There is optical band gap and electronic band gap but I am not sure how relate them to the bulk band gap value and what is the relationship between them.
Also, Is the optical band gap related to minimum photon-energy that material can absorb to make electron-hole (excitons) pair but they still held together by electrostatic coulomb force, so the electron cannot move to the conduction band due to this force?
You are right about the coulomb force holding the e-h pair. It is called the exciton binding energy. As an approximation, optical band gap plus exciton binding energy is the electronic band-gap. For most inorganic semiconductors, the exciton binding energies are usually very low, so their optical and electronic band-gaps very close. The binding energy depends on the type of excitons (Frenkel, Wannier-Mott, ...). So it really depends on the exact material.
Dear Abu Saeed
Hanyang University
I do totally agree with Pr. Abhilash Sugunan and Pr. Alexander P. Litvinchuk answers. I have just one point to add; when we talk generally about ‘bulk’ band gap it means that the dimensions of the material are large. However with increasing reduction of material size (ultra-thin film) down to quantum devices where the knowledge of material band gap becomes crucial, bulk band gap will be obsolete.
Kind regards,
Dear Dr. Abhilash, Dr. Alexander and Dr. Abderrahmane, thank you very much for ypu informative answers.
With respect to the band gap of semiconductors; This value is determined by the energy difference between the top of the valence band and the bottom of the conduction band. It is necessary to distinguish between transport (electronic) band gap and optical band gap. The transport band gap corresponds to the minimal energy of the creation of free electron and hole, while the optical band gap is related with the energy of creation of bound electron-hole pair, i.e. exciton. Therefore, the transport band gap is larger than the optical band gap. This difference is determined by the binding energy of exciton in a semiconductor.
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