A SiGe heterojunction bipolar transistor uses a silicon-germanium alloy layer as the active base material. The introduction of Ge causes a reduction of the bandgap and thus an increase in the intrinsic carrier concentration, and at the same time creates a valence band offset which impedes the flow of holes into the emitter. SiGe-HBTs fabricated using MBE in 1987 exhibited a remarkable enhancement of current gain. There have been many technological developments and innovations since then. Some recent developments, such as IBM epi-base technology, SiGe-BiCMOS technology and SiGe-HBTs on SOI, are briefly described in this chapter. DC performance of SiGE-HBTs and their behavior at low temperature are presented. Incorporation of carbon into the SiGe alloy enables attainment of many improvements in performance. This is discussed. Special techniques and processes need to be used for fabricating HBTs for high voltage and high power applications. IC processes for incorporation of SiGe-HBTs in a standard Si IC receive special attention.

RF applications are known to be the special niche for SiGe-HBTs because of their excellent high frequency performance. For application in rf circuits, several analog characteristics become important. The cutoff frequency and maximum frequency of oscillation are important figures to specify the high frequency capability. It is known that the f _T value must be 10 times higher than the operating frequency. The noise figure is another factor for rf circuit design especially for low noise amplifier applications. Flicker noise or 1/ f noise is important as it results in...