PN junction

  • P-N junction is the basic building block of many semiconductor devices like diodes transistors etc.
  • An P-N junction is formed when a P-type semiconductor is joined to a N-type semiconductor in such a way that the crystal structure remains continuous at the boundary.
  • Three basic processes used for preparation of an P-N junction are alloying , growing and diffusion out of which diffusion is generally used for the preparation of an P-N junction.
  • One very important characteristic of an P-N junction is its ability to conduct current only in one direction and the very high resistance it offers in another (or reverse direction).
  • Consider the figure given below which shows a simple P-N junction having P type material towards left hand hand side and N-type material towards right hand side.

  • P-type material have high concentration of holes and N-type material has high concentration of electrons. Along with these majority charge carriers both type of materials also consists of few minority charge carriers.
  • As soon as P-N junction is formed holes begin to diffuse from P region to N region and electrons begin to diffuse from N region to P region and electron and holes begin to recombine in pairs.
  • Due to diffusion and recombination of electrons and holes there remain no charge carriers near junction and this region known as depletion region is formed on both sides of the junction as shown in the figure.
  • The thickness of this depletion region is of the order of one-tenth of micrometer.
  • In depletion region there are stationary negative ions towards P side and positive ions towards N side.
  • This creates a potential difference between two parts of the junction where N region is at higher potential with respect to P region .
  • An electric field thus develops in this depletion region  which is directed from positive charged ions in N region towards negative charged ions in P region.
  • Under the influence of this field electrons which are minority charged carriers in P side of the junction moves to N side and holes on N side of the junction moves to the P side.
  • The motion of charged carriers due to this electric field is called drift.
  • This internal electric field helping the movement of minority charged carriers adjust itself in such a way that drift of minority charged carriers across the junction is counterbalanced by the diffusion of same number of majority charged carriers across the junction thereby creating a state of equilibrium.

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