A process called epitaxy (EPI) is used to grow a layer of single crystal silicon from vapor onto a single crystal silicon substrate at high temperatures. Trichlorosilane or silicon tetrachloride and hydrogen are combined with either diborane or phosphine gas to act as dopants.

The purpose of EPI growth is to create a layer with different, usually lower, concentration of electrically active dopant on the substrate. For example, an n-type layer on a p-type wafer.