Light-emitting diode (LED) is a widely used standard source of light in electrical equipment.

It has a wide range of applications ranging from your mobile phone to large advertising
billboards. They mostly find applications in devices that show the time and display different
types of data.

What is LED?

A light-emitting diode (LED) is a semiconductor device that emits light when an electric
current flows through it. When current passes through an LED, the electrons recombine with
holes emitting light in the process. LEDs allow the current to flow in the forward direction and
blocks the current in the reverse direction.

Light-emitting diodes are heavily doped p-n junctions. Based on the semiconductor material
used and the amount of doping, an LED will emit coloured light at a particular spectral
wavelength when forward biased. As shown in the figure, an LED is encapsulated with a
transparent cover so that emitted light can come out.
It is made from special types of semiconductor materials such as gallium, phosphorus, arsenic,
etc. The main part of the LED is a semiconductor chip that primarily forms the P-N junction.
The anvil, shown in the diagram, holds this chip. The reflective cavity emits light, which is
located at the top of the anvil. Through the reflective cavity, light emission can be directed in
one direction by reflection. A thin metal wire connects the P-type metal contact of the
semiconductor chip with the post to ensure that the voltage applied to the anode can be applied
to the P-type semiconductor of the diode. On the opposite side, you will notice that a metal
conductor inside the anvil connects with the N-type side, allowing the voltage applied to the
cathode to be applied to the N-type semiconductor. The entire system is then covered with a
transparent hard plastic. A terminal is drawn out from the anvil, called the cathode, and another
terminal is drawn out from the post, called the anode.

LED Symbol

The LED symbol is the standard symbol for a diode, with the addition of two small arrows
denoting the emission of light.

Simple LED Circuit

The figure below shows a simple LED circuit.

The circuit consists of an LED, a voltage supply and a resistor to regulate the current and
voltage.

How does an LED work?

When the diode is forward biased, the minority electrons are sent from p → n while the minority
holes are sent from n → p. At the junction boundary, the concentration of minority carriers
increases. The excess minority carriers at the junction recombine with the majority charges
carriers.

The energy is released in the form of photons on recombination. In standard diodes, the
energy is released in the form of heat. But in light-emitting diodes, the energy is released in the
form of photons. We call this phenomenon electroluminescence. Electroluminescence is an
optical phenomenon, and electrical phenomenon where a material emits light in response to an
electric current passed through it. As the forward voltage increases, the intensity of the light
increases and reaches a maximum.

What determines the colour of an LED?

The colour of an LED is determined by the material used in the semiconducting element. The
two primary materials used in LEDs are aluminium gallium indium phosphide alloys and
indium gallium nitride alloys. Aluminium alloys are used to obtain red, orange and yellow
light, and indium alloys are used to get green, blue and white light. Slight changes in the
composition of these alloys change the colour of the emitted light.

Properties of Laser Light

Laser light is monochromatic, directional and coherent.

Laser Light is Monochromatic

Unlike white light, which is made of seven colours, laser light is made of a single colour.

Laser Light is Directional

Laser light is highly directional.

Laser Light is Coherent

Laser light is coherent because the wavelengths of the laser light are in phase in space and time.

Uses of LED
LEDs find applications in various fields, including optical communication, alarm and security
systems, remote-controlled operations, robotics, etc. It finds usage in many areas because of its
long-lasting capability, low power requirements, swift response time, and fast switching
capabilities. Below are a few standards LED uses:

• Used for TV back-lighting

• Used in displays

• Used in Automotives

• LEDs used in the dimming of lights

Types of LED

Below is the list of different types of LED that are designed using semiconductors:

• Miniature LEDs

• High-Power LEDs

• Flash LED

• Bi and Tri-Colour

• Red Green Blue LEDs

• Alphanumeric LED

• Lighting LED

Advantages of LEDs over Incandescent Power Lamps

Some advantages of LEDs over Incandescent Power Lamps are:

• LEDs consume less power, and they require low operational voltage.

