An electric
insulator is a material or object which resists the flow of electrical charges
through it. They lack movable electrical charges. This is because the electrons
in the material including the outermost one are strongly bound to the atoms. When an
electrical potential difference is applied across an electrical insulator, no
free charges are exposed to the electric field so there will be no flow of
charges and an electric current cannot arise. Ideally, an electric insulator
has infinite resistance and zero conductance.
A
semiconductor is a material with an electrical conductance that is intermediate
between that of an insulator and a conductor. A
semiconductor behaves as an insulator at very low temperature but has a
significant electrical conductance at room temperature. This is however, much
lower than that of a conductor. The
energy-band structure of a solid is the series of “allowed” and “forbidden” energy
bands that it contains. This is according to the band theory which proposes the
existence of continuous ranges of energy values which electrons may occupy
(allowed) or may not occupy.
Bands have
different widths, based on the properties of the atomic orbitals from which
they arise. Also, allowed bands may overlap, producing a single large band. The band
structure determines a material’s electronic properties.
The valence
band is the highest range of electron energies where electrons are normally
present at absolute zero temperature. The
conduction band is the range of electron energy, higher than that of the
valence band, sufficient to make the electrons free to accelerate under the
influence of an applied electric field and thus constitute an electric current. In
semiconductors and insulators, there is a band gap above the
valence band, followed by the conduction band. For
insulators, the forbidden energy gap is large. If additional energy is given to
an electron in the upper level of the valence band, the electron attempts to
cross the forbidden energy gap and enter the conduction band. In metals,
the valence and conduction bands overlap. The valence band is only partially
filled with electrons. The conduction band extends beyond the upper end of the
filled valence band. There are no forbidden levels at higher energies.
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