In comparison, the highest energy band containing electrons is completely full in a good insulator. The thermal energy of the electrons is not sufficient for promotion from this band, known as the valence band, to the next band with available energy states, known as the conduction band. The distinction between an insulator and a semiconductor is one of degree. Although both have completely filled valence bands at 0 K, the band gap of a semiconductor is smaller than an insulator. For narrower band gaps, thermal energy is more capable of promoting electrons into the conduction band.
Insulators play a critical role in many aspects of technology, from large scale to the microscopic.
Electrical power transfer relies on high voltage transmssion lines for which insulators are required to prevent losses to ground. High voltage transformers rely on special insulating oils. Dielectric materials enhance the charge storage of a capacitor. Even bits of information in a computer memory require thin insulators in the form of oxide layers in increasingly smaller transistor circuits. Kittel, C. 1986 Introduction to Solid State Physics , 6th ed.