O. Y. Raisky has written:

'Investigation of photoluminescence and photocurrent in InGaAsP/InP strained multiple quantum well heterostructures' -- subject(s): Photoluminescence, Solar cells, Heterostructures

The output current of a PIN diode is proportional to the light incident . Whereas in APD due to internal multiplication (Avalanche multiplication) of primary photocurrent, higher gains are possible.

A small signal current output from the pin diode are to be amplified before applying to next associated receiver circuit. This includes thermal noise in the main signal, giving lower, receiver sensitivity. In case of APD, photocurrent is multiplied before encountering the thermal noise amplifier, giving increased receiver sensitivity.

The increased frequency increases the kinetic energy of the single electron ejected. Remember that the incident light releases a single electron when the threashod frequency is reached

The stopping voltage where the current goes to zero is proportional to the KE of the electron.

Simply detecting a photocurrent doesn't tell you the electron's energy. You could try and find the photon energy where the PEE just barely works, but what if you don't have a continuously-tunable light source? (which most people don't). But a retarding potential is easily tuned.

There are two root words. 'Photo' comes from the Greek photos, meaning light, and 'graph' comes from graphos, also Greek, meaning writing or painting.