I think it is right to talk about the band gap of semiconductors, but not gap. 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 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.

I think it is right to talk about the band gap of semiconductors, but not gap. 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 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.

Hello Karima

Do you mean both the photonic band gap and the band gap of semiconductors?

There are different gaps, optical (phonones) and acustic (phonones)

May Allah bless You Dr. Karima

About Your query I recently read this opinion for Alexander P. Litvinchuk,  University of Houston

There is not real reason to make a distinction between optical band gap and electronic band gap.

Let us think about that as follows: the real band gap in a material is due to its electronic structure. So, the band gap is "electonic" due to its nature. Its existance is independent of whether or not we are thinking about it or trying to determine its magnitude one or the other way.

Now, you should probably ask yourself the next question:  what I would be measuring using optical spectroscopy? That's where the excitonic effects, among other things will come into play.

Summarizing, I do not think the formulation "optical band gap" is a proper one. Otherwise, one might talk about "thermal band gap", "magnetic band gap", "electric band gap" etc... So, it seems to me that it is more appropriate to talk about the "electronic band gap", and "an optical measurement of the band gap".

Best regards