In case of a single isolated atom there are single energy levels but when two atoms are brought very near to each other there will be modification in energy level. Similarly in solids where the atoms are arranged in a systematic lattice space where many atoms are close by.

The closeness of atoms results in intermixing of electrons of neighbouring atoms. The valence electrons in the outermost shell are not bound strongly by the nucleus, due to this intermixing the number of permissible energy level increases.

Hence in case of solid instead of single energy level associated with single atoms,there will be band of energy levels.

A set of such closely spaced energy levels is called an energy band.

Valence Bands: In solids, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature.

Conduction Band: The conduction band is the range of electron energies enough to free an electron from binding with its atom to move freely within the atomic lattice of the material.

Energy Gap:

• Except in metals, the conduction band and valence band don’t overlap.
• In such a case there is no free electron available and conduction is not possible.
• This gap between the conduction band and valence band is called energy gap or band gap.
• Essentially this band gap determines the conductive property of the solid.
• If the valence band and conduction band overlap then there is no energy gap, which means that there are free electrons available for conduction.
• In case of semi-conductors there is a small amount of gap between the conduction band and valence band, which can be overcome by either by providing thermal energy or by doping.
• In case of insulators the energy gap is very high which makes it difficult to conduct.

Extrinsic Semi-conductor:

• In case of semi-conductors, the energy gap is small.
• Fermi level lies between the conduction band and valence band.
• With low energy gap, it means by providing more energies valence band will overlap the conduction band which will enable it to be a good conductor.
• This can be done in 2 ways, either by thermal charging it or by doping it.
• Extrinsic semi-conductor is the semi-conductor which has been doped.
• By doping we introduce impurities in the semi-conductor helping introduce free electrons and holes which help in conduction.
• A "dopant" atom is added to the lattice in order to draw electrons from the valence band.
• This atom is referred to as anacceptor.
• As more acceptors are added to the lattice, the number of holes will begin to exceed the number of negative charge carriers, eventually leading to a p-type (positive type) semiconductor. N-type semiconductors have a large number ofdonors, "dopant" atoms that donate electrons to the conduction band.