Conversation started Aug 20, 2019 at 15:07.
Aladdin
Aladdin
Aug 20, 2019 15:07
In the n type material, I don't understand what donor ions and minority carrier come from...
John Rennie
John Rennie
@Aladdin Start with pure silicon. At absolute zero this has no electrons in the conduction band so it is an insulator.
Aladdin
Aladdin
Ok
John Rennie
John Rennie
Now increase the temperature to some temperature $T$ e.g. room temperature.
The average thermal energy avaible to the electrons is $kT$ and this is less than the band gap.
Aladdin
Aladdin
Ok
John Rennie
John Rennie
But that's the average energy. Some electrons have an energy much less than $kT$ and some electrons have an energy much greater than $kT$.
A few electrons have enough energy to jump into the conduction band.
OK so far?
Aladdin
Aladdin
Aug 20, 2019 15:15
@JohnRennie ok
Was talking with mom....
John Rennie
John Rennie
So in silicon at room temperature we have a few electrons in the conduction band, and those electrons left behind holes in the valence band i.e. we have equal numbers of electrons and holes. Both electrons and holes are mobile so they can carry a current.
Aladdin
Aladdin
Ok
John Rennie
John Rennie
So at room temperature silicon is a very poor conductor, not an insulator, due to these tiny numbers of thermally generated electrons and holes.
These thermal electrons and holes are known as the minority carriers.
Aladdin
Aladdin
Aha okay
John Rennie
John Rennie
When we $n$ dope the silicon this injects loads more electrons into the conduction band, but it doesn't change the number of thermally generated electron-hole pairs. So now we have the same (small) number of holes but suddenly shedloads more electrons.
These extra electrons that come from the doping are the majority carriers.
Aladdin
Aladdin
Aug 20, 2019 15:20
Okay. Things are looking clearer now
What about donor ions
John Rennie
John Rennie
@Aladdin the way the donor atoms work is that they produce an energy state in the band gap just below the conduction band i.e. outside the valence band.
The electron from a donor atom jumps into the conduction band, where it can conduct electricity.
But the positive donor ion left behind does not form a hole because it is not in the valence band. The positive charges on the donor ions can't move so they can't carry any current.
Aladdin
Aladdin
The electron from a donor ion jumps into the conduction band, where it can conduct electricity.
Shouldn't it be donor atom?
John Rennie
John Rennie
@Aladdin oops, yes, typo!
Aladdin
Aladdin
Okay. So it makes sense
So since the electron left, the donor atom becomes positive ion
John Rennie
John Rennie
Yes.
Aladdin
Aladdin
Aug 20, 2019 15:27
@JohnRennie But the positive donor ion left behind does not form a hole because it is not in the valence band.
Can you explain this line
John Rennie
John Rennie
@Aladdin let me draw a diagram ...
user image
@Aladdin there!
On the left I've drawn a thermal electron. The electron jumps into the conduction band and leaves behind a hole in the valence band. Both the electron and the hole can move. OK so far?
Aladdin
Aladdin
Ok
John Rennie
John Rennie
Now, the outermost electron of the donor atom has an energy in the band gap just below the conduction band, as drawn on the right.
Aladdin
Aladdin
Ah okay. I get the meaning of that statement now
John Rennie
John Rennie
Because the energy of this electron is only just below the conduction band the thermal energy $kT$ is enough to excite the electron into the conduction band so almost every donor atom puts an electron in the conduction band.
@Aladdin cool :-)
Aladdin
Aladdin
Aug 20, 2019 15:38
That's why doping is so extensively used... The conductivity increase rapidly
 
Conversation ended Aug 20, 2019 at 15:38.