A photodiode consists of an active p-n junction which is operated in reverse bias. When light falls on the junction, a reverse current flows which is proportional to the illuminance. The linear response to light makes it an element in useful photodetectors for some applications. It is also used as the active element in light-activated switches.
Photodiode Mechanism
The mechanism of the photodiode is like that of a (miniaturized) solar cell. Their response time is fast, on the order of nanoseconds. As light detectors, they are reverse biased - the reverse current is linearly proportional to the illuminance striking the diode. They are not as sensitive as a phototransistor, but their linearity can make them useful in simple light meters.
Photodiode Characteristics
The reverse current through a photodiode varies linearly with illuminance once you are significantly above the dark current region.
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