Editing Insulator (electricity) (section)

==Physics of conduction in solids==
Electrical insulation is the absence of [[electrical conduction]]. Electronic [[Electronic band structure|band theory]] (a branch of physics) explains that electric charge flows when quantum states of matter are available into which electrons can be excited. This allows electrons to gain energy and thereby move through a conductor, such as a [[metal]], if an electric potential difference is applied to the material. If no such states are available, the material is an insulator.

Most insulators have a large [[band gap]]. This occurs because the "valence" band containing the highest energy electrons is full, and a large energy gap separates this band from the next band above it. There is always some voltage (called the [[breakdown voltage]]) that gives electrons enough energy to be excited into this band. Once this voltage is exceeded, electrical breakdown occurs, and the material ceases being an insulator, passing charge. This is usually accompanied by physical or chemical changes that permanently degrade the material and its insulating properties.

When the electric field applied across an insulating substance exceeds in any location the threshold breakdown field for that substance, the insulator suddenly becomes a conductor, causing a large increase in current, an [[electric arc]] through the substance. Electrical breakdown occurs when the [[electric field]] in the material is strong enough to accelerate free [[charge carrier]]s (electrons and ions, which are always present at low concentrations) to a high enough velocity to knock electrons from atoms when they strike them, [[ion]]izing the atoms. These freed electrons and ions are in turn accelerated and strike other atoms, creating more charge carriers, in a [[chain reaction]]. Rapidly the insulator becomes filled with mobile charge carriers, and its [[Electrical resistance and conductance|resistance]] drops to a low level. In a solid, the breakdown voltage is proportional to the [[band gap]] energy. When [[corona discharge]] occurs, the air in a region around a high-voltage conductor can break down and ionise without a catastrophic increase in current. However, if the region of air breakdown extends to another conductor at a different voltage it creates a conductive path between them, and a large current flows through the air, creating an ''[[electric arc]]''. Even a vacuum can suffer a sort of breakdown, but in this case the breakdown or [[vacuum arc]] involves charges ejected from the surface of metal electrodes rather than produced by the vacuum itself.

In addition, all insulators become conductors at very high temperatures as the thermal energy of the valence electrons is sufficient to put them in the conduction band.<ref name="Kakani2005">{{cite book|author=S. L. Kakani|title=Electronics Theory and Applications|url=https://books.google.com/books?id=XrSI2C9NlDIC&pg=PA7|date=1 January 2005|publisher=New Age International|isbn=978-81-224-1536-0|pages=7}}</ref><ref name="Waygood2013">{{cite book|first=Adrian|last=Waygood|title=An Introduction to Electrical Science|url=https://books.google.com/books?id=8qHGRTC7h-MC&pg=PT41|date=19 June 2013|publisher=[[Routledge]]|isbn=978-1-135-07113-4|pages=41}}</ref>

In certain capacitors, shorts between electrodes formed due to dielectric breakdown can disappear when the applied electric field is reduced.<ref>{{cite journal|last1=Klein|first1=N.|last2=Gafni|first2=H.|title=The maximum dielectric strength of thin silicon oxide films.|journal=IEEE Trans. Electron Devices|date=1966|volume=13|issue=2|page=281|doi=10.1109/T-ED.1966.15681|bibcode=1966ITED...13..281K}}</ref><ref>{{cite journal|last1=Inuishi|first1=Y.|last2=Powers|first2=D.A.|title=Electric breakdown and conduction through Mylar films.|journal=J. Appl. Phys.|date=1957|volume=58|issue=9|doi=10.1063/1.1722899|bibcode=1957JAP....28.1017I|pages=1017–1022}}</ref><ref>{{cite journal|last1=Belkin|first1=A.|last2=et.|first2=al.|title=Recovery of Alumina Nanocapacitors after High Voltage Breakdown|journal=Scientific Reports|date=2017|volume=7|issue=1|page=932|doi=10.1038/s41598-017-01007-9|bibcode=2017NatSR...7..932B|pmc=5430567|pmid=28428625}}</ref>{{Relevance inline|date=October 2017}}