Zener diode is a special type of p-n junction diode.

In a p-n junction diode, the majority carriers in p-side are the holes and that in n-side are the electrons. When a p-n junction diode is connected in such a way that the p-side is connected to the positive terminal and the n-side is connected to the negative terminal of a battery, there is a flow of current through the circuit. This condition is known as the forward bias condition of the junction.

If the connections are made in the opposite order, there is very less current that can pass through the p-n junction diode. This is called the reverse bias condition.

A Zener diode works in a reverse bias condition. It works in breakdown region of the applied voltage as a voltage regulator. Hence, it can act as simple p-n junction in forward bias