how light emitting diodes work 21 by nafees
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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.
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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"
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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
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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
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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
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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
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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.
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'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