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Energy Bands

The theory of energy bands is based on Pauli exclusion principle. In isolated atom the valence electrons can exist only in one of the allowed orbitals each of a sharply defined energy called energy levels. But when two atoms are brought nearer to each other, there are alterations in energy levels and they spread in the form of bands (Fig. 1).
Fig. 1 Origin of energy bands in a solid
Energy bands are of the following types:

Valence band

The energy band formed by a series of energy levels containing valence electrons is known as valence band (VB). At 0 K, the electrons fill the energy levels in valence band starting from lowest one.
  • This band is always filled with electrons.
  • This is the band of maximum energy.
  • Electrons are not capable of gaining energy from external electric field.
  • No flow of current due to electrons present in this band.
  • The highest energy level which can be occupied by an electron in valence band at 0 K is called fermi level.

Conduction band

The higher energy level band is called the conduction band (B).
  • It is also called empty band of minimum energy.
  • This band is partially filled by the electrons.
  • In this band the electrons can gain energy from external electric field.
  • The electrons in the conduction band are called free electrons. They are able to move anywhere within the volume of the solid.
  • Current flows due to such electrons.

Forbidden energy gap (ΔEg)

Energy gap between conduction band and valence band ΔEg = (CB)min – (VB)max
  • No free electron is present in Fig. 2 forbidden energy gap.
Fig. 2
  • Width of the forbidden energy gap depends upon the nature of substance.
  • As temperature increases (), the forbidden energy gap decreases () very slightly.

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