Semiconductors are used in electric circuits. It is capable of conducting electricity and is an essential part of most electrical circuits.
A semiconductor has a conductivity between that of an insulator and that of most metals.
This is due to either the addition of an impurity or because of temperature effects.
The reason why they are important is because they are essential components in most electronic circuits, such as silicon.
Basically, a semiconductor can conduct electricity and its conductance can vary depending upon the current or voltage applied to a control electrode, or on the intensity of irradiation by infrared, visible light, ultraviolet, or X rays.