a semiconductor

A semiconductor is a material caught between the two extremes of conduction, neither a good metal nor a true insulator, and, crucially, one whose ability to carry current can be swung by enormous factors with temperature, light, or a whisper of impurity. That tunability, not any fixed conductivity, is what makes silicon and its cousins the substance the entire digital age is built from.

The defining feature is a modest band gap, roughly 0.1 to 3 eV, between a full valence band and an empty conduction band. At absolute zero the valence band is completely filled and the material is a perfect insulator. Warm it, and a few electrons are thermally excited across the gap into the conduction band, each leaving behind a hole, a missing electron in the valence band that behaves as a mobile positive charge carrier of charge +e. The intrinsic carrier concentration, and hence the conductivity, rises steeply with temperature as exp(-E_g / 2 k_B T), the opposite of a metal, whose conductivity falls as it warms. Both electrons and holes carry current, and the material can be intrinsic (pure) or extrinsic (deliberately doped).

Semiconductors such as silicon, germanium, and gallium arsenide are the raw material of transistors, diodes, solar cells, LEDs, and lasers, and hence of essentially all modern electronics. The honest caveat is that a pure semiconductor at room temperature has a tiny intrinsic carrier density and is a rather poor conductor; every practical device relies on doping to install the carriers in controlled numbers and of a chosen sign.