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)
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.
