# Talk:Direct and indirect band gaps

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## Are indirect bandgap materials characterised by the momentum difference

Diagram top right makes it look as if there are degrees of indirectness. If so, does the momentum difference affect their properties - eg. the temperature at which they are usable ? Rod57 (talk) 11:24, 9 March 2011 (UTC)

It's all or nothing: The transition requires a phonon, or does not require a phonon. For a transition that requires a phonon, the phonon wavevector (=momentum difference) has some effect but I don't think it's a very important one, compared to other factors like the phonon frequency (which is affected by the mass of the atoms) and other things. Even if the phonon wavevector makes a difference (other things equal), it's not obvious that a smaller phonon wavevector necessarily means a stronger transition (faster light absorption and emission). It could be the other way around, as far as I know. --Steve (talk) 03:21, 10 March 2011 (UTC)
Hmmm, not knowing anything whatsoever about this, I would still suspect that statistical thermo implies that a lower phonon energy would imply relatively higher abundance of photons of the required energy? Would this not impede the release, or aid the absorption? Maury Markowitz (talk) 14:43, 6 September 2011 (UTC)
Yes, it's true, the ratio of the phonon energy to the crystal temperature is important, like you say. (A thin piece of silicon can become transparent if you put it in liquid helium!) However, it is not true in general that a larger phonon momentum means a larger phonon energy. For acoustic phonons that's true, but for optical phonons, the larger-momentum phonons have lower energy! --Steve (talk) 00:06, 7 September 2011 (UTC)

I don't doubt that there are people out there who find the K vector in the Brillion zone to be a useful shorthand way of defining direct and indirect bandgaps semiconductors...but these people aren't *reading* Wikipedia! Have pity on those of us whose physics classes were 30 years ago...--Wtshymanski (talk) 19:20, 11 August 2011 (UTC)

Yes. I was confused until I stumbled on

"Energy band gaps can be classified using the wavevectors of the states surrounding the band gap:

• Direct band gap: the lowest-energy state above the band gap has the same k as the highest-energy state beneath the band gap.
• Indirect band gap: the closest states above and beneath the band gap do not have the same k value."

at Electronic_band_structure#Crystalline_symmetry_and_wavevectors. - Rod57 (talk) 23:20, 9 November 2016 (UTC)

## Could explain why LEDs need direct band gap materials

Light-emitting_diode#Physics mentions (without sources) the need for LED materials to have a direct band gap. Could we have here an expanded explanation with sources ? - Rod57 (talk) 18:49, 29 October 2016 (UTC)