Semiconductor materials are nominally small band gap insulators.
They have a resistance-higher than typical resistance materials but still of much lower resistance than insulators. Their resistance decreases as their temperature increases, opposite that of a metal.
The defining property of a semiconductor material is that it can be "doped" with impurities that alter its electronic properties in a controllable way.
Materials used in semiconductors include: Si, Ge, Sn, SiC, Te, BN, AlN, GaN, CdTe, InN, InAs, CdSe, CdS, ZnO, ZnSe, ZnS, ZnTe and numerous other materials and alloys
Source: Wikipedia; List of Semiconductor Materials, April 4th, 2014 (http://en.wikipedia.org/wiki/List_of_semiconductor_materials)