What Is a Photodiode?
1 min read
Quick answer
A photodiode is a diode that converts the light falling on it into electric current. It can be thought of as the opposite of an LED: an LED emits light, a photodiode senses it.
How Does It Work?
When light (photons) hits its junction, electron–hole pairs form and produce a small current proportional to the light intensity. It's usually used under reverse bias.
Difference From an LED
| Property | LED | Photodiode |
|---|---|---|
| Job | Emits light | Senses light |
| Bias | Forward | Usually reverse |
| Output | Light | Current |
Operating Modes
Photoconductive mode (reverse bias) is fast and linear; photovoltaic mode (no bias) generates a voltage like a solar cell and is less noisy.
Where Is It Used?
Light sensors, optical remote-control receivers (IR), optical communication, optocouplers and analog light measurement.
Common Mistakes
- Getting the direction wrong: if the reverse-bias direction isn't connected correctly you won't get the expected response.
- Skipping the need for an amplifier: the output current is very small; an amplifier is usually needed.
- Ignoring ambient light: stray light corrupts the reading; a filter/optical arrangement may be needed.
Frequently Asked Questions
Is a photodiode the same as a solar cell?+
It works on a similar principle; a solar cell is optimized to generate energy, a photodiode to measure light.
Why is it used under reverse bias?+
Reverse bias speeds up the response and makes the light–current relationship more linear.
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