What is Light Emitting Diode : Working & Its Applications

The Light-emitting diode is a two-lead semiconductor light source. In 1962, Nick Holonyak has come up with the idea of a light-emitting diode, and he was working for the general electric company. The LED is a special type of diode and they have similar electrical characteristics to a PN junction diode. Hence the LED allows the flow of current in the forward direction and blocks the current in the reverse direction. The LED occupies a small area which is less than 1 mm2. The applications of LEDs used to make various electrical and electronic projects. In this article, we will discuss the working principle of the LED and its applications.

What is a Light Emitting Diode?

The lighting emitting diode is a p-n junction diode. It is a specially doped diode and made up of a special type of semiconductors. When the light emits in the forward biased, then it is called a light-emitting diode.

Light Emitting Diode
Light Emitting Diode

LED Symbol

The LED symbol is similar to a diode symbol except for two small arrows that specify the emission of light, thus it is called LED (light-emitting diode). The LED includes two terminals namely anode (+) and the cathode (-). The LED symbol is shown below.

LED Symbol
LED Symbol

Construction of LED

The construction of LED is very simple because it is designed through the deposition of three semiconductor material layers over a substrate. These three layers are arranged one by one where the top region is a P-type region, the middle region is active and finally, the bottom region is N-type. The three regions of semiconductor material can be observed in the construction. In the construction, the P-type region includes the holes; the N-type region includes elections whereas the active region includes both holes and electrons.

When the voltage is not applied to the LED, then there is no flow of electrons and holes so they are stable. Once the voltage is applied then the LED will forward biased, so the electrons in the N-region and holes from P-region will move to the active region. This region is also known as the depletion region. Because the charge carriers like holes include a positive charge whereas electrons have a negative charge so the light can be generated through the recombination of polarity charges.


How does the Light Emitting Diode Work?

The light-emitting diode simply, we know as a diode. When the diode is forward biased, then the electrons & holes are moving fast across the junction and they are combined constantly, removing one another out. Soon after the electrons are moving from the n-type to the p-type silicon, it combines with the holes, then it disappears. Hence it makes the complete atom & more stable and it gives the little burst of energy in the form of a tiny packet or photon of light.

Working of Light Emitting Diode
Working of Light Emitting Diode

The above diagram shows how the light-emitting diode works and the step by step process of the diagram.

  • From the above diagram, we can observe that the N-type silicon is in red color including the electrons which are indicated by the black circles.
  • The P-type silicon is in the blue color and it contains holes, they are indicated by the white circles.
  • The power supply across the p-n junction makes the diode forward biased and pushing the electrons from n-type to p-type. Pushing the holes in the opposite direction.
  • Electron and holes at the junction are combined.
  • The photons are given off as the electrons and holes are recombined.

History of Light Emitting Diode

LEDs were invented in the year 1927 but not a new invention. A short review of LED history is discussed below.

  • In the year 1927, Oleg Losev (Russian inventor) was created the first LED and published some theory on his research.
  • In the year 1952, Prof. Kurt Lechovec has tested the theories of Losers theories and explained about the first LEDs
  • In the year 1958, the first green LED was invented by Rubin Braunstein & Egon Loebner
  • In the year 1962, a red LED was developed by Nick Holonyak. So, the first LED is created.
  • In the year 1964, IBM implemented LEDs on a circuit board for the first time on a computer.
  • In the year 1968, HP (Hewlett Packard) started using LEDs in calculators.
  • In the year 1971, Jacques Pankove & Edward Miller were invented a blue LED
  • In the year 1972, M. George Crawford (Electrical Engineer) was invented the yellow color LED.
  • In the year 1986, Walden C. Rhines & Herbert Maruska from the University of Stafford invented a blue color LED with Magnesium including future standards.
  • In the year 1993, Hiroshi Amano & Physicists Isamu Akaski has developed a Gallium Nitride with high-quality blue color LEDs.
  • An electrical engineer like Shuji Nakamura was developed the first blue LED with high-brightness through Amanos & Akaski developments, which rapidly leads to the expansion of white color LEDs.
    In the year 2002, white color LEDs were used for residential purposes which charger around £80 to £100 for each bulb.
  • In the year 2008, LED lights have become very popular in offices, hospitals & schools.
  • In the year 2019, the LEDs have become the main light sources;
  • The LED development is incredible, as it is ranged from small indication to light the offices, homes, schools, hospitals, etc.

Light Emitting Diode Circuit for Biasing

Most of the LEDs have voltage ratings from 1 volt-3 volt whereas forward current ratings range from 200 mA-100 mA.

LED Biasing
LED Biasing

If the voltage (1V to 3V) is applied to the LED, then it functions properly due to the flow of current for the applied voltage will be in the operating range. Similarly, if the applied voltage to an LED is high than the operating voltage then the depletion region within the light-emitting diode will break down due to the high flow of current. This unexpected high flow of current will damage the device.

