The V-I characterisric of p-n junction diode is given below:
(i) Under the reverse bias condition, the holes of /)-side are attracted towards the negative terminal of the barter) and the electrons of the z-side are attracted towards the positive terminal of the battery. This increases the depletion layer and the induced potential barrier is also increased. However, the minority charge carriers are drifted across a junction producing a small current. At any temperature, a number of minority carriers is constant, so there is the small current at and applied potential. This is the reason for the current under reverse bias known as reverse saturation current which is almost independent ot applied
potential. At the certain level of voltage, avalanche breakdown takes place which results in a sudden flow of large current.
(ii) At the critical voltage, the voltage at which breakdown takes place, the holes in the ;z-side and conduction electrons in the />-side are accelerated due to the reverse bias voltage. These minority carriers acquire sufficient kinetic energy from the electric field and collide with a valence electron. Thus, the bond is finally broken and the valence electrons move into the conduction band resulting in enormous flow of electrons and thus, formation of electron-hole pairs.
Thus, there is a sudden increase in the current at the critical voltage.
Zener diode is a semiconductor device which operates under the reverse bias in the breakdown region.