Light Emitting Device(uday).pptx

8/10/2019 Light Emitting Device(uday).pptx

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Light Emitting Devices

Assignment:

Semiconductor Device modeling and stimulation

1

Submitted by:

Uday Kumar Rai

ECE Deptt.

ME-modular 2014

Roll no.- 141633

Submitted to:

Dr. S B L SACHAN

Professor &Head

ECE Deptt.

NITTTR-CHANDIGARH


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CONTENTS1. PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE

1.1 BASIC TRANSISTION

1.2 LUMINESCENCE EFFICIENCY

2. MATERIALS FOR OPTOELECTRONIC DEVICE

3. LIGHT EMITTING DIODE(LED)

3.1 GENERATION OF LIGHT3.2 LED EXTERNAL QUANTUM EFFICIENCY

3.3 LED DEVICE

3.4 APPLICATIONS

4. LASER DIODES

4.1 STIMULATED EMMISSION AND POPULATION

INVERSION

4.2 OPTICAL CAVITY

4.3 APPLICATIONS

2


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Photoluminescence and Electroluminescence

When excess electron and holes recombine, it result

in emission of photon known as luminescence.

Photoluminescence: Photon emission from recombination

process when excess electron and holes are created by photon

absorption, is called photoluminescence.

Electroluminescence: Photon emission when the excitation of

excess carrier is a result of an electric current caused by an applied

field.

3


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Basic transitions

Once electrons-holespair are formed, there

are several possible

process by which the

electrons and holescan recombine.

Fig 1

(a)_ Basic interband transition(b)_Possible recombination process

involving impurity or defect states

(c)_ Auger recombination process

4Fig_1 Basic transitions in a semiconductor


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The recombination shown in fig 1(a) indicate that theemission of a photon is not necessarily at a single, discrete

energy, but can occur over a range of energies.

The spontaneous emission rate is given as :-

I(v)v2(hv-Eg)1/2exp[-(hv-Eg)/kT]

Where ,

Eg=bandgap energy

h = planks constant(6.625x10-34J-s

k = Boltzmansconstant(1.38x10-23J/K

5

Basic transitions


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Light emission

6

In certain semiconductors, excited electrons can

relax by emitting light instead of producing heat.

These semiconductors are used in the construction

of light emitting diode and pn junction laser diode.

In these device electrical energy, in the form of acurrent , is converted directly into photon energy.


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Luminescent Efficiency

7

All recombination are not radiactive. An efficient is one in which radiactive transistion predominate .

For all process quantum efficiency is the ratio of radiactive

recombination rate to the total recombination rate. Given by

nq= Rr/R

where,

nq= quantum efficiencyRr= radiactive recombination rate.

R = total recombination rate

Since recombination rate is inversly proportional to life time,

the quantum efficiency in term of lifetime is given by,nq= nr/(nr+ r)

where,

nr= nonradiactive lifetime

r = radiactive lifetime


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Luminescent Efficiency

The interband recombination rate of electrons and holeswill be directly proportional to the number of electronavailable and directly proportional to the number ofavailable energy state(holes). Given by

Rr= Bnp

Where,Rr= band to band recombination rate

B = constant of proportionality

The emission of photons from direct bandgap materials

encounter reabsorption of the emitted photons. Possible solution to reabsorption is to use hetrojuction

devices

8


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Material for optoelectronic devices

Important direct bandgap semiconductor material for optical

devices are GaAsand AlxGa1-xAs. In AlxGa1-xAs the ratio of aluminum can be varied to achieve

specific characteristics as shown in fig_2

Fig_2 Bandgap energy of AlxGa1-xAs

as a function of the mole fraction 9

Fig_ 2 shows bandgap energyas the function of mole

fraction between Al and Ga.

Direct bandgap materials for

0


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Material for optoelectronic devices

Another compound semiconductor for optical device is

GaAs1-xPx

10

Fig_3 (a)Bandgap energy of AlxGa1-xPxas a function of the

mole fraction x (b) E Vs k diagram fo AlxGa1-xPxfor various x

Fig_3 shows

bandgap energy as

a function of mole

fraction x

Direct bandgap

materials for

xx0.45

Indirect bandgap

materials for x>0.45


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LED

11


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When a light-emitting diode is

forward biased, electronsare ableto recombine with holeswithin

the device, releasing energy in the

form of photons.

