Achievement of population inversion: When p-n junction diode is forward biased, then there will be injection of electrons into the conduction band along n-side and production of more holes in valence band along p-side of the junction. Thus, there will be more number of electrons in conduction band comparable to valence band, so population inversion is achieved.

Figure: Energy level diagram of semiconductor laser (No biasing)

Figure: Energy level diagram of semiconductor laser (with biasing)

Therefore, when the electrons and holes are injected into the junction region from opposite sides with forward biasing, then population inversion is achieved between levels near the bottom of the conduction band and empty levels near the top of the valence band.

Achievement of laser: When electrons recombine with the holes in junction region, then there will be release of energy in the form of photons. This release of energy in the form of photons happen only in special types of semiconductors like GaliumArsenide (GaAs). Otherwise in semiconductors like silicon and germanium, whenever holes and electrons recombine, energy is released in the form of heat, thus Si and Ge can not be used for the production of laser.

The spontaneously emitted photon during recombination in the junction region of GaAs will trigger laser action near the junction diode. The photons emitted have a wavelength from 8200 Å to 9000 Å in the infrared region.

Note: In next article, I will explain the concept of Fermi level in semiconductor laser. In case of any problem in this article or any other physics article, post in the comment section.

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Note from winnerscience: If you want the e-notes (e-book/soft copy) of all the laser  articles, then please contact winnerscience@gmail.com. You can post your queries also there.

The notes will include the topics like: basics of lasers like stimulated absorption, difference between spontaneous and stimulated emission, Einstein  Coefficients, properties and applications of lasers, complete construction and working of lasers like Ruby laser, He-Ne laser, Carbon dioxide laser, Nd:YAG laser, dye laser, semiconductor laser, holography and additional articles of Q-switching and mode locking.

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