In solid-state physics, materials are classified based on their electrical conductivity as conductors, semiconductors, and insulators. 

Conductors are materials that allow the flow of electric charge easily. They have high electrical conductivity and low resistance. Metals are examples of conductors.

Semiconductors have a moderate electrical conductivity that can be changed by adding impurities (doping). They have a band gap between the valence band and the conduction band that is smaller than insulators. This band gap is so small that even at room temperature, some electrons can jump from the valence band to the conduction band. Semiconductors are used in many electronic devices such as transistors, diodes, and solar cells. Silicon and germanium are examples of semiconductors.

Insulators have very low electrical conductivity and high resistance. They have a wide band gap between the valence band and the conduction band, which means that electrons are not able to move easily from the valence band to the conduction band. Examples of insulators include rubber, glass, and plastics.