The switching time of a diode is defined as the time which a diode takes to change its state from forward biased state to reverse biased state or in other words the forward current through diode doesn’t reduce to reverse saturation current immediately as the reverse voltage is applied. In fact it takes time for the current to reduce from forward current to reverse saturation current. This time is also called reverse recovery time.
To discuss more about the switching time, we first need to discuss charge distribution of diode in normal state, forward biased state and reverse biased state assuming doping of p-type is more than n-type.