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Why is photodiode operated in reverse bias

Why is photodiode operated in reverse bias?

a reverse-biased pn junction receives only a small leakage current. the photocurrent generated by the incoming light is therefore easy to detect. if the junction is forward biased, a large current passes through it. the extra photocurrent is much harder to detect.

a photodiode can be effectively used in reverse bias because, in reverse bias, the current in the pn junction varies almost linearly with the intensity of the light. when a reverse biased voltage of a few tenths of volts is applied, a nearly constant current (the dark current) is obtained. Now, if the light falls to the surface, additional electron-hole pairs are generated which, because of the high amplitude of the reverse voltage at the junction, are easily scattered and pass through to provide the current.

if the photodiode is live biased, it will act as an ordinary silicon diode that bypasses any voltage greater than 0.6 to 0.7v. will be reflected in the connected circuit according to the design. it is rather acceptable to use it as an effective utility component than to accept it as if it were a Zener diode only lowering 0.6 to 0.7 V and all the energy at above this barrier will be ignored and that light variations can not be measured or controlled Level.

in fact, it is very simple. First, let me help you work in a nutshell. As the light beam decreases, photon energy is delivered to generate charge carriers. these charge carriers are attracted by the opposite potentials of the battery and current flows.


see the generated charge carriers are opposite charges, they can easily recombine. They must be separated immediately after being generated, this is done by electric field / electrical potential in the junction.

as you know in reverse bias conditions, the junction potential is very large while the low in direct condition. stronger the field, easily separating so greater efficiency and smooth work without short circuit.

conclusion: a photodiode can be used in both modes, but the reverse bias is preferable because of the above reason because of the stronger field.

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