7/25/2019 How Light Emitting Diodes Work 21 by nafees

http://slidepdf.com/reader/full/how-light-emitting-diodes-work-21-by-nafees 1/8

How Light Emitting Diodes Work 

Light emitting diodes, commonly called LEDs, are real unsung heroes in the

electronics world. They do dozens of dierent jobs and are found in all kinds of

devices. mong other things, they form numbers on digital clocks, transmit

information from remote controls, light u! watches and tell you when your

a!!liances are turned on. "ollected together, they can form images on a jumbo

television screen or illuminate a tra#c light.

$asically, LEDs are just tiny light bulbs that %t easily into an electrical circuit. $ut

unlike ordinary incandescent bulbs, they don&t have a %lament that will burn out,

and they don&t get es!ecially hot. They are illuminated solely by the movement of

electrons in asemiconductor material, and they last just as long as a standard

transistor. The lifes!an of an LED sur!asses the short life of an incandescent bulb by

thousands of hours. Tiny LEDs are already re!lacing the tubes that light

u! L"D 'DT(s to make dramatically thinner televisions.

)n this article, we&ll e*amine the technology behind these ubi+uitous blinkers,

illuminating some cool !rinci!les of electricity and light in the !rocess.

7/25/2019 How Light Emitting Diodes Work 21 by nafees

http://slidepdf.com/reader/full/how-light-emitting-diodes-work-21-by-nafees 2/8

At the junction, free electrons from the N-type material ll holes from the

!-type material" #his creates an insulating layer in the middle of the diode

called the depletion $one"

'ow-tuorks

What is a Diode%

When the negati&e end of the circuit is hooked up to the N-type layer and

the positi&e end is hooked up to !-type layer, electrons and holes start

mo&ing and the depletion $one disappears"

7/25/2019 How Light Emitting Diodes Work 21 by nafees

http://slidepdf.com/reader/full/how-light-emitting-diodes-work-21-by-nafees 3/8

When the positi&e end of the circuit is hooked up to the N-type layer and

the negati&e end is hooked up to the !-type layer, free electrons collect on

one end of the diode and holes collect on the other" #he depletion $one

gets 'igger"

diode is the sim!lest sort of semiconductor device. $roadly s!eaking, a

semiconductor is a material with a varying ability to conduct electrical current. /ost

semiconductors are made of a !oor conductor that has had im!urities 0atoms of

another material1 added to it. The !rocess of adding im!urities is called do!ing.

)n the case of LEDs, the conductor material is ty!ically aluminum2gallium2arsenide0l3as1. )n !ure aluminum2gallium2arsenide, all of the atoms bond !erfectly to

their neighbors, leaving no free electrons 0negatively charged !articles1 to conduct

electric current. )n do!ed material, additional atoms change the balance, either

adding free electrons or creating holes where electrons can go. Either of these

alterations make the material more conductive.

semiconductor with e*tra electrons is called 42ty!e material, since it has e*tra

negatively charged !articles. )n 42ty!e material, free electrons move from a

negatively charged area to a !ositively charged area.

semiconductor with e*tra holes is called 52ty!e material, since it eectively hase*tra !ositively charged !articles. Electrons can jum! from hole to hole, moving

from a negatively charged area to a !ositively charged area. s a result, the holes

themselves a!!ear to move from a !ositively charged area to a negatively charged

area.

diode consists of a section of 42ty!e material bonded to a section of 52ty!e

material, with electrodes on each end. This arrangement conducts electricity in only

7/25/2019 How Light Emitting Diodes Work 21 by nafees

http://slidepdf.com/reader/full/how-light-emitting-diodes-work-21-by-nafees 4/8

one direction. hen no voltage is a!!lied to the diode, electrons from the 42ty!e

material %ll holes from the 52ty!e material along the junction between the layers,

forming a de!letion zone. )n a de!letion zone, the semiconductor material is

returned to its original insulating state 22 all of the holes are %lled, so there are no

free electrons or em!ty s!aces for electrons, and charge can&t 6ow.

