A semiconductor material, in the dark, at a fixed temperature, has a concentration of holes and electrons determined by the doping level. The product of the concentration of donors and the concentration of acceptors is a constant, ni2, for a given type of semiconductor, at a given temperature. For silicon ni equals 1E10.
Light hitting a semiconductor material creates electron-hole pairs, i.e. additional carriers, which disturb the equilibrium. The additional carriers increase the conductivity of the semiconductor. When the light shuts off, the excess recombine to return to equilibrium. The instantaneous rate of recombination is greater when the excess carrier concentration is greater, and thus the recombination process is one of exponential decay. In other words, if one were to plot the conductivity as a function of time, the resultant curve would be an exponential decay. The time constant of this curve is, by definition, the carrier lifetime. The process is called Photo Conductive Decay (PCD).
