Why photodiode is operated in reverse bias?

[hanii]

why photodiode is operated in reverse bias??

can anyone please explain why photodiode is operated only in reverse bias?? can we operate it forward bias state also??

 

[leright]

Good question. I think it's because when reverse biased the depletion region becomes wider. Only electron-hole pairs that are generated in the depletion region (or within a diffusion length of the depletion region) will be swept across the junction and contribute to reverse current. So having a large depletion region is beneficial. You can also operate the PD with no bias but it will not be as efficient.

-Matt Leright

 

[hanii]

so if we operate it in forward bias...the depletion layer length would be less compared to that as in reverse biased condition...there by reducing its sensitivity . why only the depletion layer is exposed to light... not all the device..?

 

[leright]

so if we operate it in forward bias...the depletion layer length would be less compared to that as in reverse biased condition...there by reducing its sensitivity . why only the depletion layer is exposed to light... not all the device..?

yes. Plus I think the forward bias current is much higher than the photocurrent generated.

The reason only ehps generated in the depletion region (or very close to the depletion region) contribute to current is because those are the ehps that are in an electric field. Remember the region where there is a slope in the band diagram there is an e-field. the flat regions there is no field so no current.

 

[hanii]

so biasing only helps to improve the reverse current ...?? if it is so...can we get the reverse current flow through the load and produce potential difference across it without biasing the diode...??

 

[leright]

so biasing only helps to improve the reverse current ...?? if it is so...can we get the reverse current flow through the load and produce potential difference across it without biasing the diode...??

The PD will have some reverse current when light is shined on it with no bias, yes.

-Matt