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Classifying materials
Materials can be placed into three groups according to their electrical resistance:
- conductors
- semiconductors
- insulators
Insulators and pure semiconductors have a very high resistance, while conductors have a very low resistance. This difference in conduction is explained by electron bands.
Band theory of conduction
Electrons orbit the positive nucleus of an individual atom in permitted energy levels, as shown on the left of the diagram below.
In a large collection of atoms such as a metal wire or a semiconductor crystal the energy levels become reorganised into two bands as shown on the right of the diagram.
Electrons can’t exist in the energy ‘gap’ between bands.
Conductors
In a conductor there are no band gaps and electrons can move easily using a continuous, partially filled conduction band.
Insulators
An insulator has a large gap between the lower energy levels (the valence band) and the upper conduction band.
The valence band is full as no electrons can move up the conduction band which is empty as a result.
The material can’t conduct as only the electrons in a conduction band can move easily.
Semiconductors
In a semiconductor, the gap between the valence band and conduction band is smaller and at room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band allowing some conduction to take place.
An increase in temperature increases the conductivity of a semiconductor.
The difference between insulators and semiconductors is due to a small amount of impurity added to a semiconductor which affects the energy bands. This process is called doping.
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