Zener diode is a special purpose diode which is designed to work in reverse bias condition. In contrast to ordinary diode which blocks the flow of current in reverse bias and even goes into breakdown region ( means not able to come back to previous position). Zener diode even has the capability to work in reverse-breakdown region. This is what it is designed for.
Before we go further I request you to read my article on PN junction diode. Here I will try to make a synchronization between two topics so you would be able to understand the function of zener diode more quickly and easily.
Zener Diode Symbol
Fig-1: Zener diode symbol
Figure-1 shows symbol of zener diode which looks same as general purpose diode but straight line is slightly bent from both ends.
What is the working principle of zener diode?
The zener diode works in reverse-breakdown voltage. The basis of the working principle of zener diode is that throughout its operation the potential difference between the two terminals of the zener diode remains almost constant even if there is a large current change.
As a zener diode is designed to work in reverse bias condition, so it is heavily doped as compared to general purpose PN junction diode. Due to heavy doping the depletion region is small as compared to general purpose diode (see fig-2). Another advantage of small depletion region is the electric field is much stronger as compared to ordinary diode. Even this electric field can pull out the electrons from valence shells of atoms.
Fig-2: Internal mechanism of zener diode
Refer the figure-3 to understand zener breakdown
Fig-3: Zener Breakdown
There are two types of breakdowns in diodes
- Avalanche Breakdown
- Zener Breakdown
Avalanche only happens in general purpose diode whereas in zener diode both types happen. But the zener breakdown occurs before the avalanche breakdown because zener diode is highly doped. It has a small depletion region. So for small reverse voltage zener goes in breakdown region called “Zener Breakdown Region”. The blue line in the above figure shown zener breakdown region whereas VZB shows the zener breakdown voltage at which zener breakdown occurs. You can observe VZB < VAB. VAB is avalanche breakdown voltage.
The zener breakdown region is a normal operating region of zener diode. But we need to maintain the current between IZ_min and IZ_max which are minimum zener current and maximum zener current in breakdown region respectively.
One thing you can observe if you decrease the current below IZ_min, the voltage changes largely as compared to voltage in zener breakdown region. Hence zener diode would not be able to work as a regulator below IZ_min.
Similarly if the current goes beyond IZ_max ,there are high chances that diode will go in avalanche breakdown region.
Difference between Avalanche and Zener Breakdown
In avalanche breakdown high heat energy produces due to collision between the electrons. Actually when high energy free electrons enter in the depletion region from p-type area. They collide with electrons in the valence shells of atoms in the depletion region. They produces more free electrons. Hence heat energy and large current produces which causes avalanche breakdown. But in zener breakdown the electrons are pulled out of valence shell with the help of strong electric field rather than collision.