Bands In Semiconductors

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  • have a narrow energy gap (~1-3 eV) between valence and conduction bands.
  • Not a large amount of energy is needed to excite a valence electron into the conduction band where it becomes mobile, leaving a hole in the valence band.

At low temperatures, valence electrons are not able to gain enough thermal energy to leap across the energy gap into the conduction band, the semiconducting material thus acts as an insulator.

At higher temperatures, the ambient thermal energy becomes sufficient to force electrons across the gap, and the material will conduct electricity.

  • An electron gain enough energy to move from the valence band to the conduction band. A vacant site known as a hole will be left behind in the valence band.
  • The hole acts as a positive charge carrier – a free electron from a nearby site can transfer into the hole. Whenever an electron does so, it creates a new hole at the site it abandoned.
  • Thus, the net effect is the hole migrating through the material in the opposite direction to the direction of electron movement.
  • When an external electric field is applied, both conduction electrons and holes contribute to current flow.

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