Zener Diode - Definition, Working, FAQs

A Zener diode is a semiconductor device made of silicon that allows current to flow in both directions. When a particular voltage is reached, the diode's special, severely doped p-n junction is designed to conduct in the other direction.
The Zener diode has a well-defined reverse-breakdown voltage at which it begins to conduct current and may operate in reverse-bias mode indefinitely without damage. Furthermore, over a wide range of voltages, the voltage drop across the diode remains constant, making Zener diodes suitable for voltage regulation.

What is Zener Diode?

Zener diode definition: The Zener diode is a heavily doped semiconductor diode that is designed to work in the opposite way. In other words, the Zener diode is a diode that is specifically engineered to optimise the breakdown zone.

Zener Diode Symbol

Zener Diode Diagram/Zener Diode Image

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Zener Diode Working:

The depletion layer of a PN junction diode widens when it is reverse biased. The depletion layer develops wider as the reverse biased voltage across the diode is increased over time. At the same time, minority carriers will cause a steady reverse saturation current.

The minority carriers gain sufficient kinetic energy due to the strong electric field after a particular reversal voltage across the junction. Free electrons with enough kinetic energy hit with depletion layer immobile ions, knocking off more free electrons. Due to the same electric field, these newly formed free electrons gain enough kinetic energy to collide with other free electrons, resulting in the creation of more free electrons.

Massive free electrons are generated in the depletion layer very quickly as a result of this commutative process, and the diode becomes conductive as a result. Avalanche breakdown is a sort of depletion layer collapse that isn't quite as sharp as avalanche breakdown. Zener breakdown is a sort of depletion layer breakdown that is more acute than avalanche breakdown.

When a PN junction diode is highly doped, the impurity atom concentration in the crystal is high. As a result of the greater concentration of impurity atoms in the depletion layer, the width of the depletion layer becomes smaller than in a properly doped diode for the same applied reverse biased voltage.

The voltage gradient or electric field intensity across the depletion layer is quite significant due to the thinner depletion layer. The electrons from the covalent bonds within the depletion zone come out and make the depletion region conductive if the reverse voltage is increased after a specific applied voltage. Zener breakdown is the name for this type of breakdown. Zener voltage is the voltage at which this breakdown happens. If the reverse voltage applied across the diode is greater than the Zener voltage, the diode offers a conductive route for the current passing through it, preventing further avalanche breakdown.

Zener breakdown in a diode, especially doped for Zener breakdown, occurs at a lower voltage level than avalanche breakdown. The Zener collapse is substantially more distinct than the avalanche breakdown. During production, the Zener voltage of the diode is adjusted with the help of required and proper doping. When a Zener diode is connected across a voltage source and the source voltage exceeds Zener voltage, the voltage across the diode remains constant regardless of the source voltage. Although, depending on the load connected to the diode, current through the diode can be of any value at that time. That is why we employ a Zener diode to control voltage in various circuits.

Zener Diode Characteristics

A Zener diode with a 3.4 V breakdown voltage shows V-I characteristics.

The V-I properties of a Zener diode are shown in the diagram above. When the diode is linked in forward bias, it behaves normally, but when the reverse bias voltage exceeds the Zener voltage, it undergoes a rapid breakdown. Vz is the Zener voltage in the V-I characteristics above. It's also known as the knee voltage since the current surges significantly at this moment.

Breakdown Voltage in Zener Diode

In a Zener diode, there are two types of reverse breakdown regions:

1. Avalanche breakdown
2. Zener breakdown

Avalanche breakdown:

At high reverse voltage, both normal and Zener diodes experience avalanche breakdown. When a strong reverse voltage is supplied to a p-n junction diode, the free electrons (minority carriers) gain a lot of energy and speed up. When free electrons collide with atoms at high speeds, extra electrons are knocked away. These speeding electrons collide with other atoms once again. A high number of free electrons are produced as a result of this continual collision with the atoms. As a result, the diode's electric current rapidly grows. The regular diode may be irreversibly damaged by this abrupt surge in electric current. Avalanche diodes, on the other hand, are unlikely to be destroyed because they are specifically engineered to function in the avalanche breakdown region. With a Zener voltage (Vz) greater than 6V, avalanche breakdown occurs.

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Zener breakdown:

Because of their limited depletion region, strongly doped p-n junction diodes experience Zener breakdown. The tiny depletion area provides a strong electric field when the reverse biased voltage applied to the diode is raised. The electric field in the depletion area is strong enough to draw electrons from their valence band when reverse biased voltage applied to the diode approaches Zener voltage. The valence electrons that obtain enough energy from the depletion region's strong electric field will break bonding with the parent atom. Free electrons are valence electrons that have broken their link with their parent atom. Free electrons transport electric current from one location to another. A modest increase in voltage at the Zener breakdown area significantly increases the electric current.

1. At low reverse voltage, Zener breakdown occurs, but at high reverse voltage, avalanche breakdown occurs.
2. Because Zener diodes have a very tiny depletion region, they experience Zener breakdown.
3. A Zener diode's usual functioning region is called the breakdown region.
4. Zener diodes having a Zener voltage (Vz) less than 6V experience Zener breakdown.

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