Where q=1.6x10-19C, is the electron charge, IP is the photogenerated current, ID is the photodetector dark current and Δf is the noise measurement bandwidth. Shot noise is the dominating source when operating in photoconductive (biased) mode.
Thermal or Johnson Noise
The shunt resistance in a photodetector has a Johnson noise associated with it. This is due to the thermal generation of carriers. The magnitude of this generated noise current is: