Semiconductors:

You have learned about the electrical conductors, which pass the current easily, and about insulators, which block the flow of current. A semiconductor can sometimes behave like a conductor, and at other times can behave like an insulator in same circuit.

The term semiconductor arises from ability of these materials to conduct part time. Their versatility is in the fact that the conductivity can be controlled to produce effects like amplification, oscillation, signal mixing, rectification, and switching