• No warm-up time is needed for LEDs.

• The emitted light is monochromatic.

• They exhibit long life and ruggedness.

LED Displays
• Single color, bicolor, multicolor and several other Light emitting diodes are combined
as a single package. They can be used as back lightening, strips and bar graphs. One
essential requirement of digital display devices is visual numeric display. The common
example of such single package of several LEDs is seen in seven segment displays.
 • A seven segment display, as the name suggests it consists of seven LEDs within the
single display package. It can be used for displaying the information.
• The display information may be in the digital data form of numbers, letters, characters
and also alphanumeric characters. The seven segment display usually has eight
combinations of input connections, one for each LED and the remaining one is a
common connection point for all the internal LEDs.
• If the cathodes of all the LEDs are connected together and by applying a logic HIGH
signal, then the individual segments are illuminated. In the same manner if anodes of
all the LEDs are connected together and by applying a logic LOW signal, then the
individual segments are illuminated.

Zener diodes
Discover the versatile functionality of Zener diodes, which enable the flow of current in both
forward and reverse directions. As one of the most widely used semiconductor diodes, Zener
diodes play a crucial role in electronic circuits.

A Zener Diode, also referred to as a breakdown diode, is a specially doped semiconductor

device engineered to function in the reverse direction. When the voltage across a Zener diode’s
terminals is reversed and reaches the Zener Voltage (also known as the knee voltage), the
junction experiences a breakdown, allowing current to flow in the opposite direction. This
phenomenon, known as the Zener Effect, is a key characteristic of Zener diodes.

Zener Diode Definition

A Zener diode is a highly doped semiconductor device specifically designed to function in the
reverse direction. It is engineered with a wide range of Zener voltages (Vz), and certain types
are even adjustable to achieve variable voltage regulation.

How does a Zener Diode work in reverse bias?

A Zener diode functions similarly to a regular diode when forward-biased. However, in

reverse-biased mode, a small leakage current flows through the diode. As the reverse voltage
increases and reaches the predetermined breakdown voltage (Vz), current begins to flow
through the diode. This current reaches a maximum level determined by the series resistor,
after which it stabilizes and remains constant across a wide range of applied voltages.

There are two types of breakdowns in a Zener Diode: Avalanche Breakdown and Zener
Breakdown.

Avalanche Breakdown in Zener Diode

Avalanche breakdown occurs in both normal diodes and Zener diodes when subjected to high
reverse voltage. When a significant reverse voltage is applied to the PN junction, the free
electrons gain enough energy to accelerate at high velocities. These high-velocity electrons
collide with other atoms, causing the ejection of additional electrons. This continuous collision
process generates a large number of free electrons, resulting in a rapid increase in electric
current through the diode. In the case of a normal diode, this sudden surge in current could
permanently damage it. However, a Zener diode is specifically designed to withstand avalanche
breakdown and can handle the sudden current spike. Avalanche breakdown typically occurs in
Zener diodes with a Zener voltage (Vz) greater than 6V.

Zener Breakdown in Zener Diode

When the reverse bias voltage applied to a Zener diode approaches its Zener voltage, the
electric field within the depletion region becomes strong enough to attract and remove electrons
from their valence band. These valence electrons, energized by the intense electric field, break
free from their parent atoms. This phenomenon takes place in the Zener breakdown region,
where even a slight increase in voltage leads to a rapid surge in electric current.

Avalanche Breakdown vs. Zener Breakdown

The Zener effect is predominant in voltages up to 5.6 volts, while the avalanche effect becomes
more prominent beyond that threshold. Although both effects are similar, the distinction lies in
the fact that the Zener effect is a quantum phenomenon, whereas the avalanche effect involves
the movement of electrons in the valence band, similar to an electric current. The avalanche
effect allows a larger current through the diode compared to what a Zener breakdown would
permit.

