Materials with resistivities between those of good conductors and those of insulators are called:

Semiconductors

Explanation:

The resistivity of conductors is low and the resistivity of insulators is high. The materials having resistivity between conductors and insulators are called semiconductors.

For example, silicon, gemanium, etc.

The elements having valency + 4 are the semiconductors.

By increasing the temperature os a semiconductor, the conductivity increases or by the process of dopping the conductivity of semiconductor also increases.

semiconductors ... silicon, germanium etc ...