Characteristics of a Zener Diode
In this article lets see the Characteristics of a Zener Diode (VI characteristics) in detail. VI characteristics of a Zener diode When forward biased voltage is applied to the Zener diode, it works like a normal diode. However, when reverse biased voltage is applied to the Zener diode, it works in different manner.
I-V Characteristics of a Zener Diode
- The Zener Diode is used in its “reverse bias” or reverse breakdown mode, i.e. the diodes anode connects to the negative supply.
- From the I-V characteristics curve above, we can see that the zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the zener diodes current remains between the breakdown current IZ(min) and the maximum current rating IZ(max).
Zener Diode as a Voltage Regulator
- This ability to control itself can be used to great effect to regulate or stabilise a voltage source against supply or load variations.
- The fact that the voltage across the diode in the breakdown region is almost constant turns out to be an important application of the zener diode as a voltage regulator.
- The function of a regulator is to provide a constant output voltage to a load connected in parallel with it in spite of the ripples in the supply voltage or the variation in the load current and the zener diode will continue to regulate the voltage until the diodes current falls below the minimum IZ (min) value in the reverse breakdown region.
- The point at which current flows can be very accurately controlled (to less than 1% tolerance) in the doping stage of the diodes construction giving the diode a specific zener breakdown voltage, (Vz) ranging from a few volts up to a few hundred volts. This zener breakdown voltage on the I- V curve is almost a vertical straight line
Breakdown in Characteristics of a Zener Diode
There are two types of reverse breakdown regions in a Zener diode:
- Avalanche breakdown
- Zener breakdown.
Avalanche breakdown
- The avalanche breakdown occurs in both normal diodes and zener diodes at high reverse voltage.
- When high reverse voltage is applied to the p-n junction diode, the free electrons gains large amount of energy and accelerated to greater velocities. The free electrons moving at high speed will collides with the atoms and knock off more electrons.
- These electrons are again accelerated and collide with other atoms. Because of this continuous collision with the atoms, a large number of free electrons are generated. As a result, electric current in the diode increases rapidly. This sudden increase in electric current may permanently destroys the normal diode.
- However, avalanche diodes may not be destroyed because they are carefully designed to operate in avalanche breakdown region. Avalanche breakdown occurs in zener diodes with zener voltage (Vz ) greater than 6V.
Zener Breakdown
- The Zener breakdown occurs in heavily doped p-n junction diodes because of their narrow depletion region.
- When reverse biased voltage applied to the diode is increased, the narrow depletion region generates strong electric field.
- When reverse biased voltage applied to the diode reaches close to Zener voltage, the electric field in the depletion region is strong enough to pull electrons from their valence band.
- The valence electrons which gains sufficient energy from the strong electric field of depletion region will breaks bonding with the parent atom and become free electrons.
- These free electrons carry electric current from one place to another place. At zener breakdown region, a small increase in voltage will rapidly increases the electric current.
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