What is a root cause behind the formation of a direct or an indirect bandgap in a semiconductor ? OR What decides whether a material will have a direct bandgap or an indirect bandgap?
If you're looking for the one rule to get that, we have to disappoint you. See Inmaurer's answer here, it captures the reality quite well, I'd say:
https://physics.stackexchange.com/questions/595560/what-is-the-cause-of-formation-of-indirect-band-gap-in-semiconductors
Bandgap in electronic materials can be understood by bond
2. Semiconductor: At 0k temperature, there is no free charge carriers are available for conduction process. Therefore, for the conduction we need to supply an external energy in form of temperature. Now we increase the temperature ( t>0k), then some bond will broken up and charge carriers are to be produced. Thus, bandgap is just an energy to create charge carrier via bond breaking process.
Anil Kumar Sisodia
Formulating it as a "bond-breaking" process is a bit too classically corpuscular as a way of thinking (plus, it doesn't help in understanding the direct vs. indirect question here).
Electronic structure needs to be modelled by quantum mechanics and in a solid you have indifferentiable, delocalized electrons so, as we were told over and over in our introductory lecture, "there are no blue and red electrons" which means that a statement implying that an analogy with individual electrons detaching and getting in motion doesn't fully depict reality.
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