ABSTRACT
The essential differences between the SiGe HBT and the Si BJT are best illustrated by considering a
schematic energy band diagram. For simplicity, we consider an ideal, graded-base SiGe HBT with
constant doping in the emitter, base, and collector regions. In such a device construction, the Ge
content is linearly graded from 0% near the metallurgical emitter-base (EB) junction to some maximum
value of Ge content near the metallurgical collector-base (CB) junction, and then rapidly ramped back
down to 0% Ge. The resultant overlaid energy band diagrams for both the SiGe HBT and the Si BJT,
biased identically in forward-active mode, are shown in Figure 4.1. Observe in Figure 4.1 that a Ge-
induced reduction in base bandgap occurs at the EB edge of the quasi-neutral base (DEg,Ge (x ¼ 0)), and at the CB edge of the quasi-neutral base (DEg,Ge (x ¼ Wb)). This grading of the Ge across the neutral base induces a built-in quasi-drift field ((DEg,Ge (x ¼ Wb) DEg,Ge(x ¼ 0))/Wb) in the neutral base. In this chapter, we examine the impact of Ge on the dc and ac properties of the transistor-the essential
devices physics of the SiGe HBT.