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. Van Nostrand Reinhold. Tipler, P. A. 1991 Solids. In Physics for Scientists and Engineers , 3rd ed.