Explain the formation of energy bands in solids. On the basis of energy bands distinguish between a metal, a semiconductor and an insulator.

Answer:

The band theory of solids is different from the others because the atoms are arranged very close to each other such that the energy levels of the outermost orbital electrons are affected. But the energy level of the innermost electrons is not affected by the neighbouring atoms. The three energy bands in solids are

  • Valence band
  • Conduction band
  • Forbidden band

Bond theory solids

  • In atoms, electrons are filled in respective energy orbits following Pauli’s exclusion principle.
  • In molecules, Two atomic orbitals combine together to form a molecular orbit with two distinct energy levels.
  • In solids, 1023 stacked up lines confined in a tiny space would look like a band. Thereby forming energy continuum called energy bands.
  • This theory helps to visualise the difference between conductor, semiconductors and insulator by plotting available energies for an electron in a material.

Energy Band Gaps in Materials

Conductors:

Metals are conductors in which there is no forbidden energy gap between the conduction band (CB) and valence band (VB). No extra energy required to transfer the electron from VB to CB.

Semiconductors:

In a semiconductor, the valence band is completely filled with electrons while the conduction band is empty. The energy gap between the bands is less. For electrons to jump from the valence band to the conduction band, room temperature needs to be maintained. If the temperature is 0K, there is no transfer of electrons from the valence band to the conduction band.

Insulators:

In insulators, the valence band is completely filled while the conduction band is empty. This results in a large energy gap. Since the energy gap between the conduction band and the valence band is more, there is no movement of electrons from the valence band to the conduction band.

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