Energy band theory: conduction of electrons and holes

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richie.harvy
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“We are aware that conduction in most metals is by electrons but conduction mostly by holes does occur in some metals such as beryllium, cadmium and zinc and this is due to their more complicated band structure.”


I find this explanation confusing. I was under the impression that "holes" are just positions that lack an electron where an electron could otherwise exist. But, in the aforementioned description, the author implies that there is a distinction between the conduction of electrons and holes themselves. This is confusing because I don't see how it makes sense to discuss conduction of holes or electrons as separate elements (that is, to discuss the conduction of holes as to the exclusion of the presence of electrons, or vice-versa) that can conduct without the presence of the other, which it seems to me is what the author is implying.

I would greatly appreciate it if people could please take the time to clarify this.

P.s. I’m doing this in A levels physics so please go light on me with the complicated stuff.
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uberteknik
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(Original post by richie.harvy)
“We are aware that conduction in most metals is by electrons but conduction mostly by holes does occur in some metals such as beryllium, cadmium and zinc and this is due to their more complicated band structure.”


I find this explanation confusing. I was under the impression that "holes" are just positions that lack an electron where an electron could otherwise exist. But, in the aforementioned description, the author implies that there is a distinction between the conduction of electrons and holes themselves. This is confusing because I don't see how it makes sense to discuss conduction of holes or electrons as separate elements (that is, to discuss the conduction of holes as to the exclusion of the presence of electrons, or vice-versa) that can conduct without the presence of the other, which it seems to me is what the author is implying.

I would greatly appreciate it if people could please take the time to clarify this.

P.s. I’m doing this in A levels physics so please go light on me with the complicated stuff
You are not wrong with your understanding of electron-hole pairs. Electrons are liberated from their valance band under the influence of an electric field of sufficient magnitude. Good conductors have the property that their atoms only contain one electron in the valance band, which can be liberated and free to migrate and so create a drift of electrons producing an electron current.

The vacated position in the valance band (hole) is now free to capture any free drifting electrons.

The book takes the concept to a higher 'virtual' level of abstraction which is necessary when dealing with semiconductor materials and combinations esp. things like diodes, bi-polar transistors, field effect devices etc.

Picture a conductor with all valance band positions occupied with electrons. Now place that in a circuit where a single electron can be pulled from one extreme end the conductor by say a chemical battery redox reaction. The conductor's vacated electron leaves behind a hole. So far so good.

An adjacent atom can now donate an electron from it's own valence band which migrates to the first hole and this filling that position. But, in turn, the second atom must now posses a new hole. In this way the holes 'appear' to migrate in the opposite direction to the electron migration.

At one end of the conductor, electrons migrate to fill vacant positions at the positive battery terminal. At the other end, electrons enter the conductor provided at the -ve battery terminal. The hole has apparently migrated from one end of the conductor to the other and subsequently positive charge accumulates at the anode of the battery. In reality it's depletion of electrons from that anode which leaves it more net positively charged.

So, both you and the book text are correct. It's just that at a later stage of learning, when dealing with semiconductors, electron-hole pairs, potential barriers, depletion regions etc. all rely on the abstract concept of hole-migration to aid brevity in description.

Here's a video to explain the concept in pictures:

Last edited by uberteknik; 1 month ago
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