An essentially noncrystalline form of silicon. Amorphous silicon has no long-range order of the silicon atoms in the solid, unlike a crystal. Very short-range order, up to about 30 nanometres, can be obtained. The material does not have an energy band structure as such, but can be described internally as having ranges of electron energies of relatively high mobility separated by a mobility gap. Thus doped amorphous silicon can be produced in which the majority charge carriers can be defined, and hence electronic devices can be made in amorphous silicon. The carrier mobilities are generally very low, so the speed of response of the devices is limited. It is deposited as a thin film using low-temperature or plasma-enhanced chemical vapour deposition techniques, and can be deposited very uniformly over very large areas using these techniques.

Amorphous silicon is used for making thin-film transistors, which are employed as driver devices in large-area flat-panel display technology; it can also be used for making solar cells over large areas, for high electrical output though at low efficiency.