Light Emitting Diodes,LED

A light-emitting diode (LED) is a two-lead semiconductor light source that resembles a basic pn-junction diode, except that an LED also emits light. When an LED's anode lead has a voltage that is more positive than its cathode lead by at least the LED's forward voltage drop, current flows. Electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.

Internal Functioning of Light Emitting Diodes (LEDs) Explained

The magnificent, beautiful, dazzling colors involved with LEDs may be quite bewitching to see. But do you really know how these effects are actually created in them or rather how do LED light bulbs work? The entire inside out of LEDs is explained in this article.

• What are LEDs?

   A LED (Light Emitting Diode) is basically a small light producing device that comes under “active” semiconductor electronic components. It’s quite comparable to the normal general purpose diode, with the only big difference being its capability to emit light in different colors. The two terminals (anode and cathode) of a LED when connected to a voltage source in the correct polarity, may produce lights of different colors, as per the semiconductor substance used inside it.
  From your cell phone to the large advertising display boards, the wide range of applications of these magical light bulbs can be witnessed almost everywhere. Today their popularity and applications are increasing rapidly due to some remarkable properties they have. Specifically, LEDs are very small in size, consume very little power, and are able to produce extremely high light intensity outputs.
  Unlike age-old incandescent bulbs, LEDs does not require red hot filaments to produce light. Rather it’s more effectively done through the passage of electrons and due to the band gap effect of its semiconductor material.
  Moreover, the heat generated in the process is negligibly small, thus there is no threat to the ever rising global warming problem, and LEDs are fast emerging as a better lighting solution compared to the other forms of modern lighting devices like FTLs and CFLs.
  

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  LEDs that can emit lights invisible to the naked eye in the infrared and ultra violet range are also produced largely and find major application in remote control devices. Here we will discuss the functioning of the more popular ones, i.e. the visible light emitting type of LEDs.
  Let’s move on and study how LED light bulbs work.

• How do LEDs work?

  
  
  
  The following points will explain how LEDs work and how light is actually produced through them:
  

  • Visible light may be defined as waves as well as particles travelling at a constant speed (in vacuum). Precisely speaking, light is made up of particles having zero mass and is an energy released as a by- product by an electron moving within the orbits of an atom.

  
  

  • Centuries ago this was discovered by Sir Isaac Newton, he named these light particles as photons, the fundamental unit of light.

  
  
  

  • The material used in LEDs is basically aluminum-gallium-arsenide (AlGaAs). In its original state the atoms of this material is strongly bonded. Without free electrons, conduction of electricity becomes impossible here.

  
  

  • By adding an impurity, which is known as doping, extra atoms are introduced, effectively disturbing the balance of the material.

  
  

  • These impurities in the form of additional atoms are able either to provide free electrons (N-type) into the system or suck out some of the already existing electrons from the atoms (P-Type) creating “holes” in the atomic orbits. In both ways the material is rendered more conductive. Thus in the influence of an electric current in N-type of material, the electrons are able to travel from anode (positive) to the cathode (negative) and vice versa in the P-type of material. Due to the virtue of the semiconductor property, current will never travel in opposite directions in the respective cases.

  
  

  • From the above explanation, it's clear that the intensity of light emitted from a source (LED in this case) will depend on the energy level of the emitted photons which in turn will depend on the energy released by the electrons jumping in between the atomic orbits of the semiconductor material.

  
  

  • We know that to make an electron shoot from lower orbital to higher orbita,l its energy level is required to be lifted. Conversely, if the electrons are made to fall from the higher to the lower orbitals, logically energy should be released in the process.

  
  

  • In LEDs the above phenomena is well exploited. In response to the P-type of doping, electrons in LEDs move by falling from the higher orbitals to the lower ones releasing energy in the form of photons i.e. light. The farther these orbitals are apart from each other, the greater the intensity of the emitted light.

  
  

  • Different wavelengths involved in the process determine the different colors produced from the LEDs. Modern technology has been able to dimension shorter wavelengths in them to produce a large variety of different colored LEDs.

  
  The topic regarding how LED light bulbs work is so vast that it may fill volumes and is difficult to contain in this article. But hopefully the above discussions should have sufficiently enlightened you regarding the subject. For further information feel free to leave a comment. (Comments require moderation and may take time to appear.)

An LED is often small in area (less than 1 mm2), and integrated optical components may be used to shape its radiation pattern

Advantages

•     Carbon emissions: LEDs deliver significant reductions in carbon emissions.
•     Efficiency: LEDs emit more lumens per watt than incandescent light bulbs. The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
•     Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
•     Size: LEDs can be very small (smaller than 2 mm2) and are easily attached to printed circuit boards.
•     On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a microsecond.[123] LEDs used in communications devices can have even faster response times.
•     Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail faster when cycled often, or HID lamps that require a long time before restarting.
•     Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current.[124] This pulse-width modulation is why LED lights viewed on camera, particularly headlights on cars, appear to be flashing or flickering. This is a type of stroboscopic effect.
•     Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
•     Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.    Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.
•     Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.
•     Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages total internal reflection (TIR) lenses are often used to the same effect. However, when large quantities of light are needed many light sources are usually deployed, which are difficult to focus or collimate towards the same target.

Disadvantages

•     High initial price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. As of 2010, the cost per thousand lumens (kilolumen) was about $18. The price is expected to reach $2/kilolumen by 2015.[needs update] The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed.
•     Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment – or "thermal management" properties. Over-driving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, which require low failure rates. Toshiba has produced LEDs with an operating temperature range of -40 to 100 °C, which suits the LEDs for both indoor and outdoor use in applications such as lamps, ceiling lighting, street lights, and floodlights.[88]
•     Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.    Light quality: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,[129] red surfaces being rendered particularly badly by typical phosphor-based cool-white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.
•     Area light source: Single LEDs do not approximate a point source of light giving a spherical light distribution, but rather a lambertian distribution. So LEDs are difficult to apply to uses needing a spherical light field, however different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees. In contrast, lasers can emit beams with divergences of 0.2 degrees or less.
•     Electrical polarity: Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity. To automatically match source polarity to LED devices, rectifiers can be used.
•     Electric shock hazard: There have been LED recalls because of faulty wiring that can cause electric shock, fire or burns.
•     Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1–05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.    Blue pollution: Because cool-white LEDs with high color temperature emit proportionally more blue light than conventional outdoor light sources such as high-pressure sodium vapor lamps, the strong wavelength dependence of Rayleigh scattering means that cool-white LEDs can cause more light pollution than other light sources. The International Dark-Sky Association discourages using white light sources with correlated color temperature above 3,000 K.[not in citation given]
•     Efficiency droop: The luminous efficacy of LEDs decreases as the electrical current increases above tens of milliamps. Heating also increases with higher currents which compromises the lifetime of the LED. Because of this increased heating, the lighting industry does not exceed 350 mA to power a given LED.

Applications

LED uses fall into four major categories:

•     Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning.
•     Illumination where light is reflected from objects to give visual response of these objects.
•     Measuring and interacting with processes involving no human vision.
•     Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light.