LED: The photonic devicePresented by:

Amit Kumar TagoreM.Tech. 1st year(1411NT02) Nanoscience & Technology

IIT Patna

Flow of PresentationFlow of Presentation IntroductionIntroduction What is LED?What is LED? Brief HistoryBrief History Mechanism behind photon emission in LEDs?Mechanism behind photon emission in LEDs? Materials for LEDsMaterials for LEDs Applications Applications References References

IntroductionIntroduction Photonic devices are those in which the basic particle of Photonic devices are those in which the basic particle of

light-the Photon, plays a major role.light-the Photon, plays a major role.

Photonic devices can be divided into three groups:Photonic devices can be divided into three groups:

1.1. Devices that convert electrical energy into optical Devices that convert electrical energy into optical radiation- the LED (Light Emitting Diode)radiation- the LED (Light Emitting Diode)

2.2. Devices that detect optical signals through electronic Devices that detect optical signals through electronic processes- Photo detectors, andprocesses- Photo detectors, and

3.3. Devices that convert optical radiation into electrical Devices that convert optical radiation into electrical energy- the photovoltaic device or solar cell.energy- the photovoltaic device or solar cell.

What is LED?What is LED? L- L- LLight ight

E- E- EEmittingmitting

D- D- DDiodeiode Semiconductor light source.Semiconductor light source.

Used as indicator lamps , advertising, Used as indicator lamps , advertising, traffic signals, television, etc.traffic signals, television, etc.

Early LEDs emitted low-intensity red Early LEDs emitted low-intensity red light, but modern versions are available light, but modern versions are available across the across the visible, , ultraviolet, and , and infrared wavelengths, with very high , with very high brightnessbrightness..

Brief History Brief History Electroluminescence as a phenomenon was Electroluminescence as a phenomenon was

discovered in 1907.discovered in 1907. - By H. J. Round- By H. J. Round -using a crystal of silicon carbide-using a crystal of silicon carbide Creation of first LED in 1927.Creation of first LED in 1927. -By Oleg Losev -By Oleg Losev

The The firstfirst practical visible-spectrum (red) LED was practical visible-spectrum (red) LED was developed in 1962.developed in 1962.

-By Nick Holonyak ("-By Nick Holonyak ("father of the light-father of the light-emitting diode")emitting diode")

CB

VB

When the electron falls When the electron falls down from conduction down from conduction band and fills in a hole band and fills in a hole in valence band, there in valence band, there is an obvious loss of is an obvious loss of energy.energy.

The question is; The question is; where does that energy go?where does that energy go?

In order to achieve a In order to achieve a reasonable efficiency reasonable efficiency for photon emission, the for photon emission, the semiconductor must semiconductor must have a direct band gap.have a direct band gap.

CB

VB

The question is; The question is; what is the mechanism what is the mechanism

behind photon emission in LEDs?behind photon emission in LEDs?

For example;For example;SiliconSilicon is known as an is known as an indirect band-gap indirect band-gap

material.material.

as an electron goes from the bottom of the as an electron goes from the bottom of the conduction band to the top of the valence conduction band to the top of the valence band;band;

it must also undergo a it must also undergo a significant significant change in change in momentum.momentum.

CB

VB

What this means is thatWhat this means is that

E

k

As we all know, whenever something changesAs we all know, whenever something changes

state, one must conserve not only energy, but also state, one must conserve not only energy, but also momentum.momentum.

In the case of an electron going from conduction band In the case of an electron going from conduction band to the valence band in silicon, both of these things can to the valence band in silicon, both of these things can only be conserved:only be conserved:

The transition also creates a quantized set of lattice vibrations,

called phonons, or "heat“ .

Phonons possess both energy and momentum.Phonons possess both energy and momentum. Their creation upon the recombination of an electron Their creation upon the recombination of an electron

and hole allows for complete conservation of both and hole allows for complete conservation of both energy and momentum. energy and momentum.

All of the energy which the electron gives up in going All of the energy which the electron gives up in going from the conduction band to the valence band (1.1 eV) from the conduction band to the valence band (1.1 eV) ends up in phonons, which is another way of saying ends up in phonons, which is another way of saying that the electron heats up the crystal.that the electron heats up the crystal.

In a class of materials called In a class of materials called direct band-gap direct band-gap semiconductorssemiconductors; ;

the transition from conduction band to the transition from conduction band to valence band involves essentially valence band involves essentially no no change in momentumchange in momentum..

