Why is photo diode reverse bias while LED is forward bias?
The most used applications are to emit light, while the photodiode detects the presence of light. when it is facing forward, it emits light. in reverse bias mode, it functions as a normal diode and does not emit light. for photodiode, it’s exactly the opposite of LEDs. in direct polarization mode, it functions as a norma diode without special properties. in reverse polarization mode, it starts driving when photons of light strike the p-n junction. this helps to detect the presence of light at the end of the circuit.
When the diode is forward biased, the electrons in the conduction band of the semiconductors recombine with the holes in the valence band, releasing enough energy to produce photons emitting monochromatic light (monochrome). because of this thin layer, a reasonable number of these photons can leave the junction and radiate, producing a colored luminous flux.
then we can say that, when used in a forward-biased direction, light-emitting diodes are semiconductor devices that convert electrical energy into light energy.
where, as in the photodiode, works in a different way,
the only reason is that a photodiode converts incident light into electric current more efficiently under reverse bias conditions than direct polarization. it is not very useful in the direct polarization sense, it will behave almost like a normal diode.
why is it true? it concerns the extension of the region of exhaustion. you should note that when the absorbed photons generate electron-hole pairs, only the electron-hole pairs generated in the depletion region, or very close to it, have a chance to contribute to the electric current because there is a strong electric field to separate the two different charge carriers. those outside the depletion region recombine rapidly and disappear.
Now, in a reverse-biased pn junction, the width of the depletion region increases as you increase the reverse bias voltage applied across the diode (proportional to the square root of the voltage). thus, by applying a higher voltage, more incident photons are converted into electrical current, or the efficiency increases (as long as you make sure that the increased leakage current remains at a manageable level) by cons, when you polarize into before a pn junction, the width of the depletion region decreases, so that only a small portion of the incident photons is converted into electric current.
you can transfer a photodiode diagonally and maintain the current level at the micro-amp. it still works like a photodiode, but with a fraction of efficiency compared to a reverse bias.