Not quite sure of the intent of the sentence:
Light entering the top of the photodiode stack fall off exponentially in with depth of the silicon.
But this may be correct:
The intensity of the light entering the top of the photodiode stack falls off exponentially as a function of depth.
Light entering the top of the photodiode stack fall off exponentially in with depth of the silicon.
But this may be correct:
The intensity of the light entering the top of the photodiode stack falls off exponentially as a function of depth.