Silicon, in its pure form, is:

In its pure, or "intrinsic," form, silicon forms a tightly bound crystalline structure where all of its outer valence electrons are locked into covalent bonds with neighboring atoms. Because all the electrons are tied up in these bonds, there are practically no free electrons or holes available to carry an electrical current at room temperature. As a result, pure silicon behaves as an insulator. Silicon only becomes a useful semiconductor when specific impurities are added (a process called doping) to create free charge carriers, or when sufficient external energy is applied to break the bonds.