IGCSE
Physics Notes: Conducting Electricity – Metals, Semiconductors and
Insulators
Everybody knows that metals
conduct electricity. If you apply a voltage, you get a current. Most
elements are metals. However the are two other classes of materials –
semiconductors and insulators. Which class a material falls into
depends on how free the electrons are to move. In a metal many of the
electrons are completely free. In the diagrams below, the electrons
can be considered to occupied the green shaded regions, and the
regions in which electrons can have enough energy to be able to move
are coloured blue. This does not mean there are any electrons in the
blue region. For there to be electrons free to move the blue and
green regions have to overlap.
They do overlap for metals,
hence metals can conduct electricity – the typical resistivity of a
metal isThey
don't overlap for semiconductors. Some energy may be given to the
electrons so they can move from the valence band, where they normally
are, to the conduction band, where they may conduct electricity. This
energy may be in the form of heat or light. This implies that if you
heat a semiconductor, it's resistance falls, since it can conduct
electricity better. The typical resistivity of a semiconductor is
For
insulators there is a much larger energy gap between the conduction
and valence bands. The resistance of insulators is very high. A lot
of energy must be given to the electrons to push them into the
conduction band. The typical resistivity of an insulator is
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