This effect is called

electroluminescenceand the color

of the light (corresponding to the

energy of the photon) is

determined by the energy gapof

the semiconductor.

Light Emitting diode

12
http://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Electron_holehttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Electroluminescencehttp://en.wikipedia.org/wiki/Energy_gaphttp://en.wikipedia.org/wiki/Energy_gaphttp://en.wikipedia.org/wiki/Electroluminescencehttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Electron_holehttp://en.wikipedia.org/wiki/Electrons


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UV AlGaNBlue GaN, InGaNRed, green GaPRed, yellow GaAsP

IR- GaAs


13


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Application of forward voltage across pn junction result diode

current flow.

This flow of current can produce photons and a light output in

the junction diode, which is known as LED.

LED may have a relatively wide wavelength bandwidth of

between 30-40 nm.

This emission spectrum is narrow.

Particular color light will be visible if output is in the visible

range14


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15

Color

Wavelength

(nm)

Voltage (V) Semiconductor Material

Infrared < 760 V < 1.9 Gallium arsenide(GaAs) Aluminium gallium arsenide(AlGaAs)

Red 610 < < 760 1.63 < V < 2.03 Aluminium gallium arsenide(AlGaAs) Gallium arsenide phosphide(GaAsP)

Aluminium gallium indium phosphide(AlGaInP) Gallium(III) phosphide(GaP)

Orange 590 < < 610 2.03 < V < 2.10 Gallium arsenide phosphide(GaAsP) Aluminium gallium indium phosphide

(AlGaInP)Gallium(III) phosphide(GaP)

Yellow 570 < < 590 2.10 < V < 2.18 Gallium arsenide phosphide(GaAsP) Aluminium gallium indium phosphide

(AlGaInP) Gallium(III) phosphide(GaP)

Green 500 < < 570 1.9 < V < 4.0 Indium gallium nitride(InGaN) / Gallium(III) nitride(GaN) Gallium(III)

phosphide(GaP)Aluminium gallium indium phosphide(AlGaInP) Aluminium

gallium phosphide(AlGaP)

Blue 450 < < 500 2.48 < V < 3.7 Zinc selenide(ZnSe), Indium gallium nitride(InGaN), Silicon carbide(SiC) as

substrate, Silicon(Si)

Violet 400 < < 450 2.76 < V < 4.0 Indium gallium nitride(InGaN)

Purple multiple types 2.48 < V < 3.7 Dual blue/red LEDs,blue with red phosphor,or white with purple plastic

Ultra-

violet

< 400 3.1 < V < 4.4 diamond(235 nm), Boron nitride(215 nm) , Aluminium nitride(AlN) (210 nm)

Aluminium gallium nitride(AlGaN) (AlGaInN) (to 210 nm)

White Broad

spectrum

V = 3.5 Blue/UV diode with yellow phosphor

http://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Delta_(letter)http://en.wikipedia.org/wiki/Gallium_arsenidehttp://en.wikipedia.org/wiki/Aluminium_gallium_arsenidehttp://en.wikipedia.org/wiki/Redhttp://en.wikipedia.org/wiki/Aluminium_gallium_arsenidehttp://en.wikipedia.org/wiki/Gallium_arsenide_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphidehttp://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Orange_(colour)http://en.wikipedia.org/wiki/Gallium_arsenide_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphidehttp://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Yellowhttp://en.wikipedia.org/wiki/Gallium_arsenide_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphidehttp://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Greenhttp://en.wikipedia.org/wiki/Indium_gallium_nitridehttp://en.wikipedia.org/wiki/Gallium(III)_nitridehttp://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_phosphidehttp://en.wikipedia.org/wiki/Bluehttp://en.wikipedia.org/wiki/Zinc_selenidehttp://en.wikipedia.org/wiki/Indium_gallium_nitridehttp://en.wikipedia.org/wiki/Silicon_carbidehttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Violet_(color)http://en.wikipedia.org/wiki/Indium_gallium_nitridehttp://en.wikipedia.org/wiki/Purplehttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Diamondhttp://en.wikipedia.org/wiki/Boron_nitridehttp://en.wikipedia.org/wiki/Aluminium_nitridehttp://en.wikipedia.org/wiki/Aluminium_gallium_nitridehttp://en.wikipedia.org/wiki/Aluminium_gallium_nitridehttp://en.wikipedia.org/wiki/Aluminium_nitridehttp://en.wikipedia.org/wiki/Boron_nitridehttp://en.wikipedia.org/wiki/Diamondhttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Purplehttp://en.wikipedia.org/wiki/Purplehttp://en.wikipedia.org/wiki/Indium_gallium_nitridehttp://en.wikipedia.org/wiki/Violet_(color)http://en.wikipedia.org/wiki/Violet_(color)http://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Silicon_carbidehttp://en.wikipedia.org/wiki/Indium_gallium_nitridehttp://en.wikipedia.org/wiki/Zinc_selenidehttp://en.wikipedia.org/wiki/Bluehttp://en.wikipedia.org/wiki/Bluehttp://en.wikipedia.org/wiki/Aluminium_gallium_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphidehttp://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Gallium(III)_nitridehttp://en.wikipedia.org/wiki/Indium_gallium_nitridehttp://en.wikipedia.org/wiki/Greenhttp://en.wikipedia.org/wiki/Greenhttp://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphidehttp://en.wikipedia.org/wiki/Gallium_arsenide_phosphidehttp://en.wikipedia.org/wiki/Yellowhttp://en.wikipedia.org/wiki/Yellowhttp://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphidehttp://en.wikipedia.org/wiki/Gallium_arsenide_phosphidehttp://en.wikipedia.org/wiki/Orange_(colour)http://en.wikipedia.org/wiki/Orange_(colour)http://en.wikipedia.org/wiki/Gallium(III)_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_indium_phosphidehttp://en.wikipedia.org/wiki/Gallium_arsenide_phosphidehttp://en.wikipedia.org/wiki/Aluminium_gallium_arsenidehttp://en.wikipedia.org/wiki/Redhttp://en.wikipedia.org/wiki/Redhttp://en.wikipedia.org/wiki/Aluminium_gallium_arsenidehttp://en.wikipedia.org/wiki/Gallium_arsenidehttp://en.wikipedia.org/wiki/Delta_(letter)http://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Wavelength


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16

Light Spectrum

Red, green and blue LEDs


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17

Generation of Light

E=c/v = hc/E = 1.24/E m

When voltage applied across pn junction, electron and holes areinjected across the space charge region where they become

excess minority carriers

Excess minority carrier diffuse into neutral semiconductor regionwhere they recombine with majority carrier

If this recombine process is direct band to band process, photons

are emitted

In GaAs, electroluminescence originates primarily on p side of the

junction as the efficiency of electron injection is higher than that

for hole injection


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Led external quantum efficiency

Photons can be emitted in any direction.

Emitted photon energy must be hvEg.

Emitted photons can be reabsorbed within the semiconductor

material.

The majority of photons will be actually emitted away from

the surface and reabsorbed in the semiconductor.

18Fig_4 Schematic of photon emission at the pn junction of an LED


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Photons must be emitted from the semiconductor to air ie,

transmitted across the dielectric surface as shown in fig_5.

19

The parameter n2is is the index ofrefraction for the semiconductor

and n1is the index of refraction forair.

The reflection coefficient is givenas

= [(n2-n1)/(n2+n1)]2

This effect is known as Fresnel loss

Fig_5 Schematic of incident, reflected and

transmitted photon at a dielectric interface


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Photons incident on the semiconductor-air interface at an

angle are reflected as shown in fig_6.

20

If the photons are incident on

the interface at an angle

greater than the critical angle

c, the photon experience

total internal reflection.

