silicon germanium (sige)

Silicon germanium (SiGe) is an alloy of silicon and germanium used extensively in semiconductor manufacturing for strain engineering, high-performance transistors, and heterojunction devices. Properties: tunable bandgap (Si: 1.12eV, Ge: 0.66eV, SiGe varies between), higher hole mobility than Si, lattice constant larger than Si (creates strain when grown on Si). Applications in logic: (1) Embedded SiGe S/D—epitaxially grown in source/drain cavities to create compressive channel strain for PMOS (standard since 90nm); (2) SiGe channel—replace Si channel with SiGe for higher PMOS hole mobility; (3) SiGe sacrificial layer—in GAA nanosheet fabrication, alternating Si/SiGe layers grown epitaxially, SiGe selectively removed to release Si channels; (4) SiGe virtual substrate—relaxed SiGe buffer for biaxial tensile strained Si. Typical compositions: 20-35% Ge for S/D stressors, 25-30% Ge for sacrificial layers, 15-25% Ge for channel material. Growth: selective epitaxy in S/D cavities using SiH₄/GeH₄ precursors at 500-700°C. SiGe in BiCMOS: SiGe heterojunction bipolar transistor (HBT) with graded Ge base—enables >500 GHz fT for RF/mmWave applications. Challenges: defect-free growth (misfit dislocations from lattice mismatch), Ge interdiffusion during thermal processing, selective etch chemistry (HCl-based for SiGe vs. Si selectivity). Critical material enabling strain engineering, GAA transistor fabrication, and high-frequency applications across modern semiconductor technology.