energiband

An energy band, or energy band structure, in solid-state physics, refers to the allowed ranges of energy that electrons can have while moving in a periodic lattice potential. In a crystal, the atomic orbitals overlap and, through Bloch’s theorem, discrete energy levels broaden into bands separated by band gaps where no electron states exist. The highest partially filled band at low temperature is the valence band, and the next higher unoccupied band is the conduction band.

Electrical conductivity is determined by how these bands are filled. In metals, bands overlap or are partially filled, allowing free charge transport. In semiconductors and insulators, a band gap separates valence and conduction bands; at room temperature most electrons remain in the valence band, and charge carriers must be thermally excited across the gap or introduced by doping to enable conduction.

Band structure can be described by several models, such as the nearly free electron and tight-binding approaches, and is often computed from first-principles methods like density functional theory. The nature of the gap matters: direct gaps, where the band extremum occurs at the same crystal momentum, favor optically active transitions, while indirect gaps involve momentum changes. Experimental probes include angle-resolved photoemission spectroscopy and optical absorption measurements.

Common semiconductors illustrate the concept: silicon has an indirect gap of about 1.1–1.2 eV, while gallium arsenide has a direct gap around 1.42 eV. Temperature, pressure, and strain can modify band gaps and carrier effective masses. The band-structure picture underpins the operation of electronic devices such as diodes and transistors, as well as optoelectronic technologies like LEDs and solar cells.