A diode is a fundamental electronic component that allows the flow of electricity in one direction only. There are many types of diodes and Zener diode is one of the commonly used diodes. Zener diodes not only allow current to flow from anode to cathode but also, in the reverse direction on reaching the Zener voltage. Due to this functionality, Zener diodes are the most commonly used semiconductor diodes. In this article, let us learn more about Zener diodes and its characteristics.
What is a Zener Diode?
Zener diodes, also known as breakdown diodes are heavily doped semiconductor devices that are designed to operate in the reverse direction. When the voltage across its terminals is reversed and the potential reaches the Zener Voltage (knee voltage), the junction will break down and the current flows in the reverse direction. This effect is known as the Zener Effect. Zener diodes are manufactured with a great variety of Zener voltages (Vz) and some are even made variable.
History of Zener Diodes
Clarence Melvin Zener was the first person to describe the electrical properties of Zener Diode. Clarence Zener was a theoretical physicist who worked at Bell Labs. As a result of his work, the Zener diode was named after him. He first postulated the breakdown effect that bears his name in a paper that was published in 1934.
Zener Diode Circuit Symbol
There are many ways in which a Zener diode is packaged. Some are used for high levels of power dissipation and the others are contained with surface mount formats. The most common type of Zener diode is contained within a small glass encapsulation. It has a band around one end marking the cathode side of the diode.
The Zener diode circuit symbol places two tags at the end of the bar – one in the upward direction and the other in the lower direction as shown in the figure. This helps in distinguishing Zener diodes from other forms of diodes within the circuit.
Zener Diode Specifications
Some commonly used specifications for Zener diodes are as follows:
- Zener/Breakdown Voltage – The Zener or the reverse breakdown voltage ranges from 2.4 V to 200 V, sometimes it can go up to 1 kV while the maximum for the surface-mounted device is 47 V.
- Current Iz (max) – It is the maximum current at the rated Zener Voltage (Vz – 200μA to 200 A)
- Current Iz (min) – It is the minimum value of current required for the diode to breakdown.
- Power Rating – It denotes the maximum power the Zener diode can dissipate. It is given by the product of the voltage of the diode and the current flowing through it.
- Temperature Stability – Diodes around 5 V have the best stability
- Voltage Tolerance – It is typically ±5%
- Zener Resistance (Rz) – It is the resistance to the Zener diode exhibits.
Working of Zener Diode
The Zener diode operates just like a normal diode when it is forward-biased. However, when connected in reverse biased mode, a small leakage current flows through the diode. As the reverse voltage increases to the predetermined breakdown voltage (Vz), a current starts flowing through the diode. The current increases to a maximum, which is determined by the series resistor, after which it stabilizes and remains constant over a wide range of applied voltage.
For a Zener diode there are two types of breakdown:
- Zener breakdown
- Avalanche breakdown
A conventional reverse-biased diode, when subjected to its breakdown voltage allows a significant amount of current. But when this reverse breakdown voltage is exceeded, the diode experiences an avalanche breakdown. An Avalanche breakdown is a form of electric current conduction that allows a very large current to pass through good insulators. This may permanently damage a conventional diode. A Zener diode exhibits the same properties except it is designed to have a reduced breakdown voltage. In contrast to the conventional diode, it exhibits a controlled breakdown. When we increase the voltage through Zener diode in reverse bias mode, the first current increases uniformly with it but after it reaches the breakdown state, the current increases massively for a very small or negligible change in voltage. The change is sharper in the Zener diode than the normal diode.
Zener Breakdown vs Avalanche Breakdown
- The Zener effect is dominant in voltages up to 5.6 volts and the avalanche effect takes over above that.
- They are both similar effects, the difference being that the Zener effect is a quantum phenomenon and the avalanche effect is the movement of electrons in the valence band like in any electric current.
- Avalanche effect also allows a larger current through the diode than the Zener effect
V-I Characteristics of Zener Diode
The diagram given below shows the V-I characteristics of the Zener diode.
The V-I characteristics of a Zener diode can be divided into two parts as follows:
(i) Forward Characteristics
(ii) Reverse Characteristics
The first quadrant in the graph represents the forward characteristics of a Zener diode. From the graph, we understand that it is almost identical to the forward characteristics of any other P-N junction diode.
When a reverse voltage is applied to a Zener voltage, initially a small reverse saturation current Io flows across the diode. This current is due to thermally generated minority carriers. As the reverse voltage is increased, at a certain value of reverse voltage, the reverse current increases drastically and sharply. This is an indication that the breakdown has occurred. We call this voltage breakdown voltage or Zener voltage and it is denoted by Vz.
Application of Zener Diode
Following are the applications of Zener diode:
Zener diode as a voltage regulator:
Zener diode is used as a Shunt voltage regulator for regulating voltage across small loads. The breakdown voltage of Zener diodes will be constant for a wide range of current. Zener diode is connected parallel to the load to make it reverse bias and once the Zener diode exceeds knee voltage, the voltage across the load will become constant.
Read More: Zener Diode as a Voltage Regulator
Zener diode in over-voltage protection:
When the input voltage is higher than the Zener breakage voltage, the voltage across the resistor drops resulting in a short circuit. This can be avoided by using the Zener diode.
Zener diode in clipping circuits:
Zener diode is used for modifying AC waveform clipping circuits by limiting the parts of either one or both the half cycles of an AC waveform.
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Frequently Asked Questions on Zener Diode
State true or False. Zener Diode exhibits a controlled breakdown.
True. Zener Diode does exhibit a controlled breakdown.
How to control the breakdown voltage of the Zener diode?
The breakdown voltage of the Zener diode can be controlled either by adding impurities or by increasing the doping level.
Does Zener Diode work in forward biased mode?
When a forward voltage is applied to the Zener diode, it behaves just like any normal general-purpose diode passing the rated current through it.
What are the advantages of a Zener diode?
- The size of the Zener diode is so small that it can be used in smaller circuits and also in cell phones.
- Zener diodes are less expensive when compared to other diodes.
- Zener diodes can be used for controlling, regulating, and stabilizing the voltage in the circuit.
- These diodes have a very high-performance standard.
- The compatibility of the Zener diodes is good that they are used in regulating voltage.
What are the disadvantages of using a Zener diode?
Zener diodes cancel out voltages by applying even larger voltages in the reverse direction, wasting electricity in the process. Additionally, Zener diodes have a relatively poor regulation ratio and are generally not as good as transistors.