Dictionary
Band Gap
Band gap is the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors.
Band gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, such as GaAlAs, InGaAs, and InAlAs. It is also possible to construct layered materials with alternating compositions by techniques like molecular beam epitaxy. These methods are exploited in the design of heterojunction bipolar transistors (HBTs) and laser diodes.
The distinction between semiconductors and insulators is a matter of convention. One approach is to consider semiconductors a type of insulator with a low band gap. Insulators with a higher band gap, usually greater than 3 eV, are not considered semiconductors and generally do not exhibit semiconductive behaviour under practical conditions. Mobility also plays a role in determining a material's informal classification.
Band gap decreases with increasing temperature, in a process related to thermal expansion. It also depends on pressure. Bandgaps can be either direct or indirect bandgaps, depending on the band structure.