Insulator

An insulator is a material where the energy gap between the valence and conduction bands is too great to allow the valence electrons to move to the conduction band. The valence band is filled entirely and there are no electrons in the conduction band so no conduction of mobile electrons is permitted.

References

Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
Roulston. David. J., An Introduction to the Physics of Semiconductor Devices. Oxford University Press, 1999.