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How does a semiconductor laser work?
The basic idea is very simple. It is radiative recombination in the semiconductor gap which leads eventually to the laser action why the semiconductor gap?
Because we want the probability of a transition from the bottom of the conduction band to the top of the valence band to be high (population inversion).
What else do we need?
We need a piece of material in which there are lots of electrons in the conduction band eager (متحمس) to descend and in which there are lots of empty states at the top of the valence band eager to receive the electrons. Then if a photon having energy slightly greater than the band gap energy is incident, such a photon will stimulate a large number of downward transitions of the electrons from the conduction to valence band and this would result in a coherent amplification and if the resulting medium is enclosed between two mirrors , one may have a laser beam . A homogeneous piece of semiconductor is obviously not suitable because we cannot achieve both conditions simultaneously. But this gives an idea . We can have lots of holes in a p-type semiconductor and we can have lots of electrons in a n-type semiconductor. So let us put them together , that is a p-n junction . If now we form a p-n junction in reverse bias ,then the excess electrons in the n-type region tend to flow across the junction into the p-type region and the excess holes in the p-region tends to flow to the n region. This creates an electric field opposing this flow and the flow stops. If we now connect the junction in the forward bias then one produce a flow of electons and holes in the junction region. These electrons and holes may recombine to produce radiation. If the current is large enough one may have laser beam
Advantages
· High efficiency
· Low cost
· Small size They operate at low voltage
· They have long life
· They are robust
· They may be made to work in the wavelength range in which optical fibres have favourable loss(near to minimum) and dispersion properties
· They are used in fibre optics communication

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