Explain the difference between an indirect and direct band gap semiconductor. Is the optical absorption coefficient less or more in direct band gap material and why?
In a direct band gap semiconductor the minimum of conduction band just lies above the Maxima of valence band while in case of indirect band gap semiconductor the conduction band minima does not. Examples of direct band gap semiconductor materials are InAs, GaAs etc while that of indirect are Ge, Si etc.
In case of direct band gap semiconductor absorption coefficient is more because here the conduction band minima is just above the valence which makes a electron transition easy and emission of photon common . This emitted photon will also ensure the momentum conservation here. While in case of indirect since extra energy is required for the electron to jump from valence to conduction band and thus this makes electron transition slightly less probable in these materials and hence less absorption coefficient.
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