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Dielectric breakdown

At high electric fields, a material that is normally an electrical insulator may begin to conduct electricity – i.e. it ceases to act as a dielectric. This phenomenon is known as dielectric breakdown.

The mechanism behind dielectric breakdown can best be understood using band theory. A detailed explanation of this can be found in the TLP on semiconductors although not all of this is relevant to the content of this TLP, therefore the aspects of band theory needed to understand dielectric breakdown are presented here.

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For each material, there is a characteristic field strength needed to cause dielectric breakdown. This is referred to as the breakdown field or dielectric strength. Typically values of the dielectric strength lie in the range 106 – 109 Vm-1. The exact value of the dielectric strength depends on many factors – most obviously the size of the energy gap, but also the geometry and microstructure of the sample and the conditions it is subjected to.

The phenomenon of dielectric breakdown is utilised in cigarette lighters and similar devices where a spark must be produced in order to ignite the fuel. The “spark gap” is a small air gap between two electrodes. Charge is built up on the electrodes on either side of the spark gap until the strength of the field across the spark gap exceeds the dielectric strength of air (the mechanism used to create this field is not directly relevant to this TLP, but interested readers may find an explanation of it here). At this point the air within the spark gap becomes capable of conduction, resulting in a spark.

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