How do energy bands explain electrical conduction in semiconductors?

In solid-state physics, band theory explains electrical conduction by describing how electrons occupy specific energy ranges within a crystal lattice.

Energy Bands and the Band Gap

When atoms form a crystal, their discrete energy levels spread into energy bands. The highest energy band normally occupied by electrons is the valence band, while the next higher, typically empty band is the conduction band. These bands are separated by a band gap—a forbidden energy region that electrons cannot occupy.123

Mechanism of Conduction

In semiconductors, the band gap is relatively narrow (typically 0.25 to 2.5 eV). At low temperatures, electrons are bound in the valence band and cannot conduct electricity. However, at higher temperatures, thermal energy can promote electrons across the gap into the conduction band. This process leaves behind a hole in the valence band.142

When an electric field is applied, both the free electrons in the conduction band and the holes in the valence band act as charge carriers, moving through the material to produce an electric current. Conductivity can be further enhanced through doping, which introduces impurities to increase the concentration of either free electrons (n-type) or holes (p-type).142