Let us discuss Zener breakdown
Zener diode is a heavily doped p-n junction diode where the depletion layer will be very narrow
Let's say the width of the depletion region is 200 Armstrong which is very small
Let's see the Zener diode in action. If a reverse bias voltage of just 4 volt is applied to the diode
the electric field intensity of voltage gradient in the depletion region will be 4 by 200 into 10 to the power minus 10 equals 2
2 into 10 to the power 8 V by M. That's very high intensity
Now let us come to the actual mechanism of Zener breakdown. It is made reversed biased
That means P side is connected to the low potential terminal of the battery
and the high potential terminal of battery is connected to the N side
The supplied voltage is V and the width of the depletion layer is D
Here the value of electric field E is very high since we can write E equals to V by D and D is very small
If we consider P side and N side are good conductors and hence very high electric field exists across the depletion layer
If reverse applied voltage is gradually increased after a certain voltage, the field is intense enough to pull electrons from the covalent bonds
directly inside the depletion layer and create current. As a result the reverse barrier of the diode is broken down
This type of breakdown is called Zener breakdown and the voltage at which this breakdown occurs is called Zener breakdown voltage
or in short Zener voltage