Zener Diode
What is Zener Diode?
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| Symbol and polarity of Zener diode |
Zener diode is specially designed, heavily doped diode which operates in both forward and reverse biased condition when enough voltage is applied, unlike the PN Junction diode which does not conduct when it is reverse biased. Normally in PN junction diode the breakdown voltage is so high , so when it is reverse biased it damages the diode permanently. But zener diode is designed in such a way that breakdown voltage is less. Zener Diode is also called as breakdown Diode. Diodes are manufactured with different zener voltages. It is used as a voltage regulator.
Construction of Zener diode:
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| Construction of Zener diode |
There are many methods of manufacturing Zener diode. The above method is called diffused method. In this method the N substrate and P substrate are diffused and metalic layers are used to connect cathode and anode terminals. The other methods are diffused and passivated structure and alloy diffused structure.
Working of Zener diode:
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| Forward biased |
When zener diode is forward biased that is when P type is connected to positive terminal and N type to the negative terminal, it acts same like the PN Junction diode . The value of the potential barrier of the depletion region for Si is 0.7v and for Ge is 0.3v so when the applied external voltage is more than the potential barrier value, the free electrons and holes gets the extra energy to cross the depletion region and thus the depletion region decreases in size and the current starts to flow.
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| Reverse bias of Zener diode |
when Zener diode is reverse biased that is when N type is connected to positive terminal and P type to the negative terminal only leakage current flows but after reaching zener voltage current starts to flow. The breakdown voltage of the zener diode when reverse biased is called zener voltage. In the reversed biased condition when the voltage is increased breakdown happens in the depletion region and current starts to flow.
Zener breakdown:
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| Zener breakdown |
Zener breakdown is sharper than Avalanche breakdown. Since Zener diode is heavily doped the impurities are so high so the depletion layer is very thin. So even for a small reverse voltage they start conducting. When the reverse biased voltage continues to increase the electrons in the depletion region breaks from the covalent bond and thus the depletion region becomes conductive. This is called Zener breakdown and the voltage at which zener breakdown happens is called zener voltage. Zener breakdown happens at lower voltage than Avalanche breakdown.
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| Zener breakdown |
Avalanche breakdown:
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| Avalanche breakdown |
when it is reversed biased the depletion layer increases and reverse saturation current due to minority carriers will be flowing constantly. When the voltage is increased in the reverse biased condition the minority carriers gains enough kinetic energy and collide with the immobile ions in the depletion region. This results in more free electrons. These free electrons collide with each other forming more free electrons. So this leads to huge free electrons in the depletion region and the whole diodes starts conducting.
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| Avalanche breakdown |
V-I Characteristics of Zener Diode:
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| V-I characteristics of Zener diode |
Current taken in the Y axis is the circuit current and the voltage taken in the X axis is the external voltage applied.
When the diode is forward biased, when the external applied voltage is increased and when it crosses the value of potential barrier the current increases steadily as we see in the figure.
When the diode is reverse biased when the applied voltage is increased breakdown happens in the depletion region and current starts to flow. When the reverse biased voltage continues to increase the electrons in the depletion region breaks from the covalent bond and thus the depletion region becomes conductive. This is called Zener breakdown and the voltage at which zener breakdown happens is called zener voltage
Advantages
- Small size and less expensive
- Good control over the flowing current
- It can handle high voltage
Disadvantages
- Electricity is wasted during the process, since it cancels voltage by applying larger voltages in the reverse direction.
- Poor efficiency
- Internal impedance is high
- It cannot be used for large load current
Applications
- Used as voltage regulator
- Used as meter protector
- Used in clipping circuits
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