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Zener diodes have major application for being used as a voltage regulator for providing stable reference voltages for use in power supplies, voltmeters, and other instruments. In this article, you will see how the Zener diode maintains (regulate) a nearly constant dc voltage under the proper operating conditions.

Diodes are used in many applications like half-wave, full- wave rectifiers, clippers, clampers, as voltage multipliers, and one of the most important and usable applications of a diode is voltage regulation, the behavior of the diode to regulate the voltage, due to which it has gain new name known as ZENER DIODE.

What is Zener Diode?

The Zener diode is a special type of diode that is designed to work in the reverse breakdown region. In simple words, it regulates (Maintain) the voltage across its terminal when it is in reverse biased.

It is consisting of two terminals one is an anode and another is cathode (kathode).
In Zener diode, current flows in both direction anode to cathode or cathode to the anode.

Symbol of Zener Diode:

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It is similar to the normal p-n junction diode, but with Z-shape on the vertical bar.

Why We Use Zener Diode?

A Simple p-n junction diode does not operate in breakdown region (reverse bias) because the diode will get large current which will permanently damage the diode. Due to this reason for operating a diode in reverse bias we need special purpose diode which can be operated in reverse breakdown voltage for that, we mostly use Zener because it can handle large breakdown voltage.

Working of Zener diode?

Zener acts as normal PN junction diode in forward bias.
The Zener diode is heavily doped as compared to the normal PN junction diode, hence it has very thin depletion layer, therefore, it allows more current than the normal PN junction diode.
Zener diode allows current just like a simple PN junction diode when it is forward bias but it also allows current when its reverse bias if the applied reverse voltage is greater than the Zener voltage (Zener’s have particular Zener voltage which indicates that this diode will regulate voltage up to this limit).

Read More: What is the Duty Cycle?

How is the voltage across Zener Diode constant?

A Zener Diode is a special type of diode which is designed to operate in reverse bias because it has very thin depletion region that makes this diode to handle large reverse breakdown voltage.

In forward bias It behaves like ordinary PN junction diode above 0.7v the diode conducts current in one direction.
In Reverse bias it behave like voltage regulator because when the reverse voltage is applied in the reverse bias condition Zener diode do not conduct until the reverse voltages reaches to the breakdown voltage (Knee voltages) of Zener then the voltage across the load is constant up to Knee voltages and the current increases in reverse direction as shown in V-I characteristics of Zener below.

When the voltage across the Zener diode is greater than the breakdown voltage of Zener it breaks the barrier potential and Zener starts to conduct with a fixed voltage and this condition is known as turn ON condition.

it will not conduct until the source voltage is greater than the Zener voltage.

It will regulate voltage up to its Zener voltage value. Every Zener diode has particular Zener voltage (Knee voltages) in that they can regulate voltage.

V-I Characteristics of Zener Diode:

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In forward bias Zener works as an ordinary diode but when it is operated in reverse bias its shows the specialty of voltage regulation, it must be noted that in fig given below.

The reverse voltages are increasing but reverse current extremely small (up to Vz) and remains constant till reverse voltages are reached at knee voltages (the voltages at which current rapidly increasing Is known as knee voltages).

After the knee voltages reverse current is rapidly increasing its known as Zener current (the Zener current is the effect of Zener resistance or we can say Zener impedance which decreased when Zener current rapidly increased, above the knee Zener breakdown voltages are essentially constant).

v-i characteristics of Zener diode

Zener Diode Modes of Operation

First Understand Ideal Zener Equivalent Circuits

It is like a constant voltage source.

Effect of changing the load on voltage regulation

There are two cases:

When load resistance is to be zero (Conducting State)
When load resistance is to be infinite (Open State)

In the first case when the load resistance is to be zero, the maximum current flows through the load and hence the diode is working as a voltage regulator.

In the second case when the load resistance is to be infinite, no current goes to load because of high resistance so the all of the current flows through the Zener diode, because of the heavy current diode may be destroyed in this condition hence, this condition is mostly not desirable.

Advantages of Zener Diode:

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It has very high Power dissipation capacity.
More accuracy
It has Small size
Cheap voltage regulators because these diodes are cheap in price.

Applications of Zener Diode:

In many electronic applications, it is desired that the output voltage should remain constant regardless of the variations in the input voltage or load. In order to ensure this, a voltage stabilizing device, called Zener voltage stabilizers are used.
It is used in rectifiers’ circuits to get constant voltage at the output
It is used as a voltage reference
They are used in voltage stabilizers or shunt regulators
They are used in switching operations
They are used in clippers and clamper circuits
They are used in various circuit protections
They are used in Ammeters and voltmeters.

For More Information:

Also Read:

A PN junction is said to be forward biased when the externally applied voltage is in such a direction that it cancels the potential barrier of a diode, hence, allowing the current flow.

Forward Bias of PN Junction Diode:

Forward bias is the condition that allows current through the PN junction Diode. The voltage source is connected in such a way that it produces a Forward Bias.

In order to forward bias a PN junction, the positive terminal of the battery is connected to to the p-type material and negative terminal of the battery is connected to the n-type material of PN junction diode as shown in the figure below.

Note that the -ve side of VBIAS is connected to the n-region of the diode and the +ve side is connected to the p-region. This is one requirement for forward bias.

A second requirement is that the bias voltage, V(bias), must be greater than the barrier potential.

A Barrier Potential is an internal potential a semiconductor material, in case of Silicon-based diode it is 0.7v and in case of Germanium, it is 0.3v. It means in order to forward bias the PN junction diode applied voltage should be greater than 0.7 for silicon and 0.3V for germanium.

What happens during forward bias of PN Junction Diode?

A fundamental picture of what happens when a PN junction diode is forward-biased is shown below.

As we know the N-type material is consist of Electrons and the P-type material is consist of Holes.

When forward voltage is applied to a PN junction, the free electrons in the n-type move towards the junction, leaving behind positively charged atoms.

When the P-type material is connected with a positive terminal of battery it transfers the holes (positive charge carrier), which travels from p-type material to the N-type material through (Junction).

When the N-type material is connected with a negative terminal of battery it transfers the free electrons (negatively charged carriers), which travels from n-type material to the P-type material through (junction).

These free electrons are attracted towards the positive terminal of the diode while the holes are attracted towards the negative terminal of a diode.

Forward Current in PN Junction

When VBIAS is applied across the junction in the forward bias, a current will flow continuously through this junction. That current can be calculated through the given formula.

I_S = Saturation Current (10^-9 to 10^-18 A)
V_T = Volt-equivalent temperature (≈ 26mV at room temperature)
n = Emission coefficient (1 ≤ n ≤ 2 for Si ICs)
Actually, this expression is approximated.

VI Characteristics of PN Junction Diode in Forward Bias:

When a forward-bias voltage is applied across a diode, there is current that current is called the forward current.

When the forward-bias voltage is increased to a value where the voltage across the diode reaches approximately 0.7 V (barrier potential), the forward current begins to increase rapidly, as illustrated in Figure given below. As you continue to increase the forward-bias voltage, the current continues to increase very rapidly, but the voltage across the diode is constant up to 0.7v for silicon and 0.3v for germanium.

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Category: Analog Electronics

Zener Diode , V-I Characteristics, Working, and Applications