valence band
/ VAY-luns band /
When a crystal is left to cool down to the coldest temperature imaginable, its electrons settle into the lowest energies they can, the way water settles to the bottom of a glass. They fill the energy bands from the floor up. The highest band that ends up completely full at this deep freeze is the valence band — the top of the water level, so to speak.
The valence band is the highest range of electron energies that is fully occupied at absolute zero. Its electrons are the outer, bonding electrons of the atoms — the same valence electrons that form chemical bonds — now shared across the whole crystal. Because the band is completely full, every seat is taken, and a packed band cannot carry electric current: there is nowhere for an electron to move to.
The valence band matters because the story of conduction begins right at its top edge. To make current flow you either lift electrons out of it into the empty band above or leave gaps in it. A common mix-up is to assume a full valence band means a useless material — yet emptying just a few of its states, or having an empty band sit nearby, is exactly what turns an inert crystal into a semiconductor or a metal.
In pure silicon at low temperature, the valence band is brim-full and the conduction band above it is empty, so the crystal barely conducts. Warm it up, and a few electrons hop from the valence band into the conduction band, each leaving an empty seat behind — and suddenly the silicon starts to pass a current.
A full valence band carries no current until electrons leave it or empty seats appear.
Valence band electrons are the same outer electrons that chemists call valence electrons, but spread out across the crystal as shared band states rather than belonging to any one atom.