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The electronic and optical interband transitions are categorized by direct and indirect transitions because the conduction band bottom and the valance band top contain more than one extrema. The electronic and optical band gaps can be calculated by the predominant mechanism of band to band transitions and are normally less than several volts. In the indirect transition, the electrons transit from the valence band top to the conduction band bottom with participation of photons and appropriate phonons. In this case, the absorption coefficient can be given by [1],

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where,
       B -- A constant
       E_gi -- The indirect energy gap
       E_p -- The energy of the absorbed (+) or emitted (-) phonons.

[1] El-Korashy A, El-Fadl AA (1999) Phys B 271:205.