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Post  Post subject: Direct and indirect bandgaps  |  Posted: Fri Dec 30, 2016 6:17 pm

Joined: Tue Dec 13, 2016 8:20 pm
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Hi. So, i know what direct and indirect bandgap materials are i.e. know the difference in electronic band structure diagram. My question is WHY does it happen? Why can't all atoms have direct bandgaps and just live happily ever after? Why do they need to form indirect bandgaps? I know it has something to do with bonding and antibonding orbitals but I have no idea what those are either. There's something about momentum too I think. I don't know I'm really confused :oops: Please help me :cry:


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PhDemon
Post  Post subject: Re: Direct and indirect bandgaps  |  Posted: Fri Dec 30, 2016 6:31 pm

Joined: Wed Jun 26, 2013 10:44 am
Posts: 515
Location: Newcastle-upon-Tyne

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It's nearly two decades since I studied this and I'm a bit (a lot!) rusty but it is due to the symmetry of the crystal lattice of the semiconductor (which is linked to the atomic orbitals of the atoms in the crystal). It is not momentum as such that is important but a related vector called "crystal momentum" (https://en.m.wikipedia.org/wiki/Crystal_momentum). IIRC if the crystal momentum of the electron in the valence band is the same as that of the hole in the conduction band that is a direct band gap, if they are different it is an indirect band gap.

As I say it's been a while and that is about all I remember!

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