heterojunction

A heterojunction is the interface between two semiconductor materials with different band structures. Unlike a homojunction, which joins the same material on both sides, a heterojunction combines dissimilar semiconductors with different band gaps and electron affinities, leading to discontinuities in the conduction and valence bands called band offsets. These offsets shape how electrons and holes are injected, confined, and transported across the interface.

Band alignment can be classified as type I (straddling gap), type II (staggered gap), or type III (broken gap). In type I, both carriers are confined in the same material; in type II, electrons and holes are localized in different materials, aiding charge separation. Type III features a broken gap with a more complex carrier spectrum. Lattice mismatch between materials can induce strain and defects but also enables strain engineering to tailor electronic and optical properties.

Heterojunctions are typically formed by epitaxial growth methods such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), or chemical vapor deposition (CVD). Common material systems include GaAs/AlGaAs, InP/InGaAs, Si/SiGe, and GaN/AlGaN, among others. Precise control of layer thickness and composition is essential to achieve desired band offsets and quantum confinement.

Applications include high-speed and high-efficiency devices such as heterojunction bipolar transistors (HBTs), high-electron-mobility transistors (HEMTs), quantum well lasers, light-emitting diodes, and advanced solar cells and photodetectors. The ability to engineer band alignment and confinement makes heterojunctions central to modern electronics and optoelectronics, while challenges include managing lattice mismatch and interface quality.