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Problem 3:

The degenerate occupation of band shown in Figure helps to maintain the laser requirement that emission must overcome absorption. Explain how the degeneracy prevents band-to-band absorption at the emission wavelength.

In order to exibit absorption, an electron from the valence band should hop to the conduction band, but of course if would hop only over $ F_{n}$ , thus the absorbed photons should have the energy of A (Figure 2). The opposite way around happens for the emitted photons (electron hopping-back). These conditions will be applicable only for these areas containing a large concentration of $ e^{-}$ in the conduction band and holes in the valence band (condition of population inversion). Since the levels are away from the junction (inversion region), this band-band absorption will become dominant far from the junction.

Figure 2: Band diagram of a p-n junction laser when forward biased

Deyan Levski 2013-06-21