The Zener diode is a special kind of heavily doped PN junction diode designed to conduct in reverse the reverse direction when a certain specified voltage reached.
It is a silicon semiconductor switch that permits the current to flow in both directions, either a forward or reverse direction.
The Zener diode can operate in reverse bias at well-defined reverse breakdown voltage and start conducting current.
The voltage drop of the diode remains constant for the wide range of the voltage. Therefore, the Zener diode is well situated for the application of voltage regulation.
The symbol of Zener diode is nearly similar to the PN junction diode. Below figure shows the symbol of Zener diode.
The operation of a Zener diodes depends on the doping level of the PN junction. The depletion region is very thin and the electric field is very high even for a small reverse bias voltage. And it allows the electron to move from the valence band of P-type material to the conduction band of the N-type material.
The breakdown voltage of a diode can be accurately control at the time of the doping level. The breakdown voltage of a commonly available Zener diodes
can vary from 1.2 V to 200 V.
Diodes which are lightly doped and the breakdown voltage is less than 5.6 V, the breakdown denoted by the avalanche effect rather than the Zener effect.
When the PN junction diode is reverse bias, the depletion layer becomes wider. And if the reverse breakdown voltage increase, the depletion layer begins to wider.
In this condition, the minority charge carriers get sufficient kinetic energy due to a strong electric field after a certain reverse breakdown voltage. And free electrons have sufficient kinetic energy that collides with the stationary ions of the depletion layer and releases more free electrons.
These new free electrons will collide with other stationary ions and produce more free electrons. This is a cumulative process. And in a short time, it will produce more free electrons in the depletion region and it becomes the entire diode conductive.
This type of breakdown known as an avalanche breakdown, but it is not quite sharp. The breakdown which is sharper than the avalanche breakdown is known as the Zener breakdown.
The voltage at which the breakdown occurs, that voltage known as the Zener voltage. With the help of a proper doping level, the Zener voltage can adjust at the time of manufacturing.
Zener Diode Circuit
A voltage source connected with the diode as shown in below figure. The source voltage is more than the Zener voltage.
Now, if you measure the voltage across the diode, it will remain fixed irrespective of the source voltage.
The current through the diode depends on the load connected with the diode. Therefore, the Zener diode used to control the voltage of different circuits.
V-I Characteristic of Zener Diode
When it connected in forward bias, it reacts the same as normal PN junction diode. But when it connected in the reverse bias and supply voltage is more than Zener voltage, a very sharp breakdown occurs as shown in below VI characteristics.
As shown in the above figure, in reverse condition, the current is very small for the starting. This current is due to minority charges carriers.
When the breakdown voltage increases than the Zener voltage, the Zener breakdown will occur and current will increase immediately and the diode will conduct without damage.
So, the reverse bias is different in the Zener diode and PN junction diode.
The point at which current starts flowing, that point is known as knee point or Zener knee.
Zener Diode Specifications
As per the requirement, the specification of a diode can be changed. So, which specifications of the diode we have to consider?
Normal working voltage, power dissipation, maximum reverse current, and Zener resistance.
Here we will list these specifications for a commonly used diode.
Zener Voltage: it refers to reverse breakdown voltage or Zener breakdown voltage. The range of Zener voltage can be 2.4V to 200V, it can go up to 1kV. But for surface mounted device (SMD) maximum goes to 47V.
Current: At rated Zener voltage maximum current will be in the range of 200 μA to 200A. And minimum current required for the diode to breakdown is 5mA to 10mA.
Power Rating: The power rating of the diode calculated by the product of the Zener voltage and current flowing through the diode. Typical value of power is 400mW, 500mW, 1W, and 5W.
Voltage Tolerance: Typically, +/- 5%
Temperature Stability: The best stability is around 5V.
Zener Resistance: The diode has some internal resistance in ON state, which calculate from the VI characteristic of the diode.