This voltage may be as low as 10 mV or as high as 50 V and the dark currents may vary from pA to μA depending upon the junction area and the process used. Dark current is always specified at a particular value of reverse applied voltage. (optoelectronics.perkinelmer.com)

The photodiode noise in is the sum of the thermal noise (or Johnson noise) ij of a resistor which approximates the shunt resistance and the shot noise isD and isL resulting from the dark current and the photocurrent. (Hamamatsu Corporation)

Dark current is dependent on temperature. While dark current may initially be low, it will increase as the device temperature increases. (tpub.com)