quasi-exponentially and /spl tau//sub b/ decreases with the increase in gradient of Ge fraction in the base.