A semiconductor is a solid material with electric properties in between those of conductors and insulators.
Semiconductors can be elementary crystals like silicon or germanium or compounds like gallium arsenide. These semiconductors receive particular relevance because they are insulators in pure form but receive conductivity if the crystal grid is doped with a very small amount of atoms of a different element. These atoms should have one valence electron more than the semiconductor (donators) or less (reciptors).
Semiconductors can receive surface structures by anisotropic plasma etching (RIE) and fotolithographic processes.