forbidden band

solid-state physics

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July 17, 2026

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In solid-state physics, a forbidden band is a range of electron energies that are not permitted within a solid material. This concept is central to band theory, which describes how electrons in solids can only occupy specific energy ranges, known as allowed bands.1

When atoms form a solid, their individual energy levels broaden into bands. The highest occupied band is the valence band, and the lowest empty band is the conduction band. The forbidden band, or energy gap, lies between these two bands.1

The size of the forbidden band determines a material's electrical properties:
* Metals have no forbidden bands in the energy range of their outermost electrons, allowing for easy electron movement and high conductivity.1
* Insulators have wide forbidden bands, requiring significant energy for electrons to cross, making them poor conductors.1
* Semiconductors have relatively narrow forbidden bands, allowing for intermediate conductivity that can be controlled, forming the basis of solid-state electronics.1

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band theory
…such allowed bands are called forbidden bands—i.e., electrons within the solid may not possess these energies. The band theory accounts for many of the electrical and thermal properties of solids and forms the basis of the technology of solid-state electronics.
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…such allowed bands are called forbidden bands—i.e., electrons within the solid may not possess these energies. The band theory accounts for many of the electrical and thermal properties of solids and forms the basis of the technology of solid-state electronics.
Read More