Thanks for the reply.

What i am looking for actually is that if for a material the band gap at a particular symmetry point (X) decreases with temperature as it is usually, is it possible that  the gap between (X) and (L) point increases with increase in the temperature (due to band softening or flattening).

FYI, I never observed (experimentally) any increase of the band gap with temperature (at least for large direct or indirect band gap semiconductors) but i believe that a locally compression stress could increase it.

Can you give more information about your semiconductor (alloys) ?  

This is interesting however, as you probably know, all semiconductors become metal in liquid phase, except some alloys (e.g., CdTe, ZnTe) which can keep a small band gap just above the melting point (due to local structural order).

Thanks @ Ali BenMoussa and Behnam Farid for showing the interest to help me.

My system is a half-metal Heusler alloy, metallic in one spin channel and semiconductor in other spin channel. the system in down spin channel is an indirect band-gap semiconductor between X and L points of the Brillouin Zone . I tried to study the effect of temperature on the band gaps between different symmetry points. What I observed was that with increased temperature, the direct band gaps decrease with increasing temperature, however, a small increase is also observed between the X and L (indirect band gap) points also. I am confused to see that and don't know how to explain that.

@Behnam Farid,  What if the system has a very little effect of that selected temperature range and is not enough to change the geometry of the system.

I hope you understand my problem now!!

With Regards

Ok thanks for the clarification, Idris.

From past results, I can just say that it is not easy to couple a thermal model, so good luck.

Thanks for the explanation. Yes i have used Wien2k, based on Kohn-Sham equation for band structure calculations and the code GIBBS2 whis is based on quasiharmonic approximations (Debye Model) to include the temperature. I have calculated the variation of temperature with volume and shows the increase trend. I selected some temperatures and calculated lattice parameters corresponding to those temperature values and proceeded to the band structure calculations, i.e, the band structures at different lattice parameters (increased because of temperature).

From band structure i measured the gaps at different symmetry points, i,e., the difference between the top of the valence band and the bottom of the conduction band (at different symmetry points). For each symmetry points the gap decreases with temperature but as my compound has an indirect band gap, i observed that the gap between X and L point increases not by a larger value, with increasing temperature.

(That is about the approximations and models i have used and the measurements i had)

With Regards

I thank you very much and appreciate your help dear Behnam Farid,

Yes, that is exactly the case, the code is actually considering it a different system at every temperature. Is it convenient to study like this?

or in other words it is actually the variation band gaps with lattice parameter. I have read some literature where people study different properties like magnetism and also the spin-polarization as a function of lattice parameter.

With Regards

@ Nilotpal Bhattacharjee , see the replies and you will get the answer.

What i was considering are the totally different systems and band structure calculations do not consider the effect of temperature in my code.

Thanks dear Behnam Farid,

You make me understand this very well.

Thank you a lot!!

With Regards

With T the band-gap edges are smeared. 

With temperature energy band gap decreases. With increasing temperature, carriers get extra energy to over come the gap. Thus, effective bandgap is reduced. This leads to increased leakage current in semiconductor devices such as diodes, transistors etc. Standard books on semiconductor devices cover this topic nicely.

What i was concerned about is the indirect band gap.

thanks for the answer and explanation Mr. A. Kumar.

Dear Idris,

Adding to the above answers i would like to say that the energy gap decreases with the increase in temperature as the lattice constant gets larger with increased temperature leading to lower energy levels. This is valid for the direct and indirect energy gap. The difference between the the direct and indirect transition is that the indirect transitions are phonon assisted to affect the momentum alignment. The temperature coefficient of the energy gap decrease with temperature are given in the literature. See for example physics of semicondcutor devices by S M Sze.

I would like also that refer to the link:

https://www.researchgate.net/post/How_does_band_gap_of_a_semiconductor_change_with_the_lattice_constant_Please_explain_the_physical_significance

Best wishes