I don't understand photodiodes?

  • Thread starter jeebs
  • Start date
In summary, photodiodes work by using a semiconductor material to convert photons of energy into an electron-hole pair, which can then be used in a circuit. They are made from P-I-N junctions and can be forward or reverse biased to control the flow of electrons and holes. When light is shone on the junction, electron-hole pairs are produced, and the key to efficient detection is to separate them before they can recombine. The added intrinsic layer in a PIN diode increases the collection efficiency.
  • #1
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?
 
Physics news on Phys.org
  • #2
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
 
  • #3
granpa said:
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.
 
  • #4
he doesn't need to know that to know how it works.
 

1. What is a photodiode and how does it work?

A photodiode is a semiconductor device that converts light into an electrical current. It works by creating a depletion region within the semiconductor material, which separates the positively charged holes and negatively charged electrons. When light hits the depletion region, it frees electrons from the atoms, creating a flow of current.

2. What are some common applications of photodiodes?

Photodiodes have a wide range of applications, including light sensors, optical communication devices, solar panels, and barcode readers. They are also commonly used in cameras, medical equipment, and industrial automation.

3. How does the size of a photodiode affect its performance?

The size of a photodiode can affect its sensitivity and response time. Generally, larger photodiodes have a higher sensitivity and can detect a wider range of wavelengths. However, smaller photodiodes have a faster response time and are better suited for high-speed applications.

4. What is the difference between a photodiode and a phototransistor?

A photodiode directly converts light into an electrical current, while a phototransistor uses light to control the flow of current in a separate circuit. Additionally, phototransistors have a higher gain, making them more sensitive to light, but also more susceptible to noise.

5. How can I choose the right photodiode for my project?

When choosing a photodiode, consider factors such as sensitivity, response time, size, and spectral range. It is also important to select a photodiode with a suitable package and operating temperature range for your specific application. Consulting with a supplier or doing research on different types of photodiodes can help you make an informed decision.

Suggested for: I don't understand photodiodes?

Back
Top