Semiconductors are substances that conduct electricity only under certain conditions. Germanium, Silicon and sometimes tin are types of semiconductors that occur naturally and do not require chemical doping to form crystal lattice structures.
Semiconductors' electrical conductivity increases with increasing temperatures. There are two basic classification of semiconductors, intrinsic and extrinsic semiconductors. Intrinsic semiconductors are group 14 elements that are able to form crystal lattice structures composed of atoms of that element, and are naturally occurring. Extrinsic semiconductors are not naturally occurring, they are artificially doped. This means they have impurities added to create additional electrons to the lattice so that they allow increased conduction.