- #1
jeebs
- 325
- 4
I'm trying to find out about how photodiodes work. I'm aware that they are used as light detection instruments, as they are made out of a semiconductor where photons of energy greater than the band gap can create an electron-hole pair. The electron and hole can be made to flow into a circuit - an applied electric field will send them in opposite directions, meaning that they can enter the circuit the photodiode is part of and cause a spike in current.
that much is fine.
Also, I see that they are made from "P-I-N junctions", which seem to be a thin region of undoped semiconductor bounded on either side by a region of acceptor (P) impurity and donor (N) impurity. I'm not sure what the point of these doped regions are though.
I mean, I get that if you have a P-N junction, the excess electrons near the boundary in the N region will move across and combine with the holes near the boundary in the P region (until the electric field this creates stops any further electrons moving across, right?)
If I'm not mistaken this is called the depletion region. It gets mentioned in all the stuff I've read on photodiodes so far, and I still don't see the significance of it.
Also, I keep hearing about "forward and reverse bias". I'm thinking that if you connect the P side of the photo diode to the negative terminal of a battery/source you get all the holes moving towards the negative terminal, and the N-region free electrons would go to the positive terminal- that's forward bias right?
Well, wouldn't that just make it be as if you had a completely undoped piece of semiconductor then?
Why bother doping at all if you've got an E field applied that sweeps your newly created electrons and holes into the circuit - what does the doping achieve?
that much is fine.
Also, I see that they are made from "P-I-N junctions", which seem to be a thin region of undoped semiconductor bounded on either side by a region of acceptor (P) impurity and donor (N) impurity. I'm not sure what the point of these doped regions are though.
I mean, I get that if you have a P-N junction, the excess electrons near the boundary in the N region will move across and combine with the holes near the boundary in the P region (until the electric field this creates stops any further electrons moving across, right?)
If I'm not mistaken this is called the depletion region. It gets mentioned in all the stuff I've read on photodiodes so far, and I still don't see the significance of it.
Also, I keep hearing about "forward and reverse bias". I'm thinking that if you connect the P side of the photo diode to the negative terminal of a battery/source you get all the holes moving towards the negative terminal, and the N-region free electrons would go to the positive terminal- that's forward bias right?
Well, wouldn't that just make it be as if you had a completely undoped piece of semiconductor then?
Why bother doping at all if you've got an E field applied that sweeps your newly created electrons and holes into the circuit - what does the doping achieve?