What is Zener Diodes?
Zener diodes, also known as voltage regulator diodes or breakdown diodes, are basically the same as standard PN junction diodes, but they are specially designed to have a low and specified reverse breakdown voltage, which can be used with any reverse voltage applied to it.
Zener diodes not only allow current to flow from the anode to the cathode, but also allow current to flow in the reverse direction when the Zener voltage is reached. This effect is called the Zener effect. Due to this feature, Zener diodes are the most commonly used semiconductor diodes.
It is understood that Zener diodes are made with a variety of Zener voltages (Vz), some of which are even variable. Clarence Melvin Zener was the first person to describe the electrical characteristics of Zener diodes. Clarence Zener was a theoretical physicist working at Bell Labs, and because of his work, the Zener diode was named after him.
Working Principle of Zener Diode Reverse Bias
Zener diodes work like ordinary diodes when forward biased. However, when connected in reverse bias mode, a small amount of leakage current flows through the diode.
As the reverse voltage increases to the predetermined breakdown voltage (Vz), current begins to flow through the diode. The current increases to a maximum value, which is determined by the series resistance, and then stabilizes and remains constant over a wide range of applied voltages.
There are two types of failures in Zener diodes: Avalanche breakdown and Zener breakdown
Avalanche breakdown of Zener diodes
Both normal and Zener diodes experience avalanche breakdown at high reverse voltages. When a high value of reverse voltage is applied to the PN junction, the free electrons gain enough energy and accelerate at high speeds. These high-speed moving free electrons collide with other atoms and knock off more electrons.
Due to this continuous collision, the current in the diode increases rapidly, resulting in a large number of free electrons. This sudden increase in current may permanently destroy a normal diode, however, Zener diodes are designed to operate under avalanche breakdown and can withstand sudden spikes in current. Avalanche breakdown occurs in Zener diodes with a Zener voltage (Vz) greater than 6V.
Zener Breakdown of Zener Diodes
When the applied reverse bias is close to the Zener voltage, the electric field strength of the depletion region is sufficient to pull electrons out of their valence bands.
The valence electrons that gain enough energy from the strong electric field of the depletion region break away from the parent atom. In the Zener breakdown region, a small increase in voltage results in a rapid increase in current.
Difference Between Avalanche Breakdown and Zener Breakdown
The Zener effect dominates at voltages up to 5.6 volts, while the avalanche effect dominates above that voltage.
They are both similar effects, the difference being that the Zener effect is a quantum phenomenon, while the avalanche effect is the movement of electrons in the valence band, just as in any current flow.
The avalanche effect also allows for a larger current through the diode than the Zener breakdown would allow.
Circuit Symbol for Zener Diode
Zener diodes are packaged in a variety of ways. Some are used for high power dissipation, while others are contained in a surface mount format. The most common type of Zener diode is contained in a small glass package that has a strip at one end marking the cathode side of the diode.
As you can see from the image above, the strip around the package corresponds to the line on the diode circuit symbol, which makes it easy to remember the two ends.
The Zener diode circuit symbol places two labels at the ends of the strip, one in the upward direction and the other in the downward direction, as shown in the image above. This helps to distinguish the Zener diode from other forms of diodes in the circuit.
VI Characteristics of Zener Diodes
The figure below shows the VI characteristics of a Zener diode. When a reverse bias voltage is applied to the Zener diode, it allows only a small amount of leakage current until the voltage is lower than the Zener voltage.
In fact, the VI characteristics of a Zener diode can be divided into the following two parts: Forward characteristics and Reverse Characteristics
Forward Characteristics of a Zener Diode
In the figure above, the first quadrant represents the forward characteristics of a Zener diode. From the figure, it can be seen that it is almost the same as the forward characteristics of any other PN junction diode.
Reverse Characteristics of a Zener Diode
When a reverse voltage is applied to the Zener voltage, initially there is a small reverse saturation current Io flowing through the diode. This current is caused by thermally generated minority carriers.
As the reverse voltage increases, at a certain reverse voltage value, the reverse current increases sharply and sharply. This indicates that a fault has occurred. We call this voltage the breakdown voltage or Zener voltage, denoted by Vz.
Main Specifications of Zener Diodes
Some common specifications of Zener diodes are as follows:
Zener/Breakdown Voltage: The Zener or reverse breakdown voltage ranges from 2.4 V to 200 V, sometimes up to 1 kV, while the maximum for surface mount devices is 47 V.
Current Iz (max): The maximum current at the rated Zener voltage (Vz – 200μA to 200 A).
Current Iz (min): The minimum value of the current required for the diode to break down.
Power Rating: It indicates the maximum power that the Zener diode can dissipate and is given by the product of the voltage across the diode and the current flowing through it.
Temperature Stability: Diodes around 5 V have the best stability.
Voltage Tolerance: Typically ±5%.
Zener Resistance (Rz): The resistance of the Zener diode.
Main Applications of Zener Diodes
Below are the application characteristics of Zener diodes, which are mainly in 3 aspects.
Zener Voltage Diodes as Voltage Regulators: Zener diodes are used as shunt voltage regulators to regulate the voltage on small loads. The breakdown voltage of a Zener diode is constant over a wide current range. A Zener diode is connected in parallel with the load to reverse bias it, and once the Zener diode exceeds the knee voltage, the voltage across the load becomes constant.
Zener diode in overvoltage protection: When the input voltage is higher than the Zener break voltage, the voltage across the resistor drops causing a short circuit. This can be avoided by using a Zener diode.
Zener diode in a clipping circuit: A Zener diode is used to modify the AC waveform clipping circuit by limiting a portion of one or two half cycles of the AC waveform.
Summary
In simple terms, a Zener diode is a semiconductor device that allows current to flow in either the forward or reverse direction, and is mainly used as a shunt voltage regulator. A Zener diode is connected in parallel with the load to reverse bias it, and once the Zener diode exceeds the knee voltage, the voltage across the load becomes constant.
The main difference between a Zener diode and an ordinary diode is the passage of current. An ordinary diode allows current to flow in only one direction, while a Zener diode allows current to flow in both directions.
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