On the N side, when you add dopant atoms they create gap states just under the conduction band. Electrons are thermally promoted from the gap states into the conduction band so now the conduction band contains electrons that are free to move. These electrons can carry a current. So that's N type conduction. Yes?
But when you add the dopant this creates new energy states for the electron that have an energy in between the bands. They are more like the states you get ina single atom like a hydrogen atom.
These are the gap states.
The dopants are carefully chosen to have gap states in exactly the right place i.e. just above the valence band or just below the conduction band.
On the N side, when you add dopant atoms they create gap states just under the conduction band. Electrons are thermally promoted from the gap states into the conduction band so now the conduction band contains electrons that are free to move. These electrons can carry a current. So that's N type conduction. Yes?
OK. On the P type side the gap states are electron acceptors, so electrons are thermally promoted from the valence band into the gap states. This leaves holes in the valence band. The holes allow current to flow in the valence band. So that's P type conduction.
On the N side of the diode an electron flows from the wire into the diode and it goes into the conduction band. The conduction band now has one too many electrons. Yes?
Now an electron crosses the junction and drops down to fill a hole on the P type side. This brings the number of electrons in the conduction band on the N side back to normal.
However the electron that crosses the junction fills the hole, so the valence band on the P side now has one too few holes. OK so far?
So in the final step an electron flows out of the valence band on the P side into the wire. This leaves behind a hold, and this brings the number of holes on the P side back to normal.
The end result of the three steps is that one electron flowed into the diode and one electron flowed out the other side.
On the P side we typically add trivalent atoms like gallium. These create gap states just above the top of the valence band. The gap states can accept an electron from the valence band so the gap state gets a negative charge and it leaves behind a positive hole in the valence band. The electrons in the gap states are tightly bound and cannot move, so they don't contribute to the current.
Yes. The energy difference between the top of the valence band and the gap states is less than $kT$ at room temperature, so electrons are thermally excited into the gap states.
So in the final step an electron flows out of the valence band on the P side into the wire. This leaves behind a hold, and this brings the number of holes on the P side back to normal. The end result of the three steps is that one electron flowed into the diode and one electron flowed out the other side.
@JohnRennie So why is there an electron in valence band as u said in ur first sentence
Because one electron flowed out of the wire into the conduction band on the N type side, so one electron has to flow out of the diode into the wire on the P type side. The flow of these electrons is being driven by the external voltage we are applying to the diode.
Problem Solving Strategies
Problem Solving Strategies