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PN Junction - explanation and uses

Describe how a pn junction works and how it is used in diodes, LEDs and lasers.

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PN JUNCTION
A pn junction is a piece of n-type semiconductor next to a piece of p-type, which creates certain interesting properties utilized in diodes, LEDS, lasers and solar cells.

A semiconductor can be doped by adding atoms from a neighbouring group in the periodic table so there is one more electron (or hole) per impurity atom which goes into the conduction band. If the impurity atom has an extra electron then this leaves behind a positive ion with which the electron can form a hydrogen-like, bound 'donor' state. This will have energy slightly less than that of the conduction band. Likewise if the impurity atom has one less electron we can regard this as a hole, or positively charged particle, in the conduction band which forms bound, hydrogen-like 'acceptor' states with energy slightly above that of the valence state. The semiconductor is called p-type if there are extra holes and n-type if there are extra electrons. The extra particles are the majority carriers and the opposite the minority carrier, for example in an n-type semiconductor the majority carriers are electrons and the minority carriers are holes.

The effective filling level of the states is given by the chemical potential, which at low temperature is the level where half the states are occupied. This moves towards the middle of the gap as temperature is increased, because the presence of donor or acceptor states becomes less important compared to the amount of thermally excited electrons. The chemical potential is higher for n-type than p-type ...

Solution Summary

A pn junction is a piece of n-type semiconductor next to a piece of p-type, which creates certain interesting properties utilized in diodes, LEDS, lasers and solar cells.

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