P-N Junction

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P-N Junction

  • P-type and N-type placed in contact with each other
  • current will flow readily in one direction (forward bias) but not in the other (reverse bias)
depletion region at p-n junction

Near the junction, electrons diffuse across to combine with holes, creating a depletion region.

Steps to formation of depletion region

  • In the p-type region there are holes from the acceptor impurities and in the n-type region there are extra electrons from the donor impurities.
  • When a p-n junction is formed, some of the electrons from the n-region which have reached the conduction band are free to diffuse across the junction and combine with holes.
  • Filling a hole makes a negation ion and leaves behind a positive ion on the n-side. A space charge builds up, creating a depletion region which inhibits any further electron transfer unless it is helped by putting a forward bias on the junction. (The e-field created is strong enough to prevent any more electrons from moving)

P-N junction under reverse bias (P connected to negative terminal of battery, N connected to positive terminal)

  • At N, electrons move towards the positive terminal, creating a larger area of positive charges.
  • At P, holes move towards the negative terminal, creating a large area of negative charges.
  • Hence, size of the depletion zone increases.
  • The applied voltage drives the electrons and holes away from the junction, preventing conduction.

P-N junction under forward bias (P connected to positive terminal, N connected to negative terminal)

  • The applied voltage assists electrons in overcoming the electric field due to the charges in the depletion region. Hence, electrons will flow with very small resistance in the forward direction.
  • Size of depletion zone decreases.

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