Amorphous silicon

Amorphous silicon is silicon whose atoms are jumbled rather than lined up in a neat crystal. That makes it a poorer conductor than the crystal silicon in computer chips, but it has one huge advantage: it can be laid down as a thin, even film over glass meters across, at temperatures the glass can survive.

It's made by plasma-enhanced chemical vapor deposition. Silane gas, often mixed with hydrogen, flows into a vacuum chamber over glass heated to around 300 °C. An electric field turns the gas into a glowing plasma that breaks the silane apart, and silicon atoms settle on the glass as a film a fraction of a micrometer thick. In a jumbled film, many silicon atoms are left with a broken bond, which traps electrons. Hydrogen atoms cap those broken bonds, so the material, properly called hydrogenated amorphous silicon (a-Si:H), works as a useful semiconductor.

In a thin-film transistor, the film is the channel that switches between conducting and not. Electrons move through it about a thousand times more slowly than through crystal silicon, which is still fast enough to refresh a TV picture. The same material is used in thin-film solar cells, like the small ones on calculators.