heterojunctions

Heterojunctions are interfaces between two semiconductor materials with different band structures. At the junction, the conduction and valence bands are discontinuous, creating band offsets that affect carrier confinement and transport. They enable band structure engineering not possible in a single material and lie at the heart of many electronic and optoelectronic devices.

Band alignment is classified as type I (straddling), type II (staggered), or type III (broken gap). Type I confines both electrons and holes in one material; type II spatially separates them; type III involves overlapping bands. The offsets ΔEc and ΔEv govern confinement and device behavior.

Offsets arise from electron affinities, ionization energies, and interfacial dipoles. Anderson’s rule offers a starting point, but real interfaces show effects from lattice mismatch, defects, and polarization, especially in nitrides and oxides. Strain from mismatch can modify band edges and, if large, create dislocations. Common systems include GaAs/AlGaAs, InP/InGaP, Si/SiGe, and III-nitride heterostructures.

Confinement in heterojunctions creates quantum wells and superlattices, boosts mobility, and enables efficient radiative recombination or tunneling. Applications include high-electron-mobility transistors, heterojunction bipolar transistors, LEDs, laser diodes, and multijunction solar cells. Transport mechanisms include thermionic emission, tunneling, and thermally assisted processes.

Fabrication relies on epitaxial growth methods such as molecular beam epitaxy and metal-organic chemical vapor deposition, producing abrupt or graded interfaces. Lattice matching and strain management are key considerations; characterization uses X-ray diffraction, transmission electron microscopy, and optical spectroscopy to assess band offsets and interface quality.

Heterojunctions continue to enable advances in electronics and photonics, including devices with engineered band structures and quantum confinement, as well as integration with low-dimensional materials.