Zener diode is made of heavily doped silicon semiconductor material. Dr. Clarence Melvin Zener, an American physicist, discovered the Zener effect in the 1930s. It is designed for reverse breakdown and is a special type of diode.
The Zener diode allows current to flow both forward and reverse. The Zener diode is made up of two regions of semiconductor material, one p-type and one n-type, which are joined to form a junction. The p-type region has a higher concentration of holes, while the n-type region has a higher concentration of electrons.
Unlike a normal diode that stops any current flow when reverse biased, when the reverse voltages reach a certain threshold, the Zener type begins to conduct.
Zener diodes are very economical components that comprise a high proportion of modern circuits.
Zener diode Symbol and Construction
The Zener has two terminals-the anode and the cathode. It works using a reverse bias. This means that the anode is connected to the positive terminal and the cathode to the negative terminal.
Zener diode is heavily doped with impurities. Thereby, it contains many more free charges in both p and n-regions. Hence, these free charges are very closely packed together in the diode. By controlling the doping level carefully, the breakdown voltage is set. Due to heavy doping, the width of the depletion region is narrowed.
As shown in the figure below, when the reverse voltage is applied to the Zener diode, the depletion layer is increased slightly but not as wide as the regular pn-junction diode. If the value of the reverse voltage increases, the positive terminal repels the holes in the P-region, and the negative terminal repels the electrons in the N-region, making the depletion region conductive.
There is no current flow when no voltage is applied, and the diode does not conduct. However, when a voltage that exceeds the breakdown voltage is applied, the diode begins to conduct and allows current to flow. The breakdown of the depletion regions is called the Zener breakdown, and the voltage at which the breakdown occurs is known as the Zener voltage.
Operation of the ordinary pn-type diode in the reverse bias region gets damaged.
How does a Zener diode regulate voltage?
It regulates output voltage in power supplies and rectifiers.
For example, if we have a commonly used 12V power supply and want to use it to power a 5V device, we can use a Zener diode with a 5V breakdown voltage to regulate the output. The device is protected from damage by ensuring that only 5V is supplied, regardless of the input voltage.
This type of diode in electronic devices improves their overall performance and reliability.
Read More- Zener Diode as a Voltage Regulator
Characteristic of the Zener diode
The curve that illustrates the relationship between the current and voltage of a Zener diode is called the characteristic curve and is represented below.
The first quadrant of the characteristic curve is the part where the Zener diode receives the forward voltage. This is the positive voltage across its terminals from the anode to the cathode. The diode in this region is forward-biased. During this period, the current is small until the voltage reaches a certain level, called threshold voltage, at which point it increases exponentially.
When considering Zener diodes, the third quadrant of the characteristic curve is the most significant one. A Zener diode receives voltage across its cathode and anode terminals at this region. The diode in this region is reverse-biased. Initially, the current is small when receiving reverse voltage. Only a small current, called the leakage current, flows through the diode.
When the breakdown voltage is reached, the current rapidly rises. This current rises so drastically that it is known as an “avalanche current.” The Zener voltage (Vz) is the voltage at which the Zener diode breaks down and begins to conduct current. The breakdown voltage point is also important; once the Zener diode voltage has reached, it remains constant at this voltage, although the current through it may greatly increase. Due to this, the Zener diode is useful in voltage regulation applications.
Working
If the Zener diode is biased in the forward direction, then it will act like a common diode and conduct.
If it is reversely biased, it does not conduct as long as the applied voltage is less than the Zener voltage. The above characteristic curve shows that the diode behaves like a regular diode with minimum current flow. Once the Zener voltage has been reached, the current flows from the cathode to the anode and rises sharply. A Zener voltage (Vz) is maintained across its terminals (see the above characteristic on the V-I curve).
The diode must not be considered a real component or an ideal one when analyzing it. When reverse biased, there is a current flowing in the opposite direction of the diode arrow, but very little.
Applications
- This type of diode is often used as a voltage regulator. A voltage regulator is a device that takes an input voltage and outputs regulated or constant voltage. By using a Zener diode as part of the voltage regulator circuit, the output voltage can be kept very stable, even if the input voltage varies.
- Zener can also be used in pairs, with one diode connected in reverse bias mode and the other in forward bias mode. This configuration is sometimes called a “voltage doubler.” If the input voltage is high enough, the two diodes will act together to double the output voltage.
- Many other applications include surge suppressors, reference voltages, and clipping and clamping circuits. Zener diodes are an important part of many modern electronic devices and circuits. Protecting integrated circuits in cell phones, televisions, and laptops they have vital importance in regulating the voltage throughout their entire circuit.
- It is used for reshaping a signal and protects the circuit from power supply voltage spikes. Thus, it acts as a waveform clipper.