Materials having resistivity or conductivity in between conductors and insulators, are called semiconductors.
These are the materials with the forbidden energy gap between valence and conduction band
<3eV. It is less than that of insulators
(>3eV) and more than that of conductors.
e.g.
Elemental semiconductor: Germanium, silicon, selenium, etc.
Compound semiconductors: Gallium, arsenide, Indium, phosphide, cadmium, sulphide etc.