Question

In which bias can a zener diode be used as voltage regulator?

Answer 1

A zener diode is used in reverse bias in a voltage regulator.

The function of a regulator is to provide a constant output voltage to a load connected in parallel with it in spite of the ripples in the supply voltage or the variation in the load current. As seen from characteristics of a zener diode in reverse bias, it will continue to regulate the voltage until the diodes current falls below the minimum $I_{Z_{min}}$ value in the reverse breakdown region.