Why Conduction Band? Understanding Its Significance

[snorkack]

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?

 

[mfb]

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).

 

[carlgrace]

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.

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.

 

[snorkack]

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?

 

[carlgrace]

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.