Conductivity of an Insulator Now let’s consider the conductivity of an insulator.
Consider carbon with its diamond cubic cyrstal structure. The valance electrons are all covalently bonded in sp3 orbitals. These orbitals are completely filled. The next available energy level (above these sp3 orbitals) is 6 eV above the highest occupied level.
The energy picture looks like this for the electrons of carbon:
In this picture we see that the
valance band is completely
filled. Then there is an energy gap where there are forbidden energies.
(Remember that quantized nature of energy levels. Only specific energy
levels are allowed. Others are forbidden.) This gap represents the
energy between the highest level in the
Valance Band and the next allowable energy level. For carbon, this gap is 6eV. If an electron could gain enough energy to jump over this gap into this next allowable
level, it could be a charge carrier and contribute to conduction. Hence the levels above the energy gap are called the Conduction Band.
At
absolute zero the conduction band is completely empty. At temperatures
above absolute zero, some electrons might have enough energy to get
into the conduction band, depending on the magnitude of the energy gap.
For insulators, the gap is large and so not very many electrons make it
there, even at high temperatures.
Overall Conclusion on the Conductivity of Insulators: Insulators
are not good electrical conductors because their valence band
is completely filled and the energy gap between the valance band and the
conduction band is too large for many electrons to make it into the conduction band to become charge
carriers.