 To get rid of the de!letion zone, you have to get electrons moving from the 42ty!e

area to the 52ty!e area and holes moving in the reverse direction. To do this, you

connect the 42ty!e side of the diode to the negative end of a circuit and the 52ty!e

side to the !ositive end. The free electrons in the 42ty!e material are re!elled by

the negative electrode and drawn to the !ositive electrode. The holes in the 52ty!e

material move the other way. hen the voltage dierence between the electrodes

is high enough, the electrons in the de!letion zone are boosted out of their holes

and begin moving freely again. The de!letion zone disa!!ears, and charge moves

across the diode.

)f you try to run current the other way, with the 52ty!e side connected to thenegative end of the circuit and the 42ty!e side connected to the !ositive end,

current will not 6ow. The negative electrons in the 42ty!e material are attracted to

the !ositive electrode. The !ositive holes in the 52ty!e material are attracted to the

negative electrode. 4o current 6ows across the junction because the holes and the

electrons are each moving in the wrong direction. The de!letion zone increases.

0-ee 'ow -emiconductors ork for more information on the entire !rocess.1

 The interaction between electrons and holes in this setu! has an interesting side

eect 22 it generates light7 )n the ne*t section, we&ll %nd out e*actly why this is.

How (an a Diode !roduce Light%

Light is a form of energy that can be released by an atom. )t is made u! of many

small !article2like !ackets that have energy and momentum but no mass. These

!articles, called !hotons, are the most basic units of light.

5hotons are released as a result of moving electrons. )n an atom, electrons move in

orbitals around the nucleus. Electrons in dierent orbitals have dierent amounts of

energy. 3enerally s!eaking, electrons with greater energy move in orbitals farther

away from the nucleus.

8or an electron to jum! from a lower orbital to a higher orbital, something has to

boost its energy level. "onversely, an electron releases energy when it dro!s from a

higher orbital to a lower one. This energy is released in the form of a !hoton.

greater energy dro! releases a higher2energy !hoton, which is characterized by a

higher fre+uency. 0"heck out 'ow Light orks for a full e*!lanation.1

s we saw in the last section, free electrons moving across a diode can fall into

em!ty holes from the 52ty!e layer. This involves a dro! from the conduction band to

7/25/2019 How Light Emitting Diodes Work 21 by nafees

http://slidepdf.com/reader/full/how-light-emitting-diodes-work-21-by-nafees 5/8

a lower orbital, so the electrons release energy in the form of !hotons. This ha!!ens

in any diode, but you can only see the !hotons when the diode is com!osed of

certain material. The atoms in a standard silicon diode, for e*am!le, are arranged in

such a way that the electron dro!s a relatively short distance. s a result, the

!hoton&s fre+uency is so low that it is invisible to the human eye 22 it is in the

infrared !ortion of the light s!ectrum. This isn&t necessarily a bad thing, of course9)nfrared LEDs are ideal for remote controls, among other things.

(isible light2emitting diodes 0(LEDs1, such as the ones that light u! numbers in

a digital clock, are made of materials characterized by a wider ga! between the

conduction band and the lower orbitals. The size of the ga! determines the

fre+uency of the !hoton 22 in other words, it determines the color of the light. hile

LEDs are used in everything from remote controls to the digital dis!lays on

electronics, visible LEDs are growing in !o!ularity and use thanks to their long

lifetimes and miniature size. De!ending on the materials used in LEDs, they can be

built to shine in infrared, ultraviolet, and all the colors of the visible s!ectrum in

between.

)n the ne*t section we&ll look at the advantages of LEDs.

#he interior of a LED is actually )uite simple, which is one of the reasons

this technology is so &ersatile"

LED Ad&antages

hile all diodes release light, most don&t do it very eectively. )n an ordinary diode,

the semiconductor material itself ends u! absorbing a lot of the light

energy. LEDs are s!ecially constructed to release a large number of !hotons

outward. dditionally, they are housed in a !lastic bulb that concentrates the light

7/25/2019 How Light Emitting Diodes Work 21 by nafees

http://slidepdf.com/reader/full/how-light-emitting-diodes-work-21-by-nafees 6/8

in a !articular direction. s you can see in the diagram, most of the light from the

diode bounces o the sides of the bulb, traveling on through the rounded end.

