Electronic devices with hyperpure silicon surfaces with no metallic impurities or oxygen, and an extremely perfect crystal structure are manufactured using a special deposition method, namely epitaxy (a Greek term meaning "superimposing")

This is carried out in the gas phase, using ultrapure, particle-free chlorosilanes such as trichlorosilane or silicon tetrachloride, to deposit silicon layers of a few microns thickness on a polished silicon wafer. The high purity of the chlorosilanes is also a prerequisite for a controlled and reproducible adjustment of the resistivity of the epitaxial layers.