Silicongermanium

Silicongermanium, commonly referred to as silicon–germanium or SiGe, is a solid solution alloy of silicon (Si) and germanium (Ge) with variable composition described by Si1−xGex, where 0 ≤ x ≤ 1. The alloy maintains the diamond cubic crystal structure of the parent group-IV elements and can be grown on silicon substrates. Lattice spacing increases with Ge content, enabling strain engineering in silicon devices when SiGe layers are grown pseudomorphically on Si or used as relaxed buffers.

SiGe is typically deposited using chemical vapor deposition (CVD) or molecular beam epitaxy (MBE), often with ultra-high vacuum conditions. Strained SiGe layers can modify carrier mobilities; tensile strain in the silicon channel generally enhances electron mobility, improving transistor performance. The alloy also provides a tunable bandgap, decreasing from silicon’s ~1.12 eV toward germanium’s ~0.66 eV as Ge content rises, which allows bandgap engineering and formation of SiGe heterostructures with engineered band offsets.

Applications include high-speed CMOS and heterojunction bipolar transistors (HBTs) that leverage strain and bandgap engineering to boost performance. SiGe is also employed in silicon photonics and infrared detectors, where higher Ge content enables detection in the near- to mid-infrared range. SiGe buffers are used to integrate optoelectronic components on silicon chips, supporting photonics–electronics integration.