Are there free electrons in the valence band?

  • #1
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Main Question or Discussion Point

If not: then how do the holes in a semiconductor manage to conduct electricity (if they can't travel through the solid)?

If there are:
1) then what is the difference between the valence and the conduction band?
2) then why do the electrons in a semiconductor need to reach the conduction band to conduct? (cause they should also be able to conduct the current through the valence band)

I thank you very much!

EDIT: for the purists, of course I don't mean an actual free electron, just one that can travel through the solid
 

Answers and Replies

  • #2
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I'm no expert on this, just a lowly student, but I thought that it was called being in the valence band because the electron was bound in the valence shell of its atom and couldn't move, in the sense of a net flow of charge.
I've always thought that they could move around individually because the atoms have covalent bonds, so the individual electrons can still move around between them?
Wouldn't this mean that a hole (ie. a gap where a valence electron should be?) wouldn't have any problem moving through the material either when a potential difference is applied?
I think I asked a lecturer about this a while ago and he said something along these lines anyway.
 
  • #3
Drakkith
Staff Emeritus
Science Advisor
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In semiconductor materials, the electrons normally have enough energy to occupy the valence band, but not the conduction band. Electrons in the valence band can move up to the conduction band, it simply takes a "kick" of energy to make them jump the gap between the valence band and to the conduction band. This gap is called a Band Gap. In conductors, the valence and conduction bands overlap, while in semiconductors they do not, hence the electrons needing a kick of energy provided by a potential voltage to "conduct". Once they are kicked out, they leave behind a hole which can be filled by another electron. Below a certain voltage, you will not have current flow through a semiconductor, as the electrons are not getting enough energy to make the jump. This is also mostly true in an insulating material, but the band gap is larger, which makes them poor materials to build electronic components out of. Give any material a high enough potential difference, and it WILL conduct.

Also, the electrons in the conduction band are free to move around the material with little resistance, while in the valence band the electrons are usually bound to the parent atom/molecule, but they do have some degree of freedom, just not as much as the conduction electrons. Look up valence band on wikipedia.
 
  • #4
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In conductors, the valence and conduction bands overlap, while in semiconductors they do not, hence the electrons needing a kick of energy provided by a potential voltage to "conduct". Once they are kicked out, they leave behind a hole which can be filled by another electron. Below a certain voltage, you will not have current flow through a semiconductor, as the electrons are not getting enough energy to make the jump. This is also mostly true in an insulating material, but the band gap is larger, which makes them poor materials to build electronic components out of. Give any material a high enough potential difference, and it WILL conduct.
There is some confusion here. In semiconductors the electrons are "kicked" from valence to conduction band by the thermal excitation and not by the external voltage.
What is described in the quote above is more like electrical breakdown than normal conduction. A semiconductor has a band gap low enough that the thermal motion excites a significant number of electrons in the conduction band, at room temperature. The electric field induces a drift motion of these electrons that are already in the conduction band.
An insulator has a larger gap so that there are very few electrons, at room temperature.

An there is no threshold voltage for a common semiconductor to conduct.
Maybe you mixed it with the p-n junction.
 
  • #5
Drakkith
Staff Emeritus
Science Advisor
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Ah, my mistake, i confused the two.
 

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