Classification of Solids

The solids can be classified in three parts, conductor, semiconductor and insulators.

On the electrical point of view

The atoms of conductors have their outermost orbits incomplete initially.
The valence electrons of conductors are loosely bound to the nucleus, and hence they can be freed from the atoms.
The electrical and thermal conductivity of conductors are very high.
In steady state, the conductors obey Ohm’s law.
The resistance of conductors increases with rise in temperature, it means the temperature coefficient of the conductors are positive.
Gold, silver, aluminium etc., are the examples of conductors.

The atoms of insulators have their outermost orbits saturated.
The valence electrons of insulators are tightly bound to the nucleus, and hence they can not be freed from the atoms, or practically they don’t have any free electrons.
The electrical conductivity of insulators are very small or the electrical resistivity of insulators are very high.
Glass, mica, quartz, ebonite etc., are some examples of insulators.

The atoms of semiconductors have their conductivity intermediate between conductors and insulators.
The resistivity of the semiconductors are higher than that of the conductors and lower than that of the insulators.
The conductivity of semiconductors are lower than the conductors and higher than the insulators.
Resistance and resistivity of these substances decreases with rise in temperature, it means the temperature coefficient of the semiconductors are negative.
The conductivity of semiconductors can be increased by adding some impurities in it.
Ge, Si, etc., are the examples of semiconductors.

The electrical properties of the materials can be explained by energy bands.
An electron in a solid can have only those discrete energies that lie within these energy bands, such bands are known as allowed energy bands.
The allowed energy bands are separated by some gaps in which there is no allowed energy bands, such gap is known as forbidden energy bands.
The energy bands occupied by valence electrons are known as valence band, and the energy bands occupied by conduction electrons are known as conduction band.
The gap between valence band and conduction band is energy gap or forbidden band.
Higher is the energy gap, lower is the electrical conductivity of the material.

The difference between valence band and conduction band in conductors are zero, or both the bands in conductors overlap.
There are sufficient amount of free electrons in conduction band in conductors.
In conductors, the space for moving, the free electrons is very small, or there is no free space to move.
The conductivity of conductors is very large at room temperature, and as the temperature increases their conductivity decreases.

In insulators, the valence band is completely filled, and the conduction band is completely empty.
In insulators, the difference between valence band and conduction band is very large.
It is found that with rise in temperature, the conductivity of insulators can be increased.

The semiconductors has a partially filled conduction band and partially filled valence band.
The energy gap between valence band (V.B.) and conduction band (C.B.) in semiconductors are very small (about 1 eV).
At absolute zero temperature the semiconductor behaves as insulators.
With rise in temperature the conductivity of semiconductor increases, or resistivity decreases.
At very high temperature the semiconductor behaves as conductor.

Energy band diagram of solids

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