This can be avoided by connecting a resistor in series with the voltage source & an LED. The safe voltage ratings of LEDs will be ranges from 1V to 3 V whereas safe current ratings range from 200 mA to 100 mA.

Here, the resistor which is arranged in between the voltage source and LED is known as the current limiting resistor because this resistor restricts the flow of current otherwise the LED may destroy it. So this resistor plays a key role in protecting the LED.

Mathematically, the flow of current through the LED can be written as

IF = Vs – VD/Rs


‘IF ‘is forward current

‘Vs’ is a voltage source

‘VD’ is the voltage drop across the light-emitting diode

‘Rs’ is a current limiting resistor

The amount of voltage dropped to defeat the barrier of the depletion region. The LED voltage drop will range from 2V to 3V while Si or Ge diode is 0.3 otherwise 0.7 V.

Thus, the LED can be operated by using high voltage as compared with Si or Ge diodes.
Light-emitting diodes consume more energy than silicon or germanium diodes to operate.

Types of Light Emitting Diodes

There are different types of light-emitting diodes present and some of them are mentioned below.

  • Gallium Arsenide (GaAs) – infra-red
  • Gallium Arsenide Phosphide (GaAsP) – red to infra-red, orange
  • Aluminium Gallium Arsenide Phosphide (AlGaAsP) – high-brightness red, orange-red, orange, and yellow
  • Gallium Phosphide (GaP) – red, yellow and green
  • Aluminium Gallium Phosphide (AlGaP) – green
  • Gallium Nitride (GaN) – green, emerald green
  • Gallium Indium Nitride (GaInN) – near-ultraviolet, bluish-green and blue
  • Silicon Carbide (SiC) – blue as a substrate
  • Zinc Selenide (ZnSe) – blue
  • Aluminium Gallium Nitride (AlGaN) – ultraviolet

Working Principle of LED

The working principle of the Light-emitting diode is based on the quantum theory. The quantum theory says that when the electron comes down from the higher energy level to the lower energy level then, the energy emits from the photon. The photon energy is equal to the energy gap between these two energy levels. If the PN-junction diode is in the forward biased, then the current flows through the diode.

Working Principle of LED
Working Principle of LED

The flow of current in the semiconductors is caused by the flow of holes in the opposite direction of current and the flow of electrons in the direction of the current. Hence there will be recombination due to the flow of these charge carriers.

The recombination indicates that the electrons in the conduction band jump down to the valence band. When the electrons jump from one band to another band the electrons will emit the electromagnetic energy in the form of photons and the photon energy is equal to the forbidden energy gap.

For example, let us consider the quantum theory, the energy of the photon is the product of both the Planck constant and frequency of electromagnetic radiation. The mathematical equation is shown

Eq = hf

Where his known as a Planck constant, and the velocity of electromagnetic radiation is equal to the speed of light i.e c. The frequency radiation is related to the velocity of light as an f= c / λ. λ is denoted as a wavelength of electromagnetic radiation and the above equation will become as a

Eq = he / λ

From the above equation, we can say that the wavelength of electromagnetic radiation is inversely proportional to the forbidden gap. In general silicon, germanium semiconductors this forbidden energy gap is between the condition and valence bands are such that the total radiation of electromagnetic wave during recombination is in the form of infrared radiation. We can’t see the wavelength of infrared because they are out of our visible range.

The infrared radiation is said to be as heat because the silicon and the germanium semiconductors are not direct gap semiconductors rather these are indirect gap semiconductors. But in the direct gap semiconductors, the maximum energy level of the valence band and minimum energy level of the conduction band does not occur at the same moment of electrons. Therefore, during the recombination of electrons and holes are migration of electrons from the conduction band to the valence band the momentum of the electron band will be changed.

White LEDs

The manufacturing of LEDs can be done through two techniques. In the first technique, the LED chips like red, green & blue are merged within a similar package to generate white light; whereas in the second technique, phosphorescence is utilized. Fluorescence within the phosphor can be summarized within the epoxy surrounding then the LED will be activated through the short-wavelength energy using the InGaN LED device.

The different color lights like blue, green & red lights are combined in changeable quantities to produce a different color sensation which is known as primary additive colors. These three light intensities are added equally to generate the white light.

But, to attain this combination through a combination of green, blue & red LEDs which need a complicated electro-optical design for controlling the combination & diffusion of different colors. Further, this approach can be complicated because of the changes within LED color.

The product line of white LED mainly depends on a single LED chip using a phosphor coating This coating generates white light once struck through ultraviolet otherwise blue photons. The same principle is also applied to Fluorescent bulbs; the emission of ultraviolet from an electric discharge within the tube will cause the phosphor to blink white.

