How does a photodiode behave in saturation?
OP: "...please explain how would a photo diode behave in saturation which would explain my doubt in total, is it like a close switch or a constant current source with current being peak current"

A quick Google gave me two useful results:

"Once the Saturation point is reached, the behavior of the photodiode becomes more and more non-linear until the Damage threshold point is reached. At this point the photodiode is no longer able to turn excess in incident optical power into a photocurrent output and all the extra energy received by the device would be absorbed, typically as heat. This heat inside the device can easily damage it irreversibly as it endangers the fragile wire bonds on the surface of the photodiode."

Source: http://www.osioptoelectronics.com/technology-corner/frequently-asked-questions/input-light-intensity.aspx



"As the photocurrent increases, first the non-linearity sets
in, gradually increasing with increasing photocurrent, and finally at
saturation level, the photocurrent remains constant with increasing
incident light power."

Source: http://www.osioptoelectronics.com/application-notes/an-photodiode-parameters-characteristics.pdf



PS: Seems that the question is too old. I have no idea why it appeared in recent questions. Maybe a Community edit...