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Zener Diode

Last Updated : 20 Mar, 2023
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Zener Diode is one of the most important semiconductor diodes used in our daily life. It is a specific diode that works in reverse bias conditions. It allows current to flow from anode to cathode and it also works in the reverse direction. Let’s learn about Zener Diodes their function, and their construction, in detail in this article.

Zener Diode

A heavily doped p-n junction diode that works in reverse bias conditions is called a Zener Diode. They are special semiconductor devices that allow the current to flow in both forward and backward directions. For the Zener diode, the voltage drop across the diode is always constant irrespective of the applied voltage. Thus, Zener diodes are used as a voltage regulator.

 What is Zener Diode?

A Zener diode can be considered as a highly doped p-n junction diode which is made such that it works in reverse bias condition.

A Zener diode which is also called a Breakdown diode works in reverse bias conditions. An electrical breakdown occurring in the reverse-biased condition of the PN junction diode is called the Zener effect. In this condition when the electric field increases to a high value it enables the tunnelling of electrons from the valence band to the conduction band of a semiconductor, which suddenly increases the reverse current.

History of Zener Diodes

A theoretical physicist working at Bell Labs was the first man to describe the electrical properties of Zener Diode. His name was Clarence Melvin Zener, he was the first to tell about this special diode that works on reverse bias conditions so the diode is named after him Zener Diode. He first postulated the breakdown effect in a paper published in 1934.

Zener Diode Explanation

Zener diode that is also known as a breakdown diode is a heavily doped semiconductor device that has been specially designed to operate in the reverse direction. When the potential reaches the Zener voltage which is also known as Knee voltage and the voltage across the terminal of the Zener diode is reversed, at that point time, the junction breaks down and the current starts flowing in the reverse direction. This effect is known as the Zener effect.

Zener Diode Circuit Diagram

The figure given below is the circuit diagram of the Zener diode. The Zener diode has applications in various electronic devices and it works in reverse biasing conditions. In reverse biasing, the P-type material of the diode is connected with the negative terminal of the power supply, and the n-type material is connected with the positive terminal of the power supply. The diode consists of a very thin depletion region as it is made up of heavily doped semiconductor material.

Zener Diode Circuit Diagram


A Zener diode can be packed in many ways. Some Zener diodes are used where high levels of power dissipation are required. The Zener diode which is the most commonly used is contained within a small glass encapsulation having a band around one end marking the cathode side of the diode.

There are two tags at the end of the bar in the circuit symbol of the Zener diode, one in the upward direction and the other in the lower direction, as shown in the figure given below. In this way, we can easily distinguish between the Zener diode and other diodes.

Zener Diode Working

High-level impurities are added to a Zener diode to make it more conductive and thus the Zener diodes can easily conduct electricity compared to other p-n junction diodes. These impurities reduce the depletion layer of the Zener diode and make it very thin. Thus, this diode also works even if the voltage applied is very small.

In no biassing condition of the Zener diode, all the electrons accumulate in the valence band of the p-type semiconductor material and thus no current flow occurs through the diode.

In reverse bias conditions, if the Zener voltage is equal to the supplied voltage, the diode conducts electricity in the direction of reverse bias. When the Zener voltage equals the supplied voltage the depletion layer vanishes completely.

Zener Diode Working in Reverse Biased

In forward-biased conditions, the Zener Diode works like any normal diode but in the reverse-bias condition, a small leak current flows through the diode. As we keep increasing the reverse voltage it reaches a point where the reverse voltage equals the breakdown voltage. The breakdown voltage is represented as Vz and in this condition the current start flowing in the diode. After the breakdown voltage the current increase drastically unit it reaches a stable value. 

In reverse bias condition, two kinds of breakdowns occur for Zener Diode which are,

  • Avalanche Breakdown
  • Zener Breakdown

Avalanche Breakdown

The phenomenon of Avalanche breakdown occurs both in the ordinary diode and Zener Diode at high reverse voltage. For a high value of reverse voltage, the free electron in the PN junction diode gains energy and acquires high velocity and these high-velocity electrons collide with other atoms and knock electrons from that atoms. This collision continues and new electrons are available for conducting current thus the current increase rapidly in the diode.

This phenomenon of a sudden increase in the current is called the Avalanche breakdown. This phenomenon damages the diode permanently whereas the Zener diode is a specific diode that is made to operate in this reverse voltage area. 

If the reverse voltage is greater than 6V the avalanche breakdown happens in the Zener diode.

Zener Breakdown

Zener breakdown happens in heavily doped PN junction diodes. In these diodes, if the reverse bias voltages reach closer to Zener Voltage, the electric field gets stronger and is sufficient enough to pull electrons from the valance band. These electrons then gain energy from the electric field and break free from the atom. 

Thus, for these diodes in the Zener breakdown region, a slight increase in the voltage causes a sudden increase in the current.

