At some stage, for a certain positive potential of plate A, all the
emitted electrons are  collected by the plate A and the photoelectric current
becomes maximum or  saturates If we increase the accelerating potential
of plate A further, the photocurrent does not increase This maximum
value of the photoelectric current is called  saturation current Saturation
current corresponds to the case when all the photoelectrons emitted by
the emitter plate C reach the collector plate A