What is Zener Diode?
A Zener diode is a semiconductor device that makes the current flow in the forward or in the backward direction. The diode usually consists of a p-n junction which is heavily doped. The diode is designed to conduct the flow of current in the reverse direction after reaching a specified voltage.
The Zener diode has a reverse-breakdown voltage at which the diode starts conductivity electric current, and remains continuous in the reverse-bias mode. The voltage drop across the diode always remains constant irrespective of the applied voltage, and this feature of the Zener diode makes it suitable for voltage regulation.
Zener Diode Explanation
Zener diode that is also known as 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 of 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 above figure is the circuit diagram of the Zener diode. The Zener diode has its application in reverse biassing. In reverse biassing, the P-type material of the diode is connected with the negative terminal of the supply, and the n-type material is connected with the positive terminal of the supply. The diode consists of a very thin depletion region as it is made up of heavily doped semiconductor material.
Zener Diode Working
In a Zener diode, high-level impurities are added to the semiconductor material to make it more conductive. Due to the presence of these impurities, the depletion region of the diode becomes very thin. The intensity of the electric field is increased across the depletion region, due to heavy doping even if a small voltage is applied.
When no biassing is applied across the Zener diode, the electrons accumulate in the valence band of the p-type semiconductor material and no current flow occurs through the diode. The band in which the valence electrons are present is called the valence band electron. When external energy is applied across the valence band, the electrons get easily moved from one band to another.
When the reverse bias is applied across the diode and when the Zener voltage is equal to that of the supplied voltage, the diode starts conducting in the direction of reverse bias. The Zener diode voltage is the particular voltage at which the depletion region vanishes completely.
The intensity of the electric field increases across the depletion region when the reverse bias is applied across the diode. Hence, the electrons are free to move from the valence band of the P-type semiconductor material to the conduction band of the N-type semiconductor material. This movement of electrons decreases the barrier between p-type and n-type materials. Once the depletion region vanishes completely, the diode starts conducting current in the reverse bias direction.
Zener Diode Working in Reverse Biased
When it is in forwarding, the biased Zener diode operates just like any other normal diode. But a small leakage current flows through the diode when it is connected in reverse biassed mode. As the reverse voltage starts increasing and finally reaches the predetermined breakage voltage which is represented as Vz the current starts flowing through the circuit. It is determined by a series of resistors after the current increases to its maximum limit after which it remains constant over a wide range of applied voltage as it gets stabilised.
Types of a Breakdown of Zener Diode
There are two types of breakdown that can be observed for a Zener diode:
Avalanche Breakdown:
At high reverse voltage, Avalanche breakdown occurs both in the normal diode as well as Zener diode. The free electrons gain efficient energy and accelerate to the high velocity when a high value of reverse voltage is applied to the PN junction. These high-velocity electrons then collide with the other atoms and knock off the electrons of those atoms.
Due to the constant collision of the electrons with new atoms, a large number of free electrons has generated that result in electric current that increases rapidly in the diode.
Though this phenomenon of a sudden spike of the electric current may permanently destroy the normal diode, the Zener diode is designed to operate under this kind of voltage breakdown and can withstand a spike of current suddenly.
With Zener voltage (Vz) greater than 6V, the avalanche breakdown happens in the Zener diode.
Zener Breakdown:
When the reverse bias voltage which is applied reaches closer to the Zener voltage, the depleting electric field in the region gets stronger enough to pull electrons from the valence band.
This valence electron then gains a high amount of energy from a strong electric field of the depletion region and breaks free from the parent atom.
Therefore, in the Zener breakdown region, a small increase in the voltage results in a sudden spike of the electric current.
VI Characteristics of Zener Diode
The VI characteristics of the Zener diode is described through the graph, mentioned in the figure below. This shows that the Zener diode behaves like an ordinary diode when it is connected in forward bias. But when the reverse voltage is applied across the Zener diode, such that the reverse voltage rises beyond the predetermined rating, breakdown occurs on the Zener diode.
The electric current starts to flow in the reverse direction at the breakdown voltage of the Zener diode. The graph represents that the Zener diode has resistance. Further, it is shown that the graph of Zener breakdown is not exactly vertical. The voltage across the Zener diode is represented by the equation given below:
V = VZ + IZRZ
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Applications of Zener Diode
The major use of Zener diode is in industrial and commercial applications. Below are some of the important applications of the Zener diode.
As Voltage Stabiliser – The Zener diode is used for voltage regulation. It converts the fluctuating voltage of the source to a constant voltage and supplies it to the load. The Zener diode is always connected in parallel with the load, and it maintains a constant voltage VZ, thus stabilising the voltage.
For Metre Protection – In multimeters, the Zener diode is used to control the movement of the metre against any accidental overloads. The multimeter is connected in parallel with the Zener diode. When the overload occurs across the diode, the major amount of current passes through the diode, and in this way, the diode protects the metre from damage.
For Wave Shaping – A sine wave is converted into a square wave by using the Zener diode. This is done by connecting two Zener Diodes in series with the resistance of the circuit. It should be noted that the diode must be connected back to back and in the opposite direction to each other.
FAQs on Zener Diode
1. Why does Zener's breakdown occur?
The Zener breakdown occurs either due to the Zener breakdown effect which occurs when the voltage is below 5.5 V or due to the impact of ionization which occurs above 5.5 V. Both of these mechanisms occur in the same circuit. However, they have different temperature coefficients.
The impact effect has a positive temperature coefficient, while the Zener effect has a negative temperature coefficient. The two temperature effects occur at an equal voltage of around 5.5V and cancel out each other, thus making the Zener diode operate at 5.5V.
2. Give some Zener diode specifications.
Zener diodes vary in specifications like the nominal working voltage, maximum reverse current, power dissipation, and packaging. Some of the commonly used specifications are as follows:
Voltage Vz: The Zener voltage refers to the reverse breakdown voltage, which ranges from 2.4 V to 200 V; and can go up to 1 kV in maximum case the surface-mounted device (SMD) is about 47 V).
Current Iz (min.): The minimum current required to break down the diode is 5 mA – 10 mA.
Current Iz (max.): The maximum current of the rated Zener voltage (Vz) is 200 uA – 200 A.
Voltage Tolerance: It is normally ±5%.
Power Rating: The maximum power which can be dissipated by the Zener diode is calculated by the product of the current flowing through the diode and the voltage across the diode. Normal values of power rating are 400 mW, 500 mW, 1 W, and 5 W. For surface mounting, 200 mW, 350 mW, 500 mW, and 1 W are the normal values.
Temperature Stability: Diodes operating around 5 V have the best stability.
Package: Lead device and the surface mounts either within integrated circuits or as discrete devices.
Zener Resistance (Rz): The diode exhibits some resistance which is shown from the IV characteristics.