Photons, it turns out, possess a fair Photons, it turns out, possess a fair amount of energy ( several eV/photon in amount of energy ( several eV/photon in some cases ) but they have very little some cases ) but they have very little momentum associated with them.momentum associated with them.

Thus, for a direct band gap material, the excess Thus, for a direct band gap material, the excess energy of the electron-hole recombination can either energy of the electron-hole recombination can either be taken away as heat, or more likely, as a photon of be taken away as heat, or more likely, as a photon of light.light.

This radiative transition then This radiative transition then

conserves energy and momentum conserves energy and momentum

by giving off light whenever an by giving off light whenever an

electron and hole recombine.electron and hole recombine. CB

VB

This gives rise to This gives rise to (for us) a new type (for us) a new type of device;of device;

the light emitting diode (LED).the light emitting diode (LED).

Mechanism is “injection Mechanism is “injection Electroluminescence”. Electroluminescence”. Luminescence Luminescence part tells us that we are producing photons.part tells us that we are producing photons.

Electro part tells us that Electro part tells us that the photons are being produced the photons are being produced by an electric currentby an electric current..

e-

Injection tells us that Injection tells us that photon production is by photon production is by the injection of current carriers.the injection of current carriers.

Mechanism behind photon Mechanism behind photon emission in LEDs?emission in LEDs?

e-

Producing photonProducing photon

Electrons recombine with holes.Electrons recombine with holes.

Energy of photon is the energy of Energy of photon is the energy of band gap.band gap.

CB

VB

e-

h

Method of injectionMethod of injection We need putting a lot of eWe need putting a lot of e--’s where there are lots of ’s where there are lots of

holes.holes. So electron-hole recombination can occur.So electron-hole recombination can occur. When forward bias is applied to a p-n junction, the When forward bias is applied to a p-n junction, the

injection of minority carriers across the junction can injection of minority carriers across the junction can give rise to efficient radiative recombination, since give rise to efficient radiative recombination, since electric energy can be converted photons. electric energy can be converted photons.

n-side

p-side

-

+

I

Materials for LEDsMaterials for LEDs Since, the eye is only sensitive to Since, the eye is only sensitive to

light of energy hv≥1.8eV (~0.7µm).light of energy hv≥1.8eV (~0.7µm). The semiconductor bandgap energy The semiconductor bandgap energy

defines the energy of the emitted defines the energy of the emitted photons in a LED.photons in a LED.

To fabricate LEDs that can emit To fabricate LEDs that can emit photons from the infrared to the photons from the infrared to the ultraviolet parts of the e.m. spectrum, ultraviolet parts of the e.m. spectrum, then we must consider several then we must consider several different material systems.different material systems.

No single system can span this No single system can span this energy band at present, although the energy band at present, although the 3-5 nitrides come close.3-5 nitrides come close.

CB

VB

Unfortunately, many of potentially useful 2-6 Unfortunately, many of potentially useful 2-6 group of direct band-gap semiconductors group of direct band-gap semiconductors come come naturally doped naturally doped either p-type, or n-type, but either p-type, or n-type, but they don’t like to be type-converted by they don’t like to be type-converted by overdoping.overdoping.

The material reasons behind this are The material reasons behind this are complicated and not entirely well-known.complicated and not entirely well-known.

The same problem is encountered in the 3-5 The same problem is encountered in the 3-5 nitrides and their alloys InN, GaN, AlN, nitrides and their alloys InN, GaN, AlN, InGaN, AlGaN, and InAlGaN. The amazing InGaN, AlGaN, and InAlGaN. The amazing thing about thing about 3-5 nitride alloy 3-5 nitride alloy systems is that systems is that appear to be appear to be direct gap direct gap throughout.throughout.

When we talk about light ,it is conventional to specify When we talk about light ,it is conventional to specify its wavelength, its wavelength, λλ, instead of its frequency. , instead of its frequency.

Visible light has a wavelength on the order of Visible light has a wavelength on the order of nanometers. nanometers.

Thus, Thus, a semiconductor with a 2 eV band-gapa semiconductor with a 2 eV band-gap should should give give aa light at about 620 nm light at about 620 nm (in the(in the red). A red). A 3 eV 3 eV band-gap material band-gap material would emit at would emit at 414 nm414 nm, in the violet. , in the violet.

The human eye, ofThe human eye, of course, is not equally responsive course, is not equally responsive to all colors.to all colors.

