The Light emitting diode is a two-lead semiconductor light source. In 1962, Nick Holonyak has come up with an idea of 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 of 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 the small area which is less than the 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. Light Emitting Diodes or LED´s, are among the most widely used of all the different types of semiconductor diodes available today and are commonly used in TV’s and colour displays.
They are the most visible type of diode, that emit a fairly narrow bandwidth of either visible light at different coloured wavelengths, invisible infra-red light for remote controls or laser type light when a forward current is passed through them.
Light Emitting Diode
The “Light Emitting Diode” or LED as it is more commonly called, is basically just a specialised type of diode as they have very similar electrical characteristics to a PN junction diode. This means that an LED will pass current in its forward direction but block the flow of current in the reverse direction.
Light emitting diodes are made from a very thin layer of fairly heavily doped semiconductor material and depending on the semiconductor material used and the amount of doping, when forward biased an LED will emit a coloured light at a particular spectral wavelength.
When the diode is forward biased, electrons from the semiconductors conduction band recombine with holes from the valence band releasing sufficient energy to produce photons which emit a monochromatic (single colour) of light. Because of this thin layer a reasonable number of these photons can leave the junction and radiate away producing a coloured light output.
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 combining 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
The above diagram shows how the light emitting diode works and the step by step process of the diagram.
From the diagram, we can observe that the N-type silicon is in red color and it contains the electrons, they 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.
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
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
The flow of current in the semiconductors is caused by the both flow of free electrons in the opposite direction of current and 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 an example, let us consider the quantum theory, the energy of the photon is the product of both Planck constant and frequency of electromagnetic radiation. The mathematical equation is shown
Eq = hf
Where h is 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 a f= c / λ. λ is denoted as a wavelength of an 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 the 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 a 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 conduction band does not occur at the same moment of electrons. Therefore, during the recombination of electrons and holes are a migration of electrons from the conduction band to valence band the momentum of electron band will be changed.
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
The following graph shows the approximate curves between the forward voltage and the current. Each curve in the graph indicates the different color. The table shows the summary of the LED characteristics.
Characteristics of LED
Applications of Light Emitting Diodes
There are many applications of the 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 the motorcycles and cars
These are used in the mobile phones to display the message
At the traffic light signals led’s are used
Advantages of LED’s
The cost of LED’s is less and they are tiny.
By using the LED’s the electricity is controlled.
The intensity of the LED differs with the help of the microcontroller.
In this article, we have discussed the Light-emitting diodes 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?
I-V Characteristics of LED
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