the anode bias current does not change with time: since charge transport in a junction diode is mainly due to diffusion, i.e. to   a non constant spatial charge carrier density caused by bias voltage, a charge Qs is stored in the device.
Since the anode bias current remains stable over time, it can be attributed to the fact that charge transport in a junction diode is mainly governed by diffusion, which is influenced by the non-uniform spatial distribution of charge carriers caused by the applied voltage. Consequently, a stored charge Qs persists within the device.