Conduction band filled

  • #1

Summary:

At T = 0K, the valance band of a semiconductor is completely filled, and no current can flow, acting as an insulator. Is there every a situation where the opposite can occur, such that the conduction band is filled (and the valance band is completely empty) forcing the semiconductor to be an insulator?

Main Question or Discussion Point

At T = 0K, the valance band of a semiconductor is completely filled, and no current can flow, acting as an insulator. Is there every a situation where the opposite can occur, such that the conduction band is filled (and the valance band is completely empty) forcing the semiconductor to be an insulator?
 

Answers and Replies

  • #2
411
162
Is there every a situation where the opposite can occur, such that the conduction band is filled (and the valance band is completely empty) forcing the semiconductor to be an insulator?
Light-induced transparency effects loosely fits your description. At extreme end of photon power densities, even metals like aluminum can be made transparent, likely making them insulating too. The impinged power would destroy sample before DC conductivity measurement can be made though.
https://www.sciencedaily.com/releases/2009/07/090727130814.htm
 
  • #3
So I should not think of electrons in the conduction band as “a sea of electrons” like in metals, but actually having a maximum capacity of how much electrons can fill the band?
 
  • #4
411
162
So I should not think of electrons in the conduction band as “a sea of electrons” like in metals, but actually having a maximum capacity of how much electrons can fill the band?
The number of energy levels in energy band is simply twice of number of atoms in sample. You can exceed a capacity of normally free or half-free band, but only in harshest conditions not found in typical electric circuits - i.e. by ionizing each atom in sample.
Lower energy work-around with 3rd band which give similar effect do exist too - see Gunn effect.
 
  • #5
In the example you gave, why would transparent Al be insulating?
I understand from the perspective of when the valance band and full and conduction band is empty, but why did you mention that when the conduction band is full, in the case of transparent Al, the material is insulating?
 
  • #6
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In the example you gave, why would transparent Al be insulating?
I understand from the perspective of when the valance band and full and conduction band is empty, but why did you mention that when the conduction band is full, in the case of transparent Al, the material is insulating?
Because light reflection by metals is direct consequence of their conductivity. Search for "metal plasma frequency".
 
  • #7
To clarify, I want to restate my inquiry. At zero temperature, the valance band is completely filled, and every available energy state is occupied. In order that net charge can flow, there must be empty states available for electrons to move into, which is not the case for a completely filled valance band. The exchange of electrons between any pair of states does not represent a flow of charge. In this 0K scenario, the semiconductor behaves as an insulator.

My question now is: is there a scenario where the conduction band is completely filled, and every available energy state is occupied such that no empty states exist for electrons to move into, thus forcing the material to be an insulator.

I think the scenario described is different than transparent Al. Can a filled conduction band conduct under a voltage?
 
  • #8
411
162
My question now is: is there a scenario where the conduction band is completely filled, and every available energy state is occupied such that no empty states exist for electrons to move into, thus forcing the material to be an insulator.

I think the scenario described is different than transparent Al. Can a filled conduction band conduct under a voltage?
Well, this is situation similar to p-type semiconductor under reverse (negative) bias. Depletion region would form due electrons completely filling the band where conduction possible with zero bias. The difference from your inquiry is what P-type semiconductor conduction happens in (nearly-filled) valence band, not conduction band. As i said before, completely filling conduction band (not a valence band) requires a tremendous power input, not going to happen in normal electronic circuits.
 
  • #9
analogdesign
Science Advisor
1,139
354
You can’t fill the conduction band because if you increase the energy of a charge carrier in the conduction band it leaves space for a new carrier but remains in the conduction band.
 
  • #10
113
35
In the example you gave, why would transparent Al be insulating?
I understand from the perspective of when the valance band and full and conduction band is empty, but why did you mention that when the conduction band is full, in the case of transparent Al, the material is insulating?
Aluminum in its ground state is highly reflecting up to about 15 eV. This high reflectivity is from the electrons in the highest band. Above 15 eV the reflectivity drops and the reflectivity is dominated by transitions between bands below the Fermi energy and above the Fermi energy.

Now, the news item mention that they excited a core electron from every atom. By this they mean that they excited a deep, flat band. So, in fact, they didn't do away with the conduction electrons. What did they do? Well, they mentioned that the aluminum is transparent in the Extreme UV. Our plasma edge at 15 eV is just at the beginning of the EUV where interband transitions dominate. They emptying of the band made some interband transitions not possible and thus the reflectivity drops.
 
  • #11
113
35
To clarify, I want to restate my inquiry. At zero temperature, the valance band is completely filled, and every available energy state is occupied. In order that net charge can flow, there must be empty states available for electrons to move into, which is not the case for a completely filled valance band. The exchange of electrons between any pair of states does not represent a flow of charge. In this 0K scenario, the semiconductor behaves as an insulator.

My question now is: is there a scenario where the conduction band is completely filled, and every available energy state is occupied such that no empty states exist for electrons to move into, thus forcing the material to be an insulator.

I think the scenario described is different than transparent Al. Can a filled conduction band conduct under a voltage?
Let me give you some background then answer your question.

Any completely filled band does not conduct. The reason is because band structures have a symmetry such that all the momenta of the electrons average to zero in a filled band. If instead you had some empty states then applying a voltage shifts the distribution ever so slightly and now the average momentum of the electrons is not zero and you now have an electronic current.

If you could by some process move all the valence band electrons to the conduction band, you would have an unstable state. In one sense the electrons would tend to emit photons and go back to the valence band. In another sense you've changed the nature of one of the things greatly contributing to the binding together of the metal, and the results will probably be catastrophic for the metal's structure.

If you really want to fill that band in a sense you can do it. You can add a significant amount of a particular dopant atom and it could fill the band, but in the end you've got a material with a different chemical composition and crystal structure, among other things.
 

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