What is silicon germanium (SiGe)?
SiGe is a semiconductor alloy, meaning a mixture of two elements, silicon and germanium. Since 2000 or so, SiGe has become widely used to enhance the performance of ICs of various types. SiGe can be processed on equipment nearly the same as used for ordinary silicon. SiGe doesn't have some of the drawbacks of III-V compound semiconductors like gallium arsenide (GaAs), for example it doesn't lack a native oxide (important for forming MOS structures) and doesn't suffer from mechanical fragility that limits the wafer size of GaAs. This results in costs that are only a small multiple of ordinary silicon, and so much lower than competing technologies like GaAs.

SiGe allows two main improvements compared to ordinary silicon:

First, adding germanium increases the lattice constant of the alloy. If a layer of Si is grown on top of SiGe, there will be mechanical strain induced by the lattice constant mismatch. The strained layer will have higher carrier mobility than unstrained Si. This can be used, for example, to balance the performance of PMOS and NMOS transistors, reducing the area needed for a given CMOS circuit.

Second, the SiGe alloy can be used selectively in the base region of a BJT to form a heterojunction bipolar transistor (HBT). SiGe HBT's have been demonstrated with speeds (fT) to 500 GHz, and are commercially available with fT up to 240 GHz. The SiGe HBT also has lower noise than a standard silicon BJT.