Even though this process of LED can generate different hues, differences can be controlled by screening. White LED-based devices are screened by using four exact chromaticity coordinates which are adjacent to the center of the CIE diagram.

The CIE diagram describes all achievable color coordinates within the horseshoe curve. Clean colors lie over the arc, but the white tip is within the center. The white LED output color can be represented through four points which are represented in the middle of the graph. Even though the four graph coordinates are close to clean white, these LEDs are usually not effective like a common light source to light up colored lenses.

These LEDs are mainly useful to white otherwise clear lenses, backlight opaque,. When this technology maintains to progress, white LEDs will certainly gain a reputation as an illumination source & indication.

Luminous Efficacy

The LEDs’ luminous efficacy can be defined as the produced luminous flux in lm for each unit and electrical power can be used within W. The rated internal efficacy order of Blue color LED is 75 lm/W; amber LEDs have 500 lm/W & red LEDs have 155 lm/W. Because of internal re-absorption, the losses can be taken into consideration; the order of luminous efficacy ranges from 20 to 25 lm/W for green & amber LEDs. This efficacy definition is also known as external efficacy & is analogous to the efficacy definition normally used for other types of light sources like multicolor LED.

Multicolor Light Emitting Diode

A light-emitting diode that produces one color once they connected in forward bias & produce a color once they connected in reverse bias is known as multicolor LED.

Actually, these LEDs include two PN-junctions and the connection of this can be done in parallel with the anode of one that is linked to the cathode of another.

Multicolor LEDs are normally red once they biased in one direction & green once they biased in another direction. If this LED is turned ON very fast among two polarities, then this LED will generate a third color. A green or red LED will generate a yellow color light once rapidly switched backward and forward among biasing polarities.

What is the Difference between a Diode and a LED?

The main difference between a diode and a LED includes the following.



The semiconductor device like a diode conducts simply in one direction. The LED is one type of diode, used to generate light.
The designing of the diode can be done with a semiconductor material & the flow of electrons in this material can give their energy the heat form. The LED is designed with the gallium phosphide & gallium arsenide whose electrons can generate light while transmitting the energy.


The diode changes the AC into the DC The LED changes the voltage into light
It has a high reverse breakdown voltage It has a low-reverse breakdown voltage.
The on-state voltage of the diode is 0.7v for silicon whereas, for germanium, it is 0.3v The on-state voltage of LED approximately ranges from 1.2 to 2.0 V.
The diode is used in voltage rectifiers, clipping & clamping circuits, voltage multipliers.



The applications of LED are traffic signals, automotive headlamps, in medical devices, camera flashes, etc.

I-V Characteristics of LED

There are different types of light-emitting diodes are available in the market and there are different LED characteristics which include the color light, or wavelength radiation, light intensity. The important characteristic of the LED is color. In the starting use of LED, there is the only red color. As the use of LED is increased with the help of the semiconductor process and doing the research on the new metals for LED, the different colors were formed.

I-V Characteristics of LED
I-V Characteristics of LED

The following graph shows the approximate curves between the forward voltage and the current. Each curve in the graph indicates a different color. The table shows a summary of the LED characteristics.

Characteristics of LED
Characteristics of LED

What are the two types of LED configurations?

The standard configurations of LED are two like emitters as well as COBs

The emitter is a single die that is mounted toward a circuit board, then to a heat sink. This circuit board gives electrical power toward the emitter, while also drawing away heat.

To aid in reducing cost as well as enhance light uniformity, investigators determined that the LED substrate can be detached & the single die can be mounted openly to the circuit board. So this design is called COB (chip-on-board array).

Advantages and Disadvantages of LED’s

The advantages of light-emitting diode include the following.

  • The cost of LED’s is less and they are tiny.
  • By using the LED’s electricity is controlled.
  • The intensity of the LED differs with the help of the microcontroller.
  • Long Lifetime
  • Energy efficient
  • No warm-up period
  • Rugged
  • Doesn’t affect by cold temperatures
  • Directional
  • Color Rendering is Excellent
  • Environmentally friendly
  • Controllable

The disadvantages of light-emitting diode include the following.

  • Price
  • Temperature sensitivity
  • Temperature dependence
  • Light quality
  • Electrical polarity
  • Voltage sensitivity
  • Efficiency droop
  • Impact on insects

Applications of Light Emitting Diode

There are many applications of LED and some of them are explained below.

  • LED is used as a bulb in the homes and industries
  • The light-emitting diodes are used in motorcycles and cars
  • These are used in mobile phones to display the message
  • At the traffic light signals led’s are used

Thus, this article discusses an overview of the light-emitting diode circuit working principle and application. I hope by reading this article you have gained some basic and working information of the light-emitting diode. If you have any queries about this article or about the final year electrical project, please feel free to comment in the below section. Here is a question for you, What is LED and how does it work?

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