The critical angle is determined

from Snells law and is given by

c=sin-1(n1/n2)

Fig_6 Schematic showing refraction and total internal

reflection at the critical angle at a dielectric interface


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fig_7(a) shows the external quantum efficiency plotted as a

function of p type doping concentration. Fig_7(b) shows the external quantum efficiency as a function

of junction depth below the surface

Both fig. shows that the external quantum efficiency is in therange of 1 to 3 percent.

21

Fig_7(a) External quantum efficiency of a GaP LED versus acceptor doping

(b) External quantum efficiency of a GaAs LED versus junction depth


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Led Devices

Direct bandgap material(GaAs)has a bandgap energy Eg=1.42eVand =0.873m as shown in

fig_8

The output of GaAs LED is not inthe visible range.

For visible output, the

wavelength of the signal shouldbe in the range of 0.40.72mand bandgap energy

1.7-3.1eV(approx.)

22

The wavelength of the output of an LED is determined by

the bandgap energy.

Fig_8 GaAs diode emission spectra at T=295K and T=77K


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GaAs1-xPxis a direct bandgap material for 0 x0..45 as

shown in fig_9

23

Led Devices

Fig_9(a)Bandgap energy of GaAs1-xPxas a function of

the mole fraction x

At x=0.40, the bandgap

energy is approximatelyEg=1.9eV which would

produce an optical

output in the red range


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The brightness of GaAs1-xPxdiode can be varied for

different value of x (mole fraction)

24

Led Devices

Fig_10 shows thebrightness of GaAs1-xPxdiode for different value

of x

The peak brightnessoccurs at red color.

GaAs0.6P0.4monolithic

array has beenfabricated for numericand alphanumericdisplay.

Fig_10 brightness of GaAsPdiode versus wavelength


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when the mole fraction x of GaAsP is greater than 0.45, the material

change to an indirect bandgap semiconductor so that the quantumefficiency is greatly reduced.

GaAlxAs1-xcan be used in a hetrojuction structure to form an LED.

Fig_11(a) shows the structure of GaAlAs hetrojunction LED.

Electron are injected from wide bandgap N- GaAl0.7As0.3to the narrow

bandgap p- GaAl0.7As0.3

25

Led Devices

Fig_11(a)cross section of GaAlAshetrojunction LED


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Fig_11(b) shows the thermal equilibrium energy-band

diagram of a GaAlAs hetrojunction LED

26

Led Devices

The minority carrierelectrons in the pmaterial can recombine

radiatively. Since Egp< EgN, photons

are emitted throughwide bandgap Nmaterial with no

absorption.

The wide bandgap Nmaterial act as anoptical window.

Fig_11(b) thermal equilibrium energy-band

diagram of GaAlAs hetrojunction LED


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Never connect an LED directly to a battery or a power supply!It will be destroyed almost instantly because too much current

will pass through and burn it out.

LEDs must have a resistor in series to limit the current to a safe

value, for quick testing purposes a 1kresistor is suitable for most

LEDs if your supply voltage is 12V or less.

Remember to connect the LED the correct way!

Testing of LED

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The most popular type of tri-color LED has a red and agreen LED combined in one package with three leads.

They are called tri-color because mixed red and green

light appears to be yellow.

The diagram shows the organization of a tri-color LED.

Note the different lengths of the three leads.

The central lead (k) is the common cathode for both

LEDs, the outer leads (a1 and a2) are the anodes to theLEDs allowing each one to be lit separately, or both

together to give the third color.

Tri-color LED

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An LED must have a resistor connected in series to

limit the current through the LED. The resistorvalue, R is given by:

R = (VS - VL) / I

Calculating an LED resistor value

VS = supply voltageVL = LED voltage (usually 2V, but 4V for blue and white LEDs)

I = LED current (e.g. 20mA), this must be less than the maximum permitted

If the calculated value is not available, choose the nearest standard resistor value

which is greater,to limit the current. Even greater resistor value will increase the

battery life but this will make the LED less bright.

