`Zener diode is just like a normal p-n junction diode but, heavily doped as compared to normal diode.
As i mentioned above, zener diode also follows the same graph for forward bias characteristics. But as we check reverse bias characteristics, it is not that similar as that of normal p-n junction diode.
For reverse bias characteristics, zener diode follows zener breakdown rather than following avalanche breakdown.
Why avalanche breakdown happens?
As i told you above, zener diode is heavily doped as compared to normal p-n junction diode. So, when we connect zener diode in reverse bias configuration it will follow the same graph of zener breakdown.
Now, why zener breakdown happens?
At every diode in reverse bias configuration, there exists a reverse breakdown potential. So, here also in the zener diode we get the reverse breakdown potential at 4 volts.
And this happens due to the heavily doped semiconductor. And also when the potential is applied at the terminals of the zener diode, electric field gets produced and this also contributes in the early breakdown of the diode.