Optical Absorption in Semi-Conductors

OPTICAL ABSORPTION IN SEMI-CONDUCTORS

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 gap of a semiconductor is given by

From this relation we can say that the absorption edge (λ) is inversely proportional to the energy band gap, (i.e.) the materials having small energy band gap (E_g) will absorb the lower wavelength and allow the higher wavelength of light.

Explanation: When light is allowed to fall on a zinc selenium crystal which has the band gap E_g = 4800Å, corresponding to bluish green wavelength, then it absorbs the bluish green light and emits the wavelength of light greater than bluish green (i.e.) yellow light, which is at longer wavelength side.

The wavelength that is absorbed with respect to the energy band gap of the material is called Absorption Edge.

Hence we can say that the materials having low energy band gap will absorb lower wavelength corresponding to the E_g and allow the wavelength of higher range.