A SiGe HBT is similar to a conventional Si bipolar transistor except for the base.
SiGe, a material with narrower bandgap than Si, is used as the base material.
Ge composition is typically graded across the base to create
an acclerating electric field for minority carriers moving across
the base, typically 30-50 kV/cm, as schematically shown in the Figure below.
A direct result of the Ge grading in the base is higher speed, and thus higher
operating frequency. The transistor gain is also increased compared to a Si BJT, which
can then be traded for a lower base resistance, and hence lower noise.
For the same amount of operating current, SiGe HBT has a higher gain, lower RF noise,
and low 1/f noise than an identically constructed Si BJT. The higher raw speed can be
traded for lower power consumption as well.
