Bandgap

Notes

Silicon 1.12 eV, GaN 3.4 eV, 4H-SiC 3.26 eV, beta-gallium oxide 4.8 eV and diamond 5.5 eV.

Overview

Bandgap is the energy difference between the top of a semiconductor's valence band, where electrons are bound, and the bottom of its conduction band, where they are free to carry current, usually given in electronvolts (eV). It is the minimum energy an electron must gain, from heat or light, before it can conduct, so it shapes how a material copes with temperature and which light it absorbs or emits. Silicon's bandgap is about 1.1 eV, while gallium nitride's is about 3.4 eV, roughly three times larger.

How it works

When an electron gains at least the bandgap energy it jumps into the conduction band and leaves behind a hole in the valence band, and both can carry current. Heat does this continuously, so a narrow-gap material generates more unwanted carriers as it warms, while a wide-gap one can work at higher temperature. The gap itself shrinks slightly as temperature rises.

Light interacts through the same gap. A photon with less energy than the bandgap passes through; one with more can be absorbed and create an electron-hole pair, which is how photodiodes and solar cells work. In a direct-gap material, such as gallium arsenide, gallium nitride or indium phosphide, an electron can drop back across the gap and emit a photon efficiently, which makes these materials suitable for LEDs and lasers, with the colour set by the gap. Silicon has an indirect gap, so it emits light poorly.

The gap can be tuned. Alloys of compound semiconductors, such as indium gallium arsenide, have gaps between those of their parent materials, which lets device makers choose wavelengths for lasers and detectors. Very wide gaps shade into insulators, and where a semiconductor ends and an insulator begins is a matter of convention rather than a fixed value.

Examples

• Silicon, with an indirect gap of about 1.12 eV, which dominates integrated circuits and solar cells.
• Gallium nitride, at about 3.4 eV, used in blue LEDs and lasers and in power transistors for roughly 30 to 600 V applications.
• Gallium arsenide, with a direct gap of about 1.42 eV, used for near-infrared LEDs and fast electronics.
• Indium phosphide epitaxial wafers for lasers and photodetectors in optical fibre networks.

Why it matters

Bandgap is one of the first numbers device physicists, materials researchers and power device engineers look at, because it frames what a material can do: wide-bandgap gallium nitride is used for power switching as well as RF, and direct-gap compounds suit light emission. Photonics and image sensor engineers rely on it to know which wavelengths a material will detect.

Related terms

• WBG
• Critical breakdown field
• SiC
• GaN
• Ga2O3
• Semiconductor

Where it appears on the Hub

Used on 16 pages of the Hub. In each group, the pages that mention it most come first.