Energy bands

Energy bands

• In case of a single isolated atom an electron in any orbit has definite energy.

• When atoms are brought together as in solids, an atom is influenced by the forces from other atoms. Hence an electron in any orbit can have a range of energies rather than single energy. These range of energy levels are known as Energy bands.

• Within any material there are two distinct energy bands in which electrons may exist viz Valence band and conduction band.

• The range of energies possessed by valence electrons is called valence band.

• The range of energies possessed by free electrons is called conduction band.

• Valence band and conduction band are separated by an energy gap in which no electrons normally exist this gap is called forbidden gap.

Electrons in conduction band are either escaped from their atoms (free electrons) or only weakly held to the nucleus. Thereby by the electrons in conduction band may be easily moved around within the material by applying relatively small amount of energy. (either by increasing the temperature or by focusing light on the material etc. ) This is the reason why the conductivity of the material increases with increase in temperature.

But much larger amount of energy must be applied in order to extract an electron from the valence band because electrons in valence band are usually in the normal orbit around a nucleus. For any given material, the forbidden gap may be large, small or non-existent.

Classification of materials based on Energy band theory

Based on the width of the forbidden gap, materials are broadly classified as conductors, Insulators and semiconductors.


• Conductors are those substances, which allow electric current to pass through them.
Example: Copper, Al, salt solutions, etc.

• In terms of energy bands, conductors are those substances in which there is no forbidden gap. Valence and conduction band overlap as shown in fig (a).

• For this reason, very large number of electrons are available for conduction even at extremely low temperatures. Thus, conduction is possible even by a very weak electric field.


• Insulators are those substances, which do not allow electric current to pass through them.
Example: Rubber, glass, wood etc.

• In terms of energy bands, insulators are those substances in which the forbidden gap is very large.

• Thus valence and conduction band are widely separated as shown in fig (b). Therefore insulators do not conduct electricity even with the application of a large electric field or by heating or at very high temperatures.


• Semiconductors are those substances whose conductivity lies in between that of a conductor and Insulator.
Example: Silicon, germanium, Cealenium, Gallium, arsenide etc.

• In terms of energy bands, semiconductors are those substances in which the forbidden gap is narrow.

• Thus valence and conduction bands are moderately separated as shown in fig(C).

• In semiconductors, the valence band is partially filled, the conduction band is also partially filled, and the energy gap between conduction band and valence band is narrow.

• Therefore, comparatively smaller electric field is required to push the electrons from valence band to conduction band . At low temperatures the valence band is completely filled and conduction band is completely empty. Therefore, at very low temperature a semi-conductor actually behaves as an insulator.
Conduction in solids

• Conduction in any given material occurs when a voltage of suitable magnitude is applied to it, which causes the charge carriers within the material to move in a desired direction.

• This may be due to electron motion or hole transfer or both.

Electron motion

Free electrons in the conduction band are moved under the influence of the applied electric field. Since electrons have negative charge they are repelled by the negative terminal of the applied voltage and attracted towards the positive terminal.