I don't understand photodiodes?

[jeebs]

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?

 

[granpa]

you are overthinking it.

when forward biased current flows easily and electrons and holes recombine at the junction
when reverse biased current flows very poorly and electrons and holes are produced at the junction.
when you shine light on the junction the photons produce electron hole pairs at the junction.

thats all you really need to know

 

[caffenta]

when forward biased current flows easily and electrons and holes recombine at the junction
when reverse biased current flows very poorly and electrons and holes are produced at the junction.
when you shine light on the junction the photons produce electron hole pairs at the junction.

thats all you really need to know

I'd say that's underthinking it.

The key to making an efficient photodiode is to separate the generated electron/hole before they can recombine. Inside the PN junction, the built-in electric field is high, so that's where the detection is most efficient. Reverse biasing the junction makes the field even higher. The added intrinsic layer in the PIN diode makes the junction wider and increases the collection efficiency.

 

[granpa]

he doesn't need to know that to know how it works.