P-N Junction

One of the crucial keys to solid state electronics is the nature of the 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 material alone. Specifically, current will flow readily in one direction (forward biased) but not in the other (reverse biased), creating the basic diode. This non-reversing behavior arises from the nature of the charge transport process in the two types of materials.

Depletion Region

When a p-n junction is formed, some of the free electrons in the 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 Details

	In the p-type region there are holes from the acceptor impurities and in the n-type region there are extra electrons.
	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 negative 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.

Bias effect on electrons in depletion zone

PN	Equilibrium ofjunction Coulomb force from ions prevents further migration across the p-n junction.
PN	Reverse biasApplied electric field further impedes the flow of electrons across the junction.
PN	Forward bias Applied electric field assists electrons in overcoming the coulomb barrier of the space charge in depletion region.