As we know, Kronig Penney model explains the existence of band gap at the same 'k', i.e. direct band gap. However, there is nothing like indirect band gap in KP model. Then, how can we explain the existence of indirect band gap other than momentum conservation by phonons?
The orientation dependence of the bandgap arises from the symmetries of the crystal lattice. Because the atomic spacing is different along different directions, so too is the bandgap. This can led to a situation where the minimum of the conduction band and maximum of the valence band occur along different directions, giving rise to an indirect bandgap.
Excellently simplified by Rob Elliman. Recently, came across the excerpt from the book “The Oxford Solid State Basics" by Steven H. Simon. Please find the attachment and may shed your thoughts. Thank you All.
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