Light communication, medical etc. LEDs can be utilised as

Light Emitting Diode (LED): The Best Alternatefor IlluminationDr Manoj Kumar Srivastava, Department ofPhysics, Army Cadet College,IMA,[email protected]

comAbstract:It was day of celebration to all in general andto Thomas Alva Edison in particular, when he succeeded to produce light withoutburning in 1879 by passing current in a carbon filament. Though incandescencewas demonstrated even before Edison but credit to invent electric bulb goes to Edisononly. Since then, lighting sources have history of long journey with continuousevolution in the field with entries of Discharge Lamps, Compact FluorescentLamps (CFL) and Light Emitting Diodes LEDs.

LED, after its birth hascontinuously evolved and finally won the race of lighting sources today 1.Now, we have LEDs of different colors including white light LED available. Aspredicted by Roland Haitz (called Haitz’s law), LEDs since the birth in 1962reduced their cost to 10 times with increase in efficiency 20 times in everydecade. LED technology has again gave the impetus to electronics which hasslowed down in the last couple of years.

Keywords: LED; alloy semiconductor;electroluminescence; band gap IntroductionIt was Oct 1879 whenThomas Alva Edison invented an incandescent bulb and light could be producedwithout burning very first time. These light sources continued with the entriesof other sources such as discharge lamps in 1950’s and LEDs in 1960’s. LEDtechnology was quite expensive in the beginning but with advancements, itsprice is decreasing day by day with more and more efficiency. Since electricalconsumption in illumination is about 20% of the total consumption worldwide,LEDs have accepted this challenge to reduce the power consumption inillumination.

Now LED’s are well ahead to their rival, incandescent anddischarge lamps.  Figure-1                    Figure-2 Courtesy: Googleimages    Figure-3 Courtesy: Google images Light emitting diode isa solid state electronic device which has numerous applications in the fields suchas an efficient lighting 3, backlighting of display, communication, medical etc.LEDs can be utilised as a point source (zero dimension), line source (onedimension) and surface or areal source (implying LEDs in two dimensions). Aproperly designed LED has extremely low power consumption with high energy efficiencyin which voltage requirement is as low as 4 volts and current upto 0.7A.  LEDs have a wider temperature tolerance range2 from -200C to 85 0C, hence, can be generally usedanywhere on the surface of the earth. These are the sources of light whichconverts electrical energy directly to light unlike the other conventionalsources of light.

LEDs are foreseen as the best lighting source of the future5. Principle of operation LED is simply a P-Njunction diode (symbolised as Fig.1) made up of such semiconducting materialwhose energy band gap falls approximately between 3.

09 eV to 1.58 eV to producelight in the visible region of electromagnetic spectrum. When LED is forwardbiased (i.e. anode of battery is connected to p-side and cathode to n-side ofthe diode), majority charge carriers (holes) in p-side and electrons in n-sidecross the junction to meet each other i.e.

an electron from n-side moves top-side. As per the band theory of semiconductors, an electron in the conductionband conduction band falls to valence band to occupy the vacancy of electroni.e. hole and an energy equal to the gap of both these bands is released.

Thisphenomenon is called electro luminescence. If photon of electromagnetic energyemitted has wavelength in visible region of electromagnetic spectrum i.e.between 400 nm to 780 nm, it gives out light. Therefore, color of lightreleased by LED depends upon the band gap of semiconducting material used tofabricate the diode.Wavelength of lightreleased (?) Historyof LED development Electricalluminescence was discovered in 1907 by the British scientist H J Round atMarconi lab in Britain using a crystal of silicon carbide. Ovlosev, a Russianscientist first time proposed the correct theory of LED with its practicalapplications such as electroluminescence in 1927. It took more than threedecades to commercialise this phenomena, when Biard and Pittman madesemiconductor radiant diode using a zinc diffused P-N junction diode underforward bias condition for emission of infrared light4.

Nick Holoyankdeveloped the first visible LED of red colour in 1962 using GaAs1-xPx.In 1972, Holoyank’sstudent M George Craford invented the first yellow LED. In the beginning, theseLEDs were extremely costly approximately 200 US$ per unit. The prices reduceddrastically to 5 cent per unit in mid 70s. The brightness of red LED in thebeginning was so poor that it can only be used in indicators not to illuminatespace. The first commercial use of LED was in seven segment display. The blueLED was discovered by Herbert Paul Marushaka in 1972 using Gallium Nitride(GaN) on sapphire substrate but brightness of both gallium nitride and siliconcarbide blue LED were not good. The first high brightness blue LED was madeusing indium gallium nitride (InGaN) 7,8,9 in 90’s by a Japan born Americanscientist Shuji Nakamura 7 with  Akashaki and H Amano grabbed Nobel Prize forhis work in 2014.

With the discovery ofblue LED, a new way to produce white light LED source was set up. Hence, blueLED paved the path for its illuminating applications replacing incandescentlamps of 20th century. This white light LED has revolutionized the field ofilluminating sources.  Fabricationof LED Generally, a flatsurface LED chip emits light in a conical shape towards semiconductor surface.The cone of light emission is called Escape Cone 11. The cone angle, whenexceeds the critical angle of semiconductor – air interface, light gets totalinternally reflected inside the semiconductor and does not come out, when LEDchip is in cuboidal shape i.e.

all the surfaces of semiconductor is at 90 degreesangled surfaces.A LED as a package(Fig.2) include, LED wafer (which can be made up of different semiconductingmaterials such as Sapphire, GaAs, Si, SiC, GaN etc.) and many epitaxial layers grownon this bare wafer. Different colors of LED are obtained by using differenttypes of these epilayers.