Circuit Symbol of Zener Diode

Zener diodes come in various packaging options, depending on their power dissipation
requirements. Some are designed for high-power applications, while others are available in
surface mount formats. The most commonly used Zener diode is packaged in a small glass
enclosure, with a distinctive band indicating the cathode side of the diode.

The symbol used to represent a Zener diode in circuit diagrams is similar to that of a regular
diode, but with a unique addition. It consists of a triangle or arrowhead pointing towards the
cathode side (the side with the band) of the diode. This triangle is accompanied by two
perpendicular lines at the cathode end, one extending upwards and the other extending
downwards. These lines indicate the specific behaviour of the Zener diode and help
distinguish it from other types of diodes in circuit diagrams. The symbol provides a visual
representation that allows engineers and technicians to easily identify and understand the
presence of a Zener diode in a circuit.

V-I Characteristics of Zener Diode

The diagram given below shows the V-I characteristics of the Zener diode.

The V-I characteristics of a Zener diode can be divided into two parts as follows:
(i) Forward Characteristics
(ii) Reverse Characteristics

Forward Characteristics of Zener Diode

The first quadrant in the graph represents the forward characteristics of a Zener diode. From
the graph, we understand that it is almost identical to the forward characteristics of P-N
junction diode.

Reverse Characteristics of Zener Diode

When a reverse voltage is applied to a Zener voltage, a small reverse saturation current Io
flows across the diode. This current is due to thermally generated minority carriers. As the
reverse voltage increases, at a certain value of reverse voltage, the reverse current increases
drastically and sharply. This is an indication that the breakdown has occurred. We call this
voltage breakdown voltage or Zener voltage, and Vz denotes it.

Zener Diode Specifications

Some commonly used specifications for Zener diodes are as follows:

• Zener/Breakdown Voltage – The Zener or the reverse breakdown voltage ranges

from 2.4 V to 200 V, sometimes it can go up to 1 kV while the maximum for the
surface-mounted device is 47 V.

• Current Iz (max) – It is the maximum current at the rated Zener Voltage (Vz –
200μA to 200 A)

• Current Iz (min) – It is the minimum value of current required for the diode to break
down.

• Power Rating – It denotes the maximum power the Zener diode can dissipate. It is
given by the product of the voltage of the diode and the current flowing through it.

• Temperature Stability – Diodes around 5 V have the best stability

• Voltage Tolerance – It is typically ±5%

• Zener Resistance (Rz) – It is the resistance to the Zener diode exhibits.

Application of Zener Diode

Following are the applications of Zener diode:

Zener diode as a voltage regulator

Zener diode in over-voltage protection

Zener diode in clipping circuits

• Zener diode as a voltage regulator:

A Zener diode is defined as a specially designed diodes that works mainly in reverse bias
conditions. These diodes are more heavily doped than ordinary ones, giving them a narrow
depletion region. Unlike regular diodes that get damaged when the voltage exceeds the
reverse breakdown voltage, Zener diodes function in this region. The depletion region in a
Zener diode returns to normal when the reverse voltage is removed. This makes Zener diodes
useful as voltage regulators.
 There is a series resistor connected to the circuit in order to limit the current into the
diode. It is connected to the positive terminal of the d.c. It works in such a way the reverse-
biased can also work in breakdown conditions. We do not use ordinary junction diode because
the low power rating diode can get damaged when we apply reverse bias above its breakdown
voltage. When the minimum input voltage and the maximum load current is applied, the Zener
diode current should always be minimum.

Since the input voltage and the required output voltage is known, it is easier to choose a Zener
diode with a voltage approximately equal to the load voltage, i.e. VZ = VL.

The circuit diagram of a voltage regulator using a Zener diode is shown:

The value of the series resistor is written as RS = (VL − VZ)IL.

Current through the diode increases when the voltage across the diode tends to increase
which results in the voltage drop across the resistor. Similarly, the current through the diode
decreases when the voltage across the diode tends to decrease. Here, the voltage drop across
the resistor is very less, and the output voltage results normally.