An electrical insulator resists the flow of electricity. Application of a voltage difference across a good insulator results in negligible electrical current. In comparison, a conductor allows current to flow readily. Controlling the flow of current in electrical wiring and electronic circuits requires both insulators and conductors. For example, wires typically consist of a current-carrying metallic core sheathed in an insulating coating.
Resistivity is the measure of a material's effectiveness in resisting current flow. Materials with resistivities higher than 108 ohm-m are usually considered to be good insulators; these include glass, rubber, and many plastics. Resistivities as high as 1016 ohm-m can be achieved in exceptional insulating materials. Normal conductors may have resistivities as low as 10-8 ohm-m. The enormous variation in room-temperature resistivity is one of the largest for any physical attribute of matter.
The charge carriers responsible for current in most conductors are electrons, moving relatively freely in a metal. In insulators, electrons cannot move freely. When atoms of simple metals combine to form a solid, the outer valence electrons become free for conduction. In an ideal insulator, all electrons stay tightly bound to the atoms, so there are no electrons that can be readily moved through the material for conduction.
A more complete understanding of insulators and conductors requires consideration of electronic band structure. The electrons in an isolated atom possess discrete energies, a consequence of quantum mechanics. These discrete levels evolve into bands of allowed energies when the atoms condense into a solid. Forbidden regions separate the allowed bands, as shown schematically in the figure. The electrons in the solid fill in the bands, from lower to higher energy.
The distinction between an insulator and a conductor lies in how the electrons fill in the allowed bands. For a simple metal, the highest band containing electrons will be only half full. The thermal energy (at ordinary temperatures) will be sufficient to generate conduction electrons -- electron states of slightly higher energy are available in the incompletely filled band.
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 gap between the valence and conduction band, known as the band gap, is at least several electron-volts (eV) wide in an insulator -- thermal electron energies are 100 times smaller.
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.
George H. Watson
University of Delaware
Last updated Oct. 31, 1996.
Copyright George Watson, Univ. of Delaware, 1996.