bandalignments

Band alignment refers to the relative positions of the conduction and valence bands at the interface between two semiconducting materials. The alignment determines how electrons and holes distribute themselves across the interface and governs carrier confinement, transport, and recombination in heterostructures such as quantum wells and multilayer devices.

There are three canonical types of band alignment. Type I, or straddling gap, occurs when the bandgap of one material lies within the bandgap of the other, confining both electrons and holes in the same region. Type II, or staggered gap, has the conduction-band minimum and valence-band maximum located in different materials, promoting spatial separation of electrons and holes. Type III, or broken gap, features an overlap where the conduction band of one material lies below the valence band of the other, leading to unique carrier dynamics. Real interfaces may exhibit intermediate or mixed characteristics depending on material quality and strain.

Band offsets, typically denoted as the conduction-band offset ΔEc and the valence-band offset ΔEv, quantify how the band edges shift across the interface. Their values influence carrier confinement and transfer and often satisfy ΔEc + ΔEv ≈ Eg1 − Eg2. Band offsets can be estimated or measured using techniques such as X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and internal photoemission, complemented by theoretical models and first-principles calculations.

Applications span optoelectronics and photovoltaics, including quantum wells, light-emitting diodes, lasers, detectors, and high-electron-mobility transistors. Common material systems include GaAs/AlGaAs, InAs/GaSb, Si/SiGe, and, in the 2D regime, van der Waals heterostructures such as MoS2/WSe2. Real-world behavior is influenced by interface quality, defects, strain, and interdiffusion, which can modify the effective alignment.