Led or Light emitting diode is a semiconductor device that emits or produces light when an electric current passes through it. Led lighting can be more versatile, efficient and long lasting as compare to compact fluorescent lightning. In our previous tutorials, we explain diodes, Zener diode, a p-n junction diode in detail. Here we are going to provide information regarding Led’s working, bulbs, construction and its types.

Led image

This type of diode emits narrow bandwidth of visible light or invisible light at different colour wavelengths for remote control. It is preferable because it is small in area and to shape its radiation pattern many optical components may be used.

Light emitting Diode construction:

Led construction image

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Light emitting diode construction differs from normal standard diode in many aspects. As shown in the figure above, the p-n junction of LED is enclosed by a transparent, hard plastic epoxy resin hemispherical-shaped shell or body which protects the Led from shock and vibration. LEDs have two terminals; a cathode and anode i.e. cathode terminal is identified by either a flat spot on the body or notch or by one of the leads shorter than the other. The domed top of the LED is just like a lens concentrating the amount of light. The epoxy resin shell is constructed in such a way that the photons of light emitted by the junction are reflected away from the substrate base to which the diode is attached because Led junction does not emit that much light. Due to this, the brightest light will be emitted at the top of the Led.

Led Working Principle:

Working principle of an LED image

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A Led consists of two semiconducting material i.e. p-type material and n-type material. By connecting these two types of materials, a p-n junction forms. When p-n junction is forward biased, the majority carriers either electrons or holes start moving across the junction. As shown in the figure above, the electrons start moving from the n region and the holes start moving from the p region. When they moved from their regions they start to recombine across the depletion region. Free electrons will remain in the conduction band of energy level while holes remain in the valence band of energy level. The Energy level of the electrons is high than holes because electrons are more mobile than holes i.e. current conduction due to electrons are more. During the recombination of electrons and holes, some portion of energy must be dissipated or emitted in the form of heat and light. The phenomenon into which light emitted from the semiconductor under the influence of electric field is known as electroluminescence. Always remember that the majority of light is produced from the junction nearer to the p-type region. So diode designing is done in such a way that this area is kept as close to the surface of the device to ensure that the minimum amount of light is absorbed.

The electrons dissipate energy in different forms depends on the nature of the diode used. Like for silicon and germanium diodes it dissipates energy in the form of heat and for gallium phosphide (GaP) and gallium arsenide phosphide (GaAsP) semiconductors, it dissipates energy by emitting photons. For the emission of different colours different semiconductors are used like phosphorus for a red light, gallium phosphide for green light and aluminium indium gallium phosphide for yellow and orange light.

Led Types:

LED’s are the mostly used component in the electronics industry. It is available in a variety of sizes and shapes. There are different types of led available in the market as per the user requirement.

(i) Depending on their electrical properties:

The main types of Leds present on the basis of electrical properties are:

(a) AC Driven Leds:  Without any need of DC converter these Leds can operate on AC power. Seoul semiconductor released a high voltage LED namely Acrich MJT, capable of driven from ac power with a simple controlling circuit. An example of this type of Led is HP-LED has an efficacy of 40 lm/W.

(b) Miniature: Small size Leds are mostly used these days because of its performance speed and good efficiency. For optical communications, efficient lightning, nano lasers researchers have invented a thinnest Led made up of 2D flexible material which is 10 to 20 times thinner than 3D Leds. Three main properties of miniature single die Led’s are low current typically rated for 2 mA, the ultra high output of 20mA at approx 2 or 4,5V etc.

(c) High power Leds: These type of leds can be driven at currents from hundred of mA to more than an ampere. For heat dissipation, high power led must be mounted on a heat sink because if heat from Hp-Led is not removed, it may cause damage to the device. It can be easily set in an array to form a powerful Led lamp.

(ii) Depending on the material:

Light emitting diodes are current dependent devices depending upon the semiconductor compound i.e. its light colour and forward biased Led current. By mixing a variety of semiconductor, metal and gas compounds, a large number of LEDs are obtained. Following  are:

  • Zinc selenide (ZnSe).
  • Gallium Nitride( GaN)
  • Gallium Phosphide (GaP)
  • Silicon Carbide( SiC)
  • Gallium Arsenide (Ga As)
  • Gallium Arsenide Phosphide (Ga AsP)

Led Bulbs:

Led bulb is a lightning device uses LEDs to produce light when an electric current passes through it. These bulbs can work for 50000 hours if run within the specified temperature range. For replacement of 60W incandescent, these bulbs use 8-11 watts of power. The two most kinds of bulbs are:

(a) Incandescent Bulb: It is a source of electric light into which filament connected is heated by passing an electric current. Filament used is made up of tungsten (piece of metal) which got heated and glow the light when electric current is flows through it. As the metal glows, it gives out a bright white light.

(b) Fluorescent bulb: These bulbs are the energy saving bulbs known as the compact fluorescent light bulb. Inside it, a small amount of mercury is present which got vapourises when electric current passes through it. As the gas is heated, particles bounce off a phosphorus coating that is applied to the internal section of the bulb which produces light.

Advantages of LED lights:

Light emitting diodes have a variety of advantages over other light sources. Following of its advantages are:

  • Easily controlled and programmed.
  • Large Life span.
  • High efficiency.
  • Low radiated heat.
  • High levels of brightness and intensity.
  • High reliability.
  • No UV rays
  • Low voltage and current requirements.
  • Less wiring required.
  • Low maintenance cost.
  • Instant lightning.

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