Light Emitting Diode (LED)

An LED or a Light Emitting Diode is semiconductor device that emits light due to Electroluminescence effect. An LED is basically a PN Junction Diode, which emits light when forward biased.

Light Emitting Diodes are almost everywhere. You can find LEDs in Cars, Bikes, Street Lights, Home Lighting, Office Lighting, Mobile Phones, Televisions and many more. The reason for such wide range of implementation of LEDs is its advantages over traditional incandescent bulbs and the recent compact fluorescent lamps (CFL). Few advantages of LEDs over incandescent and CFL light sources are mentioned below:

  • Low Power Consumption
  • Small Size
  • Fast Switching
  • Physically Robust
  • Long Lasting

Because of these advantages, LEDs have become quite popular among a large set of people. Electronics Engineers, Electronic Hobbyists and Electronics Enthusiasts often work with LEDs for various projects.

Basics of LED (Light Emitting Diode)

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.

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.
  • The light emitted by an LED is usually monochromatic i.e. of single color and the color is dependent on the energy band gap of the semiconductor.

Light Emitting Diodes can be manufactured to emit all the wavelengths of visible spectrum i.e. from Red (620nm to 750nm) to blue – violet (380nm to 490nm).

The electrical symbol of an LED is similar to that of a PN Junction Diode. The following image shows a Red LED along with symbols of PN Junction Diode and LED.

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.

The flow of current in the semiconductors is caused by the both flow of holes 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.

Different Colours 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

Characteristics of LED (Light Emitting Diode)

Before connecting an LED is a circuit and start using it, there are few characteristics of LED that are worth knowing (actually, they are very important). If you refer to any of the datasheets provided by the manufacturer, you can find a lot specification corresponding to electrical characteristics, absolute maximum ratings, physical dimensions etc. The important characteristics are the Polarity, Forward Voltage and Forward Current.

Polarity of LED

Polarity is an indication of symmetricity of an electronic component. A Light Emitting Diode, similar to a PN Junction Diode, is not symmetric i.e. it allows current to flow only in one direction.

In an LED, the positive terminal is called as Anode and the negative terminal is called as Cathode. For the LED to work properly, the Anode of the LED should be at a higher potential than the Cathode as the current in LED flows from Anode to Cathode. What happens if we connect the LED in reverse direction? Well, nothing happens as the LED would not conduct. You can easily identify the Anode terminal of an LED as they usually have longer leads.

Forward Current of LED

LEDs are very sensitive devices and the amount of current flowing through an LED is very important. Also, the brightness of an LED depends on the amount of current drawn by the LED. Every LED is rated with a maximum forward current that is safe to pass through it without burning off the LED. Yes. Allowing current more than the rated current will actually burn the LED. For example, most commonly used 5mm LEDs have a current rating of 20mA to 30mA and the 8mm LEDs have a current rating of 150mA (refer to the datasheet for exact values). How to we regulate the current flowing through an LED? In order to control the current flowing through an LED, we make use of current limiting series resistors. More information about LEDs and Current Limiting Resistors SIMPLE LED CIRCUITS.

Forward Voltage of LED

Light Emitting Diodes are also rated for forward voltage i.e. the amount of voltage required for the LED to conduct electricity. For example, all 5mm LEDs have a current rating of 20mA but the forward voltage varies one LED to another. Red LEDs have a maximum voltage rating of 2.2V, Blue LEDs have a maximum voltage rating of 3.4V and White LEDs have a maximum voltage rating of 3.6V.

Simple LED Circuit

The following image shows the circuit of a simple LED Circuit consisting of a 5mm White LED with a 5V power supply.

Since it is a White LED, the current and voltage ratings are as follows: typical forward current is 20mA and typical forward voltage is 2V.

So, in order to regulate the current and voltage, we have used a 180 Ω Resister rated for ¼ Watts of Power Dissipation.

Types of LED

Through-hole LEDs

These are available in different shapes and sizes and the most common ones being 3mm, 5mm and 8mm LEDs. These LEDs are available in different colors like Red, Blue, Yellow, Green, White, etc.

SMD LEDs (Surface Mount Light Emitting Diodes)

Surface Mount or SMD LEDs are a special packages that can be easily surface mount on a PCB. SMD LEDs are usually differentiated based on their physical dimensions. For example, the most common SMD LEDs are 3528 and 5050.

Bi-color LEDs

The next type of LEDs are Bi-color LEDs, as the name suggests, can emit two colors. Bi-color LEDs have three leads, usually two anodes and a common cathode. Depending on the configuration of the leads, the color will be activated.

RGB LED (Red – Blue – Green LED)

RGB LEDs are the most favorite and most popular LEDs among hobbyists and designers. Even computer builds are very popular for implementing RGB LEDs in Computer Cases, Motherboards, RAMs, etc.

RGB LED contains 3 LEDs on a single chip and by a technique called PWM (Pulse Width Modulation), we can control the output of the RGB LED to produce a wide range of colors.

High – Power LEDs

An LED with power rating greater than or equal to 1 Watt is called a High Power LED. This is because, normal LEDs have a power dissipation of few mill watts. High – Power LEDs are very bright and are often used in Flashlights, Automobile Headlamps, Spotlights, etc. Since the power dissipation of High – power LEDs is high, proper cooling and usage of heat sinks is required. Also, the input power requirement for these LEDs will be usually very high.

Applications of Light Emitting Diodes

  • 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.
Laser Diode
IR Receiver

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