The minimum energy required to excite a system from its ground state.The energy gap is the minimum energy required to excite a system from its ground state, the lowest-energy configuration it can occupy. In physics, this concept is most familiar in semiconductors and insulators, where the energy gap (or band gap) separates the filled valence band from the empty conduction band. It determines whether a material conducts electricity, acts as an insulator, or behaves as a semiconductor, and it also governs phenomena like the color of materials and the operation of transistors and solar cells.
The concept of the energy gap was developed in the early 20th century with the advent of quantum mechanics. The band theory of solids, which explains the gap, was formulated by Felix Bloch and others in the late 1920s, and it provided the theoretical foundation for the semiconductor revolution.
In a crystal, electrons occupy energy bands. The energy gap is the forbidden zone between these bands where no electron states exist. To conduct electricity, an electron must jump across this gap, requiring an input of energy at least equal to the gap size. If the gap is large, the material is an insulator; if it is small, it is a semiconductor; if there is no gap, it is a metal. The gap can be measured by shining light on the material and observing which wavelengths are absorbed.
It is the principle behind all modern electronics: transistors, diodes, and solar cells rely on the controlled excitation of electrons across the energy gap. The size of the gap also determines the color of LEDs and the efficiency of photovoltaic cells.
The energy gap of a semiconductor is not fixed; it can be tuned by changing the material's temperature, pressure, or by adding impurities, which is how engineers design LEDs to emit specific colors.
| E | variable | quasiparticle energy — energy joule |
| ε | variable | single-particle energy relative to chemical potential — energy joule |
| μ | variable | chemical potential — chemical potential joule |
| Δ | variable | superconducting gap parameter — energy gap joule |