LEDs have several advantages over conventional incandescent lam!s. 8or one

thing, they don&t have a %lament that will burn out, so they last much longer.

dditionally, their small !lastic bulb makes them a lot more durable. They also %tmore easily into modern electronic circuits.

$ut the main advantage is e*ciency. )n conventional incandescent bulbs, the light2

!roduction !rocess involves generating a lot of heat 0the %lament must be warmed1.

 This is com!letely wasted energy, unless you&re using the lam! as a heater,

because a huge !ortion of the available electricity isn&t going toward !roducing

visible light. LEDs generate very little heat, relatively s!eaking. much higher

!ercentage of the electrical !ower is going directly to generating light, which cuts

down on the electricity demands considerably.

5er2watt, LEDs out!ut more lumens of light than regular incandescent bulbs. Light

emitting diodes have a higher luminous e*cacy 0how e#ciently electricity is

converted to visible light1 than incandescents 22 for e*am!le, -ewell&s EvoLu* LED

bulb !roduces :;.< lumens !er watt com!ared to an incandescent bulb&s =: lm>

?source9 -ewell@. nd they last9 LEDs can have lifetimes of +, hours or

more ?source9 Design Aecycle )nc@.

B! until recently, LEDs were too e*!ensive to use for most lighting a!!lications

because they&re built around advanced semiconductor material. The !rice of

semiconductor devices has !lummeted since the year C, however, making LEDs

a more cost2eective lighting o!tion for a wide range of situations. hile they may

be more e*!ensive than incandescent lights u! front, their lower cost in the longrun can make them a better buy. -everal com!anies have begun selling LED light

bulbs designed to com!ete with incandescent and com!act 6uorescents that

!romise to deliver long lives of bright light and amazing energy e#ciency.

ver the ne*t cou!le of !ages we&ll take a look at the future of LEDs in our homes.

ne day they may be !lugged into our light bulb sockets, lighting u! our digital

readouts and illuminating the millions of !i*els that make u! our high2de%nition

televisions.

LED Light ul's &s" .ncan descents and /luorescents

8or decades, =2watt incandescent light bulbs have lit u! hallways and bedroomsF

;2watt incandescents have shone softer light from reading lam!s and closets. $ut

incandescent lights have some !roblems. They&re ine#cient, wasting lots of energy

as heat, and have shorter lifes!ans than 6uorescent lam!s. Aecently, com!act

6uorescent 0"8L1 bulbs have become !o!ular alternatives to incandescent bulbs

thanks to lower !ower consum!tion. here incandescent lights last an average of

around =, hours, "8Ls can last G, hours. Bnfortunately, (/Ls contain to0ic

7/25/2019 How Light Emitting Diodes Work 21 by nafees

http://slidepdf.com/reader/full/how-light-emitting-diodes-work-21-by-nafees 7/8

mercury that makes them !otentially hazardous and a !ain to dis!ose of

?source9 Design Aecyle )nc@.

Enter the LED light bulb. LEDs oer the advantages of "8Ls 22 lower !ower

consum!tion and longer lifetimes 22 without the downside of to*ic mercury. 8or

e*am!le, a ;2watt incandescent light bulb draws more than HI worth ofelectricity !er year and !rovides about G lumens of lightF an e+uivalent com!act

6uorescent uses less than =J watts and costs only about H:J of electricity !er year.

