(i) Zener diode: In the usual junction diodes, when the applied reverse bias increase a particular large value, the reverse current increase suddenly on increasing the reverse bias even slightly. This particular large value of reverse bias is called break down voltage. An ordinary junction diode of low power rating will get destroyed on applying reverse bias above its break down voltage. This specially designed junction diodes, which can operate in the reverse breakdown voltage region continuously without being damaged, are called zener diode. Symbol of zener diode is