When light is allowed to fall over semi-conductors the electrons in the valence band absorb the energy from light and it moves from valence band to conduction band. The transition depends purely on the energy band gap between the valence band and conduction band. The energy band gab of a semi conductor is given by.
From this relation we can say that the absorption edge is inversely proportional to the energy band gab, the materials having small energy band gab will absorb the lower wavelength and allow the higher wavelength and allow the higher wavelength of light .
Explanation:
When light is allowed to fall on a zinc selenium crystal which has the band gab , corresponding to bluish green wavelength; then it absorbs the bluish green light and emits the wavelength of light greater than bluish green (ie) yellow light, which at longer wavelength side.
‘The wavelength that is absorbed with respect to the energy band gab of the material is called absorption edge’
Hence we can say that the material having low energy band gab will absorb lower wavelength corresponding to the Eg and allow the wave length of higher range.
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