Hi,
Good question. I think delta source would be ideal because you can easily specify the number of electron hole pairs (ehp) to be injected in the simulation. With only one ehp simulated I expect the current to be very low. The delta source works with transient simulation so you should be able to obtain I-t.
You can also obtain the collection probability of the ehp vs it's location; this can be done by enabling the "map current collection probability" option in the contacts. This approach is used in our example here (please refer to step 4): https://optics.ansys.com/hc/en-us/articles/360042358574-CMOS-image-sensor-Angular-response-3D
Let us know if you have further questions.
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