The influence of nonlinear time-dependent transport on charging effect
s in small semiconducting quantum dots is studied. Due to absorption o
r emission of energy quanta, additional resonant features are obtained
in the Coulomb oscillations and the Coulomb staircase which might by
accessible to experimental observation. For an AC-bias voltage the sys
tem is shown to act like an electron pump whereas Ohm's law is obtaine
d for strong time dependent coupling.