LEDs - Presentation 16_9

7/30/2019 LEDs - Presentation 16_9

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Light EmittingDiodes (LEDs)

IN-214 Semiconductor Devices and Circuits

Panel Discussion #2

Group - IV


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LEDs are semiconducjunctions, that under

bias conditions can emit

by electroluminescence

UV, visible or infrared re

the electromagnetic spec

The quanta of light

released is appro

proportional to the band

the semiconductor.


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Direct Band Gap Semiconductors


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Luminescence: the process behind light em

Luminescence is a term used todescribe the emission of radiationfrom a solid when the solid issupplied with some form of energy.

In electro-luminescence excitationresults from the application of anelectric field.

In a p-n junction diode, injectionelectro-luminescence occurs resultingin light emission when the junction isforward biased.


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Injection Luminescence in LED

eVo

Eg

p n+

h =Eg

Eg

p n+

Electrons in CB

Holes in VB

Under forward bias majority carriers from both sides of the junction

the depletion region and entering the material at the other side.

Minority carriers will diffuse and recombine with the majority carrier.

The recombination causes light to be emitted.

Such process is termed radiative recombination.


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Ideal LED will have all injection electrons to take pa

recombination process.

In real device not all electron will recombine with

radiate light.

Sometimes recombination occurs but no light is being(non-radiative).

Efficiency of the device therefore can be described.

Efficiency is the rate of photon emission over the rate o

electrons

Recombination and Efficiency


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Emission wavelength,

The number of radiative recombination is proportional to t

injection rate.

Carrier injection rate is related to the current flowing in the

If the transition take place between states (conduction an

bands),

the emission wavelength, =hc

(ECEV)

we know, EC-EV = Eg

thus, = hc/Eg


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LED Construction

Efficient light emitter is also an efficient absorbers of therefore, a shallow p-n junction required.

Active materials (n and p) will be grown on a lattice substrate.

The p-n junction will be forward biased with contacts m

metallization to the upper and lower surfaces. Ought to leave the upper part clear so photon can escape.


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Efficient LED

Need a p-n junction - preferably the same semiconductoronly different dopants.

Recombination must occur: Radiative transmission to giveright colored LED.

Right colored LED : hc/ = Ec-Ev = Egso choose material with the right E

g Direct band gap semiconductors to allow efficient recombina

All photons created must be able to leave the semiconductor

Little or no reabsorption of photons.


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Correct BandDirect Bandgap

Material can

easily doped

Efficient radiative

pathways must exist

Materials

Requirements


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Candidate

Materials

Direct band gap

materials

e.g. GaAs not Si

UV-LED ~0.5-400

Eg > 3.25eV

V-LED ~450-650nm

Eg = 3.1eV

IR-LED ~

750nm- 1Eg = 1.65e

Readily doped n and pMaterials with refractive

index that could allow light

to get out


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Visible LEDs

LED which could emit visible light, the band gap of the materials

use must be in the region of visible wavelength = 390 - 770ncoincides with the energy value of 3.18eV - 1.61eV correspo

different colors.

Violet ~ 3.17eV

Blue ~ 2.73eVGreen ~ 2.52eVYellow ~ 2.15eVOrange ~ 2.08eV

Red ~ 1.62eV

The band gap, Egthe semiconduc

must posses to eeach light


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Candidate Materials for LEDs


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Semiconductors of Interest for Visible-LED


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Color Wavelength [nm] Semiconductor material

Infrared > 760Gallium Arsenide (GaAs)

Aluminium Gallium Arsenide (AlGaAs)

Red 610 < < 760

Aluminium Gallium Arsenide (AlGaAs)

Gallium Arsenide Phosphide (GaAsP)

Gallium(III) Phosphide (GaP)

Green 500 < < 570

Gallium(III) Nitride (GaN)

Gallium(III) Phosphide (GaP)

Aluminium Gallium Phosphide (AlGaP)

Blue 450 < < 500Zinc Selenide (ZnSe)

Indium Gallium Nitride (InGaN)

Ultraviolet < 400

Diamond (235 nm)

Boron Nitride (215 nm)

Aluminium Nitride (AlN) (210 nm)

White Broad spectrum Blue/UV diode with yellow phosphor

Conventional Materials for LEDs


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OLED Device Structures


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Typical Materials for OLEDs

Color Wavelength [nm] Semiconductor material

Red 610 < < 760

DCM-doped Alq3

DCJTB-doped Alq3

DCJTB- and rubrene-doped Alq3

Green 500 < < 570

Alq3

QA-doped Alq3

coumarin-doped Alq3

DMQA-doped Alq3

Blue 450 < < 500

DSA-doped DPVBiBCzVB-doped CBP

BCzVB-doped DPVBi

DPVBi

Salq

White Broad spectrum

DPVBi/Alq3

DCJTB-doped SAlq

PAP-ph + Alq3 + DCM-doped Alq3


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Formulae Supplement for the Materials

Alq3 = tris(8-hydroxyquinolato)aluminium(III).

BCzVB = 4, 4-(bis(9-ethyl-3-carbazovinylene)-1,1-phenyl. CBP = 4, 4-N,N-dicarbazole-biphenyl.

DCJTB = 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-4H-pyran.

DCM = 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-py

DMQA = N,N-dimethylquinacridone.

DPVBi = 4,

4-bis(2,2

-diphenylvinyl)-1,1

-biphenyl. DSA = distryrylarylene.

PAP-ph = 1,7-diphenyl-4-biphenyl-3,5-dimethyl-1,7-dihydrodipyrazolo[3,43-e]pyridine.

QA = quinacridone.

SAlq = bis(2-methyl-8-quinolato)-(triphenylsiloxy)aluminium(III).


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AnyQuestions


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THANK YOU

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