Don't get the conduction band in solid

luuurey

I was reading about electronic band structure in wiki and I didn't understand why the conduction bands are necessary for flowing current in a material. If all the atoms in a solid shares valence band and all its eletrons, then these electrons may travel through the solid. I didn't get why the electrons in the valence bands can't freely travel within solid and so make current.

Thanks for explanation!

daveyrocket

Because the valence band is symmetric in momentum. That is, in a semiconductor with a filled valence band for each electron with momentum +k there is another electron with momentum -k, so there is no net current. Even if one applies an electric field to bias the current in a certain direction, this basic momentum balancing effect is still present.

Once you get some electrons in the conduction band and have a partially filled band, then the application of an electric field can break this symmetry.

luuurey

Thank you very much for your help. But there is a thing. Why can't we break this symmetry also in a valence band ? There's no different between them, right? Why can't an electric field break symmetri also in the valence band?

daveyrocket

Because the valence band is full, which means that for every available momentum k, there is an electron with momentum k and -k. That's the definition of a full band. If you apply an electric field, that doesn't change which momenta are available, it just lowers the energies of momenta which point in the opposite direction. But if the band is full then this doesn't change the momenta of any electrons because there are no available states for electrons to shift around.

Now if you remove some electrons from the valence band, then when the E field is electrons will move into the empty states that go opposite the E field, providing a net current. When this happens in the valence band this is referred to as hole current. In the conduction band it is electron current.

luuurey

Thanks. Can you please recommend me any material, webside or text where I can read about it more?

Thank you very much.

yinx

Hi,

you might be interested in this series of lectures by keio university

http://www.youtube.com/watch?v=aOVSOIAtjoA

yinx

luuurey

I'm not sure, but he's describing in the video that an electron in valence band cannot move because it's traped. If the electron gets to conduction band then it can flow because it's actually got from the trap.

I'm really not sure of this assertion. I think it's not so simple. I have thought electrons in a valence band can move throught a solid, can't they?