why is Jenner diode operated in reverse in bias?

Zener diode is a pn junction diode so fabricated that it operates in reverse bias under breakdown region. Its primary purpose is to function as a voltage regulator.

 

Once the junction barrier is broken, a large current can flow through the diode and a constant voltage is developed across the diode. Hence, in order to use a diode to give a specific output voltage, a zener diode is used.

 

It has a heavily doped p and n junction, which makes the depletion region very thin. Hence, until a certain level of voltage is reached, the diode does not operate.