We know semiconductors have conductivity not as high as conductors and not as low as insulators. They have found a large number of application due to this unique behavior. We have categorized them into intrinsic and extrinsic semiconductors depending upon if they are in pure form or contain trivalent or pentavalent impurity. Those semiconductors with trivalent impurity are called as p type semi conductors and ones with pentavalent impurity are called as n type semi conductors. P type has holes in excess while n type has electrons in excess. We know diffusion is movement of particles due to concentration differences between two points. Particles move from high to low concentration. When n and p types are brought in contact electrons move from n side to p side due to concentration difference. Similarly holes move from p to n side due to concentration gradient. This happens due to thermal energy of electrons which exhibit itself in the form of kinetic energy of electrons. This diffusion leads to formation of depletion region in such materials.
For more understanding and short video explanation of charge carrier diffusion
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