The photodiodes are mainly used as a switch, that is to say when the light comes on. now the current generated by a photon is very small.
If the diode is live-biased, there will be no noticeable change in the current. but if it is reverse biased, the current changes tremendously, it can be used as a switch to detect light.
all diodes have a leakage current in the opposite direction. it’s necessarily small. otherwise, the device would not be a diode.
leakage current is very non-linear with respect to applied voltage, temperature, elbow voltage, etc. The cause of these various sensibilities is well understood and, for any particular construction, it is very reproducible. Diode manufacturers are able to optimize the construction taking into account certain characteristics.
Small signal diodes, such as those used in AM detectors, precision rectifiers, etc., are optimized so that leakage characteristics are as linear as possible for the first volts, in order to preserve a predictable ratio between currents going on and off in the amp-op circuits.
Rectifier diodes are designed to minimize leakage. Zener diodes are designed with an abnormally low bend voltage, which can be exploited as a voltage reference. The temperature sensing diodes are optimized for a relatively large leak, with a deep offset when the temperature changes. the temperature offset in other diodes is normally minimized
all diodes are photo-sensitive to a greater or lesser extent, incoming photons encouraging leaks. this effect is normally eliminated and then eliminated by the packaging. in the photo diodes, it is optimized as much as possible in the construction of the doping and the die, and a transparent package is used so that the photosensitive behavior of the photo diodes occurs in this quadrant of applied voltage compared to the observed current which is the biased condition in reverse.
this is how we configure the circuit to exploit the characteristic that is important to us. they are still a diode and would still transmit a much higher current if they were live biased. but it is the minimal changes in the smallest reverse-biased current that interest us. So we designed the circuit to access it.