The PN junction diode acts like a perfect conductor in forward biased configuration and acts like a perfect insulator in reverse biased configuration. During the switching time from forward to reverse biased condition the current flow switches and remains constant at the same level. This time duration over which the current reverses and maintains constant level is called as storage time (Ts).
The time taken by the electrons to move from P – type back to N- type and holes to move from N – type back to P – type is the storage time. This value can be determined by the geometry of the PN junction. During this storage time the diode behaves as a short circuit.
Transition Time :
The time for the current to decrease to a reverse leakage current value after it remains at a constant level is called as a transition time. It is denoted as the transition time value is determined by the geometry of the PN junction and concentration of doping levels of the P – type and N – type materials.
Reverse Recovery time :
The sum of the storage time and transition time is termed as reverse recovery time. It is the time taken by the diode to raise the applied current signal to 10% of the constant state value from the reverse leakage current. The reverse recovery time value for PN junction diode is usually of the order of microseconds.
Its value for a widely used small signal diode rectifier 1N4148 is usually 4 ns and for general purpose rectifier diode it is 2 μs. The fast switching speeds can be achieved by the high value of reverse leakage currents and high forward voltage drops. It is denoted by Trr.
