Now let’s analyze that what would happen when we change diode state from forward biase from reverse biase. This state change takes time which is known as reverse recovery time. Consider the following circuit of diode to analyze the switching time of diode.
When the diode is in forward biase, the minority charge distribution of diode is as:
And when diode is in reverse biase position, the minority charge distribution is as:
So to change state from forward to reverse biase, the whole minority charge distribution needs to be inverted as we can see from the figures above.
Now let’s see what happens during the period in which state changes.
Firstly we are in forward biase state when voltage applied is +V. So there are many minority carriers near the junction and then there is an exponential decrease in the concentration of minority carriers and there is a continuous flow of majority carriers across the junction. We assume the current as I in the forward biase. We depict this in the following graph of current across the junction with time:
Now we change the applied voltage to –V at time t=t1 i.e. diode is now reverse biased. As minority carrier concentration in both sides was large near junction in the forward biase, when we have instantly changed the state to reverse biased, those minority carriers start moving in the opposite direction. And due to large concentration of such minority carriers, the amount of current flowing remains the same, only direction changes as shown below:
*Red coloured lines are of reverse biased region.
But the high reverse current continues for small time because the concentration of the stored minority carriers start decreasing and the current also starts decreasing exponentially as shown below:
The time gap t2 – t1 in which the reverse current is high (i.e. equal to I) is known as storage time and the time gap from t2 to t3 i.e. the time reverse current becomes equal to reverse saturation current is known as transient time. The total time from t1 to t3 is known as reverse recovery time.