Trichlorosilane

Trichlorosilane is a colorless liquid made of one silicon atom bonded to one hydrogen atom and three chlorine atoms, HSiCl₃. It boils at about 32 °C, just above room temperature, and it fumes and reacts in moist air. Its whole reason to exist is purification: it carries silicon in a form that can be cleaned far better than any solid.

It's made by blowing hydrogen chloride gas through a hot bed of powdered metallurgical-grade silicon at around 300 °C. The silicon reacts to form trichlorosilane and hydrogen gas, along with some silicon tetrachloride. Impurities in the raw silicon, such as iron, aluminum and boron, form chlorides of their own. The mixture is then distilled again and again in tall columns. Because each chloride boils at its own temperature, the impurities can be separated until the remaining contaminants are measured in parts per billion or less.

Pure trichlorosilane is fed with hydrogen into a reactor where it breaks down on hot rods, depositing polysilicon. It can also be rearranged over a catalyst into silane and silicon tetrachloride. Silane is used to make the amorphous silicon and silicon nitride films in a TV's thin-film transistors.