SiGe

Silicon-germanium (SiGe) is a silicon-germanium alloy that forms a continuous solid solution with a variable germanium content. It is commonly grown as epitaxial layers on silicon substrates to tailor electronic and optical properties for semiconductor devices.

The lattice constant of SiGe increases with the Ge fraction, roughly following Vegard’s law. Because Si and Ge have different lattice constants, SiGe layers can be strained when grown on Si, enabling strain engineering to modify carrier mobilities. The bandgap of SiGe also decreases as the Ge content rises, allowing tuning of electronic and optical behavior. The alloy remains primarily indirect in its band structure, which limits efficient light emission, but the adjustable bandgap and band offsets are valuable for device engineering. Carrier mobilities can benefit from strain, and SiGe can form heterostructures and quantum wells used in high-speed electronics and photonics.

Fabrication methods include chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Growth often employs graded buffer layers to accommodate lattice mismatch and reduce defect formation. Relaxed or pseudo-relaxed SiGe buffers enable thicker layers for device integration, while precise control of Ge content and strain is essential for performance.

Applications span high-speed transistors and integrated BiCMOS technologies, where SiGe bases in heterojunction bipolar transistors improve gain and frequency performance. In CMOS, strained SiGe channels and strain engineering can enhance hole mobility in p-type devices. SiGe is also used in photodetectors and optical modulators in silicon photonics, and in research toward silicon-compatible light sources, though direct-bandgap emission remains a challenge.