allowed band

In solid-state physics, band theory describes how electrons in solid materials can only have specific energy ranges, known as allowed bands. These are separated by forbidden bands where electrons cannot exist. This theory explains the electrical and thermal properties of solids and is fundamental to solid-state electronics.

When atoms form a solid, their individual electron energy levels broaden into bands. The valence band contains electrons involved in bonding, while the conduction band is typically empty. Electrons can move between these bands, influencing a material's conductivity.

Materials are classified based on their band structure:
* Metals have no forbidden bands in the energy range of their outermost electrons, allowing for good electrical conductivity.
* Insulators have wide forbidden energy gaps, preventing free electron movement and resulting in poor conductivity.
* Semiconductors have narrower forbidden gaps, allowing for intermediate conductivity.12