on what principle does a semiconductor laser works?


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The semiconductor materials have valence band V and conduction band C, the energy level of conduction band is Eg (Eg>0) higher than that of valence band. To make things simple, we start our analysis supposing the temperature to be 0 K. It can be proved that the conclusions we draw under 0 K applies to normal temperatures.
Under this assumption for non-degenerate semiconductor, initially the conduction band is completely empty and the valence band is completely filled. Now we excite some electrons from valence band to conduction band, after about 1 ps, electrons in the conduction band drop to the lowest unoccupied levels of this band, we name the upper boundary of the electron energy levels in the conduction band the quasi-Fermi level Efc. Meanwhile holes appear in the valence band and electrons near the top of the valence band drop to the lowest energy levels of the unoccupied valence energy levels, leave on the top of the valence band an empty part. We call the new upper boundary energy level of the valence band quasi-Fermi level Efv. When electrons in the conduction band run into the valence band, they will combine with the holes, in the same time they emit photons. This is the recombination radiation. Our task is to make this recombination radiation to lase. Then several conditions must be met.
First for the radiation to be amplified, the light energy hnmust satisfy:
Efc- Efv³hn³Eg
From this relation we have Efc- Efv³Eg. This decides the critical condition. The value of Efcand Efvis influenced by the pumping process, i.e., by the intensity (N) of the electrons being raised to the conduction band. When N is increased, Efcincreases and Efvdecreases. The N satisfies Efc- Efv= Eg is named Ntr. We inject carriers into the semiconductor material to make the free electron intensity to be larger than Nth, then the semiconductor exhibits a net gain. We put this active medium in a suitable cavity, laser action occurs when this net gain overcome losses. The pumping of semiconductor lasers can be realized by the beam of another laser, or by an electron beam, but the most convenient way is by using electrical current that flows through the semiconductor junctions. This uses the semiconductor laser in the form of diode.
Thanks and Regards
Apoorva Arora
IIT Roorkee
askIITians Faculty
2 months ago

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