Dielectric - Conduction currents

In this article, we're interested in the movement of charges over distances far greater than a few interatomic distances. In fact, the currents we'll be measuring are largely due to carriers which, injected from one of the electrodes, will penetrate the thickness of the material. It's important to remember, however, that a localized movement of a carrier will give rise to a measurable current in the external circuit, even if the carrier does not leave the dielectric.

Measuring conduction in a dielectric may, at first sight, appear paradoxical or pointless, since its main function is, in general, to provide insulation. However, what are commonly referred to as insulators are materials whose resistivity varies from a few 10 ⁸ Ω · cm up to 10 ¹⁸ Ω · cm or more. This variation of more than 10 decades strongly suggests that several conduction phenomena or mechanisms can generate currents.

In addition, from a component and system point of view, two aspects are extremely important:

• the use of very low-loss materials (e.g. capacitors);

• the case where the evolution of very low conduction currents is the precursor of a deterioration in the properties of the insulating material, which will eventually put an end to the life of the device.

It is therefore useful, and in some cases essential, to be able to measure, analyze and understand the origin of conduction currents that appear in insulators when they are intentionally or unintentionally subjected to an electric field. This need is reinforced by the fact that current developments in electrical construction equipment are tending towards higher power densities and operating temperatures. This leads to increased stresses, particularly electrical stresses, on insulating materials, and means that the easy solution of increasing insulation thickness is no longer an option. Instead, we are obliged to use dielectric materials that are closer to their performance limits.