LED bulbs are even better, drawing less than G watts of !ower, costing about HI

!er year, and lasting J, hours or longer ?source9 Design Aecyle )nc@. There are

only G,:; hours in a whole year 22 imagine how long an LED bulb would last in the

average home7

 That makes LEDs sound !retty great 22 and they are 22 but there&s a reason

incandescent and com!act 6uorescent bulbs are still around. LED bulbs !resent a

high u!2front cost com!ared to other bulbs. )ncandescent bulbs sell in !ackages for

only a few bucks. s of mid2C==, -ewell&s EvoLu* LED bulbs sold for more than H:a!iece7 'owever, because of their longer life s!ans and dramatically lower !ower

usage, LED bulbs make u! for the high barrier of entry. -ince there&s no to*ic

mercury in an LED, they&re also easier and chea!er to dis!ose of than "8Ls. nd

since LEDs can be built to light u! in a variety of colors, they don&t need %lters like

other bulbs.

LED lighting obviously isn&t !erfect yet. )n addition to the high cost barrier, LEDs are

vulnerable to high tem!eratures. )f LED circuitry gets too hot, more current will !ass

through the junction mentioned earlier in this article. hen too much current

courses through the junction, it will cause irreversible burn2out often called LED

meltdown ?source9 8un2LED2Light@.

LEDs and 6uorescents !ut o KcoolK or bluish light com!ared to the Kwarm,K

yellowish light ty!ical of incandescent bulbs. The dierence in lighting ty!es can

take some adjustment, but LEDs obviously oer numerous advantages over

incandescents. LEDs are even easy to dim and are !erfect for encouraging !lant

growth, since they e#ciently !ut o tons of light without !roducing heat that could

!otentially be damaging to !lant life.

LED #1s and the /uture of Light Emitting Diodes

LEDs have come a long way since the early days of lighting u! digital clock faces. )nthe Cs, L"D T(s took over the high de%nition market and re!resented a huge

ste! over old standard de%nition "AT televisions. L"D dis!lays were even a major

ste! above 'D rear2!rojection sets that weighed well over = !ounds 0 J. kilos1.

4ow LEDs are !oised to make a similar jum!. hile L"Ds are far thinner and lighter

than massive rear2!rojection sets, they still use cold cathode 6uorescent tubes to

!roject a white light onto the !i*els that make u! the screen. Those add weight and

thickness to the television set. LEDs solve both !roblems.

7/25/2019 How Light Emitting Diodes Work 21 by nafees

http://slidepdf.com/reader/full/how-light-emitting-diodes-work-21-by-nafees 8/8

'ave you ever seen a a gigantic 6atscreen T( barely an inch thickM )f you have,

you&ve seen an LED television. 'ere&s where the acronyms get a bit confusing9 those

LED T(s are still L"D T(s, because the screens themselves are com!rised of li+uid

crystals. Technically, they&re LED-'acklit L"D T(s. )nstead of 6uorescent tubes,

LEDs shine light from behind the screen, illuminating the !i*els to create an image.

Due to the small size and low !ower consum!tion of LEDs, LED2backlit T(s are farthinner than regular L"D sets and are also more energy e#cient. They can also

!rovide a wider color gamut, !roducing more vivid !ictures.

$ecause LED T(s are still in their infancy, several dierent ty!es of LED2blacklit sets

are on the market 22 and not all LED T(s are created e+ual. /any sets use

white LED edge lighting to shine light across the dis!lay. The only real advantage

aorded by these sets is thinness. 23 LED-'acklit sets, on the other hand,

!rovide im!roved color. -ome con%gurations even allow for a techni+ue called local

dimming, where LEDs in dierent !arts of the dis!lay can be brightened or dimmed

inde!endently to create a more dynamic !icture ?source9 LED Tele@. nd that

highlights one more great advantage of LEDs over com!act 6uorescent lights9

$ecause the LEDs can actually be instantly toggled on and o, they !roduce

awesome black levels in dark scenes. -ince the white 6uorescent lam!s have to

remain on during T( use, some light tends to bleed through and lighten the !icture

in dark scenes.

)n the future, some of the most incredible uses of LEDs will actually come

from organic light emitting diodes, or 4LEDs. The organic materials used to create

these semiconductors are 6e*ible, allowing scientists to create bendable lights and

dis!lays. -omeday, LEDs will !ave the way for the ne*t generation of T(s and

smart !hones 22 can you imagine rolling your T( u! like a !oster and carrying it with

you anywhereM