Zener Diode - Definition, Working, FAQs

What is Zener Diode?

It is invented by C. Zener. A Zener diode is a p-n junction semiconductor device designed to operate in the reverse breakdown region. It is a highly doped p-n junction which is not damaged by high reverse current. It can operate continuously, without being damaged in the region of reverse background voltage. It forms a very thin depletion region and an extremely high electric field across the junction even for a small reverse bias voltage (~5 V).

In the forward bias, the Zener diode acts as an ordinary diode.

Symbol of Zener diode

The symbol of the Zener diode is shown in the below figure.

VI characteristics of Zener diode

• Zener Breakdown
  When a reverse bias is increased the electric field at the junction also increases. At some stage, the electric field becomes so high that it breaks the covalent bonds creating electron-hole pairs. Thus a large number of carriers are generated. This causes a large current to flow. This mechanism is known as Zener breakdown.

• Avalanche breakdown
  At high reverse voltage, due to the high electric field, the minority charge carriers, while crossing the junction acquire very high velocities. These by collision break down the covalent bonds, generating more carriers. A chain reaction is established, giving rise to a high current.

This mechanism is known as Avalanche breakdown.

The VI characteristics of a Zener diode are shown in the figure below.

When forward-biased voltage is applied to the Zener diode, it works like a normal diode.

When reverse-biased voltage is applied to a Zener diode, it allows only a small amount of leakage current until the voltage is less than the Zener voltage (V_z). As the reverse bias voltage (V) reaches the breakdown voltage of the Zener diode (Vz), there is a large change in current. Also, note that for a negligible change in the reverse bias voltage, a large change in current is produced.

Working of Zener Diode

A Zener diode is a specially designed p–n junction diode that operates in the reverse bias region to provide a constant voltage.

Working Principle

A Zener diode is connected in reverse bias (positive terminal to n-side and negative to p-side).

When the reverse voltage is small, only a tiny leakage current flows.

As the reverse voltage increases and reaches a specific value called the Zener breakdown voltage (Vz), the diode starts conducting heavily.

After breakdown, the voltage across the diode remains almost constant even if the current changes.

This property makes the Zener diode useful as a voltage regulator.

Applications of Zener Diode

• Used as a voltage regulator in power supply circuits.
• Maintains a constant output voltage despite input variations.
• Used as a reference voltage source in electronic circuits.
• Provides overvoltage protection to sensitive components.
• Used in clipping circuits to limit voltage levels.
• Used in clamping circuits for signal shaping.
• Protects measuring instruments from excess voltage.

Learn Zener Diode From the Video Given Below

Solved Examples Based on Zener Diode

Example 1: Identify the semiconductor devices whose characteristics are given below, in the order (a), (b), (c), (d) :

1) Simple diode, Zener diode, Solar cell, Light-dependent resistance

2) Zener diode, Simple diode, Light-dependent resistance, Solar cell

3) Solar cell, Light-dependent resistance, Zener diode, Simple diode

4) Zener diode, Solar cell, Simple diode, Light-dependent resistance

Solution:

Zener diode

It can operate continuously without being damaged in the region of reverse-bias

wherein

1) It acts as a voltage regulator

2) In forward biasing it acts as an ordinary diode.

Solar cells

It is based on the photovoltaic effect. It converts solar energy into electrical energy.

wherein

One of the semiconductor regions is made so thin that the light incident on it reaches the PN junction and gets absorbed.

(a) represents a simple diode.

(b) represents zener diode

(c) solar cell

(d) light-dependent resistance.

Hence, the answer is option (1).

Example 2: In the given circuit, the current (in mA) through the zener diode is :

1) 3.3

2) 6.7

3) 2.5

4) 5.5

Solution:

Given:
Potential difference across $R_2=10 \mathrm{~V}$
Potential difference across $R_1=5 \mathrm{~V}$

$
\begin{aligned}
& R_1=500 \Omega \\
& R_2=1500 \Omega
\end{aligned}
$

First, calculate the current through $R_1$ :

$
\begin{gathered}
I_{R 1}=\frac{V}{R}=\frac{5}{500} \\
I_{R 1}=0.01 A=\frac{1}{100} A
\end{gathered}
$

Next, calculate the current through $R_2$ :

$
\begin{aligned}
I_{R 2} & =\frac{10}{1500} \\
I_{R 2} & =\frac{1}{150} A
\end{aligned}
$

Now, the current through the Zener diode is the difference between the two currents:

$
\begin{aligned}
I_Z & =I_{R 1}-I_{R 2} \\
I_Z & =\frac{1}{100}-\frac{1}{150}
\end{aligned}
$

Taking LCM (300):

$
\begin{gathered}
I_Z=\frac{3-2}{300} \\
I_Z=\frac{1}{300} A \\
I_Z=0.0033 A=3.3 \mathrm{~mA}
\end{gathered}
$

Example 3: The value of the resistor, RS′ needed in the dc voltage regulator circuit shown here, equals :

1. $\frac{V_i-V_L}{n I_L}$
2. $\frac{V_i+V_L}{n I_L}$
3. $\frac{V_i-V_L}{(n+1) I_L}$
4. $\frac{V_i+V_L}{(n+1) I_L}$

Solution
Let
$V_i=$ Input voltage
$V_L=$ Load voltage
$I_L=$ Load current
$n=$ Number of loads
Each load draws current $I_L$. Therefore, total current flowing through the series resistor $R_S$ is:

$
I_{\text {total }}=(n+1) I_L
$

Voltage drop across the series resistor:

$
V_{R S}=V_i-V_L
$

Using Ohm's Law:

$
\begin{gathered}
R_S=\frac{V_{R S}}{I_{\text {total }}} \\
R_S=\frac{V_i-V_L}{(n+1) I_L}
\end{gathered}
$

Hence, the answer is option (3).

Example 4: In the given circuit the current (in mA) through the Zener Diode is close to:

1) 5

2) 6

3) 4

4) 0

Solution:

Zener diode operates continuously without being damaged in the region of reverse-biased

wherein

1) It acts as the voltage regulator.

2) In forward biasing it acts as an ordinary diode.

Since the voltage across the zener must be less than 10 V, therefore it will not work in the breakdown region its resistance is infinite and the current through it is zero.

Hence, the answer is option (4).

Summary

A Zener diode is a special kind of diode which allows current to pass in the reverse direction only if a particular voltage known as the Zener breakdown voltage is reached. While typical diodes prevent reverse current flow, a Zener diode is meant to function under the reverse bias condition. Hence it becomes useful in voltage regulation in electronic circuits where the output voltage should be kept constant despite changes in input voltage level or power consumption.