Doping is the process of adding a small impurity to a pure semiconductor to change its electrical properties. Lightly and moderately doped semiconductors are called extrinsic. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as degenerate. Most semiconductors are made out of silicon crystals. Pure silicon has little use but doped silicon is the basis for most semiconductors. Silicon Valley was named for the large number of semiconductor startup companies that were located there.