Avalanche Breakdown vs Zener Breakdown

There is a clear difference between Avalanche Breakdown and Zener Breakdown which can easily be understood by the table discussed below,

Avalanche Breakdown

Zener Breakdown

Avalanche breakdown occurs when the high voltage increase the free electron in the semiconductor and a sudden increase in current is seen. Zener breakdown happens when electrons from the valance band gain energy and reaches the conduction band which then conducts electricity.
Avalanche breakdown is seen in the diodes having breakdown voltage greater than 8 volts. Zener breakdown is seen in the diodes having breakdown voltage in the range of 5 to 8 volts.
Avalanche breakdown is observed in diodes that are lightly doped. Zener breakdown is observed in diodes that are highly doped.
In the Avalanche breakdown, the VI characteristics curve is not as sharp as the VI characteristics curve in the Zener breakdown.
Zener Breakdown has a sharp VI characteristics curve.
For Avalanche breakdown increase in temperature increases the breakdown voltage. For Zener breakdown increase in temperature decreases the breakdown voltage.

VI Characteristics of Zener Diode

The graph given underneath shows the V-I characteristics of the Zener diode.

VI Characteristics of Zener Diode


 V-I characteristics of a Zener Diode can be studied under the following two headings,

Forward Characteristics of Zener Diode

Forward characteristics of the Zener Diode are similar to the forward characteristics of any normal diode. It is clearly evident from the above diagram in the first quadrant that the VI forward characteristics are similar to other P-N junction diodes.

Reverse Characteristics of Zener Diode

In reverse voltage conditions a small amount of current flows through the Zener diode. This current is because of the electrons which are thermally generated in the Zener diode. As we keep increasing the reverse voltage at any particular value of reverse voltage the reverse current increases suddenly at the breakdown point this voltage is called Zener Voltage and is represented as Vz.

Applications of  Zener Diode

Zener diode is a very useful diode. Due to its ability to allow current to flow in reverse bias conditions, it is used widely for various purposes. Some of the common uses of Zener Diode are discussed below,

Zener diode as Voltage Regulator

Zener diode is utilized as a Shunt voltage controller for managing voltage across little loads. The breakdown voltage of Zener diodes will be steady for a wide scope of current. The Zener diode is associated with corresponding to the heap to make it switch predisposition and when the Zener diode surpasses knee voltage, the voltage across the heap will become consistent.

Zener Diode in Over-Voltage Protection

At the point when the info voltage is higher than the Zener breakage voltage, the voltage across the resistor drops bringing about a short-out. This can be kept away from by utilizing the Zener diode.

Zener Diode in Clipping Circuits

Zener diode is utilized for adjusting AC waveform cutting circuits by restricting the pieces of it is possible that one or both the half patterns of an AC waveform.

Zener Diode Specifications

Zener Diode is one other most commonly used diode and some of the specifications of Zener diode are,

  • Zener Voltage: The voltage at which Zener breakdown occurs in the Zener diode is called as Zener Voltage. It is denoted by Vz generally it ranges from 2.4 volts to 200 volts.
  • Current Iz (max): The maximum current that the diode can achieve at the Zener Voltage is called max current. It ranges from 200μA to 200 A
  • Current Iz (min): The minimum current required for the diode to break down is called min current.
  • Power Rating: The maximum power the Zener diode can dissipate is the power rating of that diode. Power is calculated by taking the product of the breakdown voltage and the value of current at that time.
  • Temperature Stability: Temperature stability of the Zener diode is greatest at 5V.
  • Voltage Tolerance: Voltage Tolerance for any Zener diode is normally ±5%
  • Zener Resistance (Rz): The resistance exhibited by the Zener diode is called Zener Resistance.

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FAQs on Zener Diode

Question 1: What is a Zener diode?


A Zener Diode, otherwise called a breakdown diode, is a highly doped diode that is intended to work in reverse bias cndition.

Question 2: Why is Zener Diode used as a regulator?


The voltage across Zener Diode always remains constant and thus Zener diode is used as a voltage regulator. Zener diode also works in reverse bias conditions.

Question 3: Who was the first person to describe the electrical properties of the Zener diode?


The first person to describe the electrical properties of the Zener diode was an American scientist Clarence Melvin Zener working at Bell Labs. Zener diode is named in his honour.

Question 4: What are the types of breakdowns for a Zener Diode?


The two types of breakdowns for a Zener Diode are,

  • Avalanche Breakdown
  • Zener Breakdown

Question 5: What is the other name of the Zener diode?


The other name of the Zener diode is Breakdown Diode.

Question 6: What is the difference between a Zener diode and a normal diode?


The difference Zener diode and an normal diode is that a normal diode allows to flow the current  in one direcrtion whereas the zener diode allow the current to flow in both directions.

Question 7: What is the voltage tolerance of a Zener diode?


The voltage tolerance of a Zener diode is close to ±5%.

Question 8: What is Avalanche Breakdown in the Zener diode?


Avalanche breakdown happens in presence of a high electric field. In a reverse biased condition if a high electric field is applied, the electrons start to gain high kinetic energy. These energised electrons breaks other covalent bonds and creates electron-hole pairs which cause a sudden surge in current this is called Avalanche Breakdown.

Question 9: What are the Applications of the Zener diode?


Following are the applications of Zener diode:

  • Zener diode is used as a voltage regulator
  • Zener diode in over-voltage protection
  • Zener diode in clipping circuits

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