https://doi.org/10.1351/goldbook.14005
Semiconductor or dielectric materials whose first band-to-band transition occurs from the highest level in the valence band to the lowest level in the conduction band possessing the same electron momentum, i.e., \(\boldsymbol{p}_{\rm{max}} = \boldsymbol{p}_{\rm{min}}\), with \(\boldsymbol{p}_{\rm{max}}\) and \(\boldsymbol{p}_{\rm{min}}\) the momentum vectors of electrons of the highest level in the valence band and the lowest level in the conduction band, respectively.
Note: On photoexcitation, the electrons change their energy owing to the absorption of photons and maintain the same momentum.
Compare: indirect bandgap semiconductor