Why Conduction Band? Understanding Its Significance

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Discussion Overview

The discussion centers around the concept of the conduction band in insulators, exploring its significance and the behavior of electrons and defects within this framework. Participants examine the conditions under which electrons can become delocalized and conductive, as well as the implications of defects in the band structure.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question why insulators are said to possess a conduction band if it is typically empty, raising concerns about the delocalization of electrons after crossing the band gap.
  • There is a suggestion that even if an electron gains enough energy to cross the band gap, it could lose energy through phonon interactions and become trapped as a defect, remaining immobile and nonconducting.
  • One participant notes that while an electron could become trapped, it would leave the conduction band and would require an empty state at a lower energy to do so.
  • Another participant emphasizes that all solids have a valence and conduction band, and the distinction between insulators and conductors lies in the occupancy of these bands.
  • Discussion includes the idea that occupied states for immovable electron defects exist in the bandgap, and that the presence of defects can affect the overall conductivity of the material.
  • Questions are raised about the location of immovable hole defects in relation to mobile charge carrier holes, with some suggesting that both types of defects are found in the bandgap.
  • It is mentioned that holes typically occupy states near the valence band edge, while electrons occupy states near the conduction band edge.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the behavior of electrons and defects in the conduction band, indicating that multiple competing views remain and the discussion is unresolved.

Contextual Notes

There are limitations regarding the assumptions made about the behavior of electrons and defects, as well as the definitions of states within the bandgap. The discussion does not resolve the complexities surrounding these concepts.

snorkack
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Why are insulators supposed to possesses a "conduction band", even if usually empty?

If you do take the energy to cross the "band gap" and displace an electron, just why should it become delocalized/conductive? Couldn´t it just lose energy rapidly by exciting phonons until it gets trapped somewhere as an electron or anion defect, completely immobile and nonconducting in a weak field?
 
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If you do take the energy to cross the "band gap" and displace an electron, just why should it become delocalized/conductive?
As a simplified model: it has so much energy that it is not bound to individual atoms.

Couldn´t it just lose energy rapidly by exciting phonons until it gets trapped somewhere as an electron or anion defect, completely immobile and nonconducting in a weak field?
Yes, but then it leaves the conduction band (and it needs an empty place with lower energy to do that).
 
snorkack said:
Why are insulators supposed to possesses a "conduction band", even if usually empty?

All solids contain a valence and conduction band. Whether it is normal empty or normally occupied is by definition the difference between an insulator and a conductor. It gives a unified treatment of all solids.

snorkack said:
If you do take the energy to cross the "band gap" and displace an electron, just why should it become delocalized/conductive? Couldn´t it just lose energy rapidly by exciting phonons until it gets trapped somewhere as an electron or anion defect, completely immobile and nonconducting in a weak field?

To add a bit to mfb's explanation, the "somewhere" you mentioned is typically a state located in the bandgap. The bandgap is only free of energy states when the solid is defect free.
 
So the occupied states for immovable electron defects are in the bandgap - as are the unoccupied states for immovable electron defects that might exist but do not.

Where are immovable hole defects, compared to the mobile charge carrier holes? Are both in the valence band?
 
snorkack said:
So the occupied states for immovable electron defects are in the bandgap - as are the unoccupied states for immovable electron defects that might exist but do not.

Where are immovable hole defects, compared to the mobile charge carrier holes? Are both in the valence band?

The hole defects are also in the bandgap. Typically holes occupy states close to the valence band edge and electrons occupy states close to the conduction band edge.
 

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