( )( )

hcnm

E eV

Relative response of the human eye to various Relative response of the human eye to various colorscolors

350 400 450 500 550 600 650 700 750

100

10-1

10-2

10-3

10-4

Relative eye responseRelative eye response

Wavelength in nanometers

The materials which are used for important light emitting diodes (LEDs) for each of the different spectral regions.

GaN

GaN

Zn

Se

Zn

Se

violet blue

GaP

:NG

aP:Ngreen yellow

GaA

sG

aAs .1

4.1

4PP868

6

GaA

sG

aAs .3

5.3

5pp656

5

redorange

GaA

sG

aAs .6.6

pp44

A number of the important LEDs are based on the GaAsP system.

GaAsGaAs is a direct band-gap S/C with a band gap of 1.42 eV1.42 eV (in the infrared).

GaPGaP is an indirect band-gap material with a band gap of 2.26 eV2.26 eV (550nm, or green).

GaAsGaP

1.42 eV

Materials for visible wavelength LEDsMaterials for visible wavelength LEDs

We see them almost everyday, either on calculator We see them almost everyday, either on calculator displays or indicator panels.displays or indicator panels.

Red LED use as “ power on” indicatorRed LED use as “ power on” indicator Yellow, green and orange LEDs are also widely available Yellow, green and orange LEDs are also widely available

but very few of we will have seen a blue LED.but very few of we will have seen a blue LED.

Red LEDsRed LEDs

can be made in the GaAsP can be made in the GaAsP (gallium arsenide phosphide). (gallium arsenide phosphide).

GaAsGaAs1-x1-xPPx x

for 0<x<0.45 has direct-gapfor 0<x<0.45 has direct-gap for x>0.45 the gap goes for x>0.45 the gap goes

indirect andindirect and for x=0.45 the band gap for x=0.45 the band gap

energy is 1.98 eV.energy is 1.98 eV. Hence it is useful for red Hence it is useful for red

LEDs.LEDs.

N-GaAs substrate

N-GaAsP P = 40 %

p-GaAsP region

Ohmic Contacts

Dielectric(oxide or nitride)

Fig. GaAsP RED LED on a GaAs sub.

Color NameColor Name WavelengthWavelength(Nanometers)(Nanometers)

SemiconductorSemiconductorCompositionComposition

InfraredInfrared 880880 GaAlAs/GaAsGaAlAs/GaAs

Ultra RedUltra Red 660660 GaAlAs/GaAlAsGaAlAs/GaAlAs

Super RedSuper Red 633633 AlGaInPAlGaInP

Super OrangeSuper Orange 612612 AlGaInPAlGaInP

OrangeOrange 605605 GaAsP/GaPGaAsP/GaP

YellowYellow 585585 GaAsP/GaPGaAsP/GaP

Pure GreenPure Green 555555 GaP/GaPGaP/GaP

Super BlueSuper Blue 470470 GaN/SiCGaN/SiC

Blue VioletBlue Violet 430430 GaN/SiCGaN/SiC

UltravioletUltraviolet 395395 InGaN/SiCInGaN/SiC

ApplicationsApplications

Large-area Large-area LED displays are used as stadium are used as stadium displays and as dynamic decorative displays. displays and as dynamic decorative displays.

One-color light is well suited for One-color light is well suited for traffic lights and and signals, signals, exit signs, , emergency vehicle lighting, , ships' navigation lights ships' navigation lights

LEDs are used as LEDs are used as street lights and in other and in other architectural lighting where color changing is used. where color changing is used. The mechanical robustness and long lifetime is The mechanical robustness and long lifetime is used in used in automotive lighting on cars, motorcycles, on cars, motorcycles, and and bicycle lights..

Screens for TV and computer displays can be Screens for TV and computer displays can be made thinner using LEDs for made thinner using LEDs for backlightingbacklighting..

Cont…Cont…

LEDs are used for infrared illumination in LEDs are used for infrared illumination in night vision uses including uses including security cameras. .

light from LEDs can be modulated very light from LEDs can be modulated very quickly so they are used extensively in quickly so they are used extensively in optical fiber and and free space optics communications. This includes communications. This includes remote controls, such as for TVs, VCRs, , such as for TVs, VCRs, and LED Computers.and LED Computers.

ReferencesReferences

Physics of Semiconductor DevicesPhysics of Semiconductor Devices

-By S.M.Sze-By S.M.Sze google.comgoogle.com googleimages.comgoogleimages.com wikipedia.comwikipedia.com

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