P-N Junction

When p-type and n-type materials are placed in contact with each other, the junction behaves very differently than either type of materials alone.

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

Depletion Region process: step 1
     When a p-n junction is formed, some of the free electrons in n-region diffuse across the junction and combine with holes to form negative ions.
In so doing they leave behind positive ions at the donor impurity sites.

Depletion Region process: step 2
      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.

Depletion Region process: step 3
      Filling a hole makes a negative ion and leaves behind a positive ion on the n-side.
Space charge builds up, creating a depletion region which inhibits any further electron transfer unless it is helped by putting a bias or supply on the junction.

Depletion Region process: Summary