Different types of these epilayers are InGaN andAlGaN for producing green, blue and UV light LEDs. For red and yellow lightLEDs, these epilayers are of InAlGaP. AlGaAs produce red and Infrared LEDs 6.Implanting electrical contacts in these epilayers and cutting these into LEDdies is called LED packaging to finally grow a LED chip as shown in figure.A convoluted chipsurface with inclined faces can reduce TIR effect which will increase lightoutput.

A perfect shape of semiconductor chip in a LED for maximum light outputwould be a hemisphere with flat bottom and the point of emission of light isexactly at the centre. Now, all the rays reaching on the semiconductor-airinterface will be normal and TIR is entirely avoided. At the same time, flatbottom will cause TIR and light coming towards bottom of the sphere, will bereflected back to increase the efficiency of LED.

A LED package, otherthan active components as discussed above, also composed of metal heatradiator, housing, bond wires, die attaches, lead frames and solder joints(Fig.3). Housing to protect LED’s die is made up of Liquid Crystal Polymers.Encapsulation on the housing is done using a resin (epoxy or Si) in the shapeof hemispherical dome. The die is a compound semiconductor as told earlier, isresponsible for the color of the LED.

Die attaches are to thermally connect theLED die to metal heat radiators for cooling. In white light LEDs, phosphors aredispersed inside the encapsulating material which emits white light afterabsorbing the light from different color LED dies.Materials used to fabricate LEDLED can be made outof direct band gap semiconductors (such as III-V alloy semiconductors e.

g.GaAs, InP, GaAsP etc) only. In direct band gap semiconductors, both electronsin conduction band and holes in valence band are same state. Hence, an electroncan jump directly into the valence band releasing a photon. However, inindirect band gap semiconductors such as Si, Ge, AlSb etc. a transitingelectron jumps to valence band via an intermediate state and transfer momentumto crystal lattice by a non-radiative transition.

These transitions do notproduce any optical emission. Though, indirect band gap semiconductors can bealso be used as LED material with certain impurity addition such as addition ofNitrogen in GaP produce red LED. LED development started with IR and reddevices made up of gallium arsenide and with advancements in the materialsciences, LEDs of even shorter wavelength with variety of colours are realisedtoday.Gallium Arsenide (GaAs)is a direct band-gap semiconductor with energy band-gap of 1.44eV and producesInfrared LED of wavelength 860nm. Gallium Phosphide (GaP), though an indirectband-gap semiconductor with poor radiation recombination produces green LEDwith wavelength 565nm by adding nitrogen to both p and n-side of the GaP. GaPdoped with Zn and O produce the emission of light with red color. Dopant Znreplaces Ga atoms and O replaces P atoms.

Ga As 1-xPx13 has band- gap between 1.44eV when x=0 and 2.26eV when x=1. Thisemits light from IR(?=860nm) to green (?= 548nm). It is a direct band gap alloysemiconductor with 00.44. Indirect band-gap semiconducting materials can be used for LED afteraddition of nitrogen as impurity.

GaAs 0.35P0.65 Nand  Ga As 0.15P0.

85 Nareused to produce orange and yellow LEDs respectively. A new material GaxAl1-x As is also in use to fabricate LED and different colors areemitted by changing the x from 0.7 to 1.

Flexible light-emittingdiodes made from soluble conducting polymers were also reported 14.Classificationof LEDsAccording to theircolor outputs, LEDs are of two types, white and RGB LEDs. Now LEDs of differentcolors such as red, blue, green and yellow are available.

According to mixingof different colors and design of its cover, LED is also available in widerspectral bandwidth of visible spectrum with different luminous intensities. LEDin its original concept emits single color only, depending upon the materialused to fabricate it. But appropriate mixing of different colors with envelopedesign, leads to different shades of LEDs including white light LED, which ismost widely used as an illuminating source. Another way ofclassifying LEDs is on the basis of their power output. Low power LEDs have apower rating of less than 1W and current is approximately 20mA.

Medium powerLED has power rating between 1W to 3W with current range from 30mA to 150mA.These medium power LEDs are also categorized as high brightness LEDs. Ultrahighbrightness LEDs or high power LEDs, power consumption range is more than 3 Wwith current equivalent in the range 350mA to 1A. By observing the graphbetween time span and lumen output, one can estimate the life span of a LEDwhich may go upto 80000 hrs or even more.Different color LEDs availablealong with material used is as under Figure-4 Courtesy: Google images  Whitelight LEDSuitable LED light toilluminate the space should be white light. However, light display panels use green,red, blue or yellow LEDs but light sources with illuminating capacity are ofhighest demand.

With the discovery of blue LED in 1995, path for bright whitelight LED sources paved. White light LED is fabricated using the mixture ofthree primary colour i.e. blue green and red in appropriate ratio with suitableenvelope design. An ultra high efficiency white light LED was reported in 200610.  There are three ways in whichwhite light LED can be produced 1. Mixing of light emitted from red, green andblue LEDs.

This mixture produces white light but intensity of this light ispoor hence, primarily used for backlighting the displays.2. Mixing of near UV or are UV LED with RGBphosphor encapsulation. Here UV LED is used to excite RGB phosphors.3. Mixing of blue LED with yellow LED.

Sincethese are complementary colours, hence produce white light.  This method of producing white LED is mostcommonly used because of its high efficiency.ConclusionOptoelectronics withdevelopment of LED technology has widely covered the fields of mobile phones,computers, TV screens and display panels etc. other than its primary use inlighting 12. LEDs with their luminous efficacy at 120-200 lm/W are winningthe race amongst incandescent, halogens and discharge lamps as lighting source.As predicted by Roland Haitz (called Haitz’s law), LEDs since the birth in 1962reduced their cost to 10 times with increase in efficiency 20 times in everydecade. LED technology has again gave the impetus to electronics which has sloweddown in the last couple of years.References:1.

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