Dielectric Materials for Electrical Engineering

Chapter 2

Physics of Charged Dielectrics: Mobility and Charge Trapping 1

2.1. Introduction

For the lay person, an insulating material is a material which does not conduct electricity. In fact, however, it is necessary to analyse this assertion more closely.

The conductivity of a material is defined by:

[2.1]

where n is the density of the charge carriers and μ the mobility.

Table 2.1 gives the values, at room temperature, of the conductivity and mobility of the electrons in high purity copper and alumina (aluminum oxide). We note that there are 23 regions between the two conductivities, but the mobility is twice as high in alumina as in copper. These values, which could be surprising, are due to the fact that, from [2.1], alumina contains much less than an electron per cm3 which is used for conduction while copper has about 4.5 × 1022 electrons per cm3 used for conduction (Table 2.1).

Alumina has, therefore, a very weak conductivity because it has practically no free charges to conduct (and not because it is inept at conducting charges). For a pure material with a large band gap, we have, then, an insulating state, through lack of charge carriers.

For conduction to be observed in an insulator, we need to inject charges into it. Unlike a metal, the bulk of which remains neutral during conduction, an insulating material is thus charged. The injection of charges can be done in many ...