Metals, Insulators and Semiconductors
A material is
able to conduct electricity, if it contains free electrons in the conduction
band when external electric field if applied to it. The free electrons thus
work as charge carriers.
Metals
Metals have a large
number of free electrons at room temperature. There is no gap between the
valance and conduction band. The conduction band and valence band overlap eachother
so valence band energies and conduction band energies are same. It is very easy
for valence electron to transit to conduction band and become free electron
even without supply of external energy. Metals have very low resistivity and
high conductivity.
ρ (Resistivity)
≈ 10-2 – 10-8 Ω
m
σ
(Conductivity) ≈ 102 – 109 Sm-1
Insulators
Insulators have
completely filled valence band and conduction band is empty. There is a large
energy gap (Eg > 5 eV) between valence and conduction bands due to which it
is practically impossible for an electron in the valence band to jump to
conduction band. Only at very high temperature or very high voltage, an
electron can jump the gap which is known as the breakdown of insulator. At room
temperature, no electron is present in conduction band. Insulators have very
high resistivity and low conductivity.
ρ
(Resistivity) ≈ 1011 – 1019
Ω m
σ
(Conductivity) ≈ 10-11 – 10-19 Sm-1
Semiconductors
Semiconductors have
properties intermediate to metals and insulators. There is a finite small
energy gap (Eg< 3 eV) between valence and conduction band. Because of the
small gap, at room temperature some electrons from valence band can acquire
enough energy to cross the energy gap and enter the conduction band. They have
resistivity and conductivity intermediate to metals and insulators.
ρ
(Resistivity) ≈ 10-5 – 106
Ω m
σ
(Conductivity) ≈ 105 – 10-6 Sm-1
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