IIT JEE , AIEEE Forum - Physics , Modern Physics

drritu

why photo diode is operated in reverse bias and LED in forward bias?

eragon007

the photo diode is operated in reverse bias but the led is operated in forward bias ..

ans the photo diode are kept in rev bias becuz when light falls on the junction many e-p pairs are created near the junc these minority carriers cross the junction and contribute to the current generated by the heat and so the current gets amplified

in LEDs the electrons(as current) moving in the forward biased diode cross the junction the get combined with the holes and release a lot of energy in the form of light

i hope u got it

vsraghuvar

hai. im a learner plz correct me if my ans. is not apt.
A photodiode is a p-n junction or p-i-n structure. When a photon of sufficient energy strikes the diode, it excites an electron thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region, producing a photocurrent.

Photodiodes can be used under either zero bias (photovoltaic mode) or reverse bias (photoconductive mode). In zero bias, light falling on the diode causes a current across the device, leading to forward bias which in turn induces "dark current" in the opposite direction to the photocurrent. This is called the photovoltaic effect, and is the basis for solar cells ? in fact, a solar cell is just a large number of big photodiodes.

Reverse bias induces only little current (known as saturation or back current) along its direction. But a more important effect of reverse bias is widening of the depletion layer (therefore expanding the reaction volume) and strengthening the photocurrent. Circuits based on this effect are more sensitive to light than ones based on the photovoltaic effect and also tend to have lower capacitance, which improves the speed of their time response. On the other hand, the photovoltaic mode tends to exhibit less electronic noise.

Extraa information(may be useful):

Avalanche photodiodes have a similar structure, but they are operated with much higher reverse bias. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device.

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