Functioning of IR-Photodiodes
Exposing a photodiode to light shifts its I-V curve:



(source: Wikipedia)

When the applied bias on the diode is zero or negative (i.e. the cathode is biased higher than the anode), this makes the diode operate approximately as a current source, with output proportional to the incident optical power. The shunt conductance of this current source is fairly low, and does not depend strongly on the optical power.