For example

If the supply voltage VS = 9V, and you have a red LED (VL = 2V), requiring a current

I = 20mA = 0.020A,

R = (9V - 2V) / 0.02A = 350, so choose 390 (the nearest greater standard value).29


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If you wish to have several LEDs on at thesame time, connect them in series.

This prolongs battery life by lighting several

LEDs with the same current as just one LED.

The power supply must have sufficient

voltage to provide about 2V for each LED (4V

for blue and white) plus at least another 2V

for the resistor.

To work out a value for the resistor you must

add up all the LED voltages and use this for

VL.

Connecting LEDs in series

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LED displays are packages of many LEDs arranged in a pattern, the

most familiar pattern being the 7-segment displays for showing

numbers (digits 0-9).

LED Displays It is a common anode display sinceall anodes are joined together and

go to the positive supply.

The cathodes are connected

individually to resistors limiting the

current through each diode to a safe

value.

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Wireless telemedicine

The PillCam is a swallow

diagnostic device, taking

high-quality, high-speed

photos as it passes through

the esophagus.

PillCam transmits 14

pictures/sec. to a receiver

worn by the patient.

This enables diagnosis of

throat disease and related

ailments.

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PN JUNCTION-LASER DIODE

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Laser diode

Laser stand for Light Amplification by StimulatedEmission of Radiation.

Laser diode produces a coherent spectral output with a

bandwidth of wavelength less than 0.1nm.

Laser diode are the modified LED in its structure and

operating condition

There are many different type of laser, one of them are pn

junction laser diode.

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Fig_12 shows the different process of stimulatedemission and population inversion

35

Stimulated Emission and population Inversion

Induced absorption: when an

incident photon is absorbed and

an electron is elevated from E1to

E2

Spontaneous emission: theelectron spontaneously make the

transition back to the lower

energy level with a photon being

emitted. Stimulated emission : there is an

incident photon at a time when

an electron is in the higher

energy state.

Fig_12(a) induced absorption

(b) spontaneous emission

(c) stimulated emission


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In thermal equilibrium, the electron distribution in a

semiconductor is determine by the Fermi-dirac statistics. If

the Boltzmann approximation applies, then we can write

N2/N1= exp[-(E2-E1)/kT]

where

N1=electron concentration in energy level E1

N2=electron concentration in energy level E2

E2>E1

In thermal equilibrium

N2


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In thermal equilibrium (N2N1. this is called population

inversion.

We cannot achieve lasing action at thermal equilibrium.

37


l d d l


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Fig_13 shows the two energy levels with a light wave atan intensity Iv, propagating in the z direction.

The change in intensity of z direction can be written as:-

dlv/dz (3 photons emitted/cm33 photons absorbed/cm3)

or

dlv/dz=N2Wi.hv- N1Wi.hv

38


where

Wi= induced transition

probability The equation assume no

loss mechenism and neglects

the spontaneous transition Fig_13 light propagation in z directionthrough a matrial with two energy levels

l d d l


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We have

dlv/dz=N

2W

i.hv- N

1Wi.hv --------------(1)

which can be written as,

dlv/dz =(v)Iv -------------------------(2)

where,

(v)Iv (N2-N1) is the amplification factorFrom equation (2),

Iv= Iv(0)e(v)z -------------------------------(3)

Amplification occurs when,

(v)>0 and

Absorption occurs when,

(v)


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we can achieve population inversion and lasing in a

forward-biased pn hetrojunction diode, if both sides of thejunction are degenerately doped.

40


Fig_14(a) shows theenergy band diagram of

a degenerately dopedpn junction in thermalequilibrium.

The Fermi level is in the

conduction band in then region and the Fermilevel is in the valenceband in the p region.

Fig_14(a) degenerateely doped pn

junction at zero biased

S i l d E i i d l i I i


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Fig_14(b) shows the energy bands of the pn junction when aforward bias is applied with photon emission.

41


The gain factor in a pn hetrojunction

diode is given by:-

(v) {1-exp[(hv-(Efn-EFp))/kT]}--(4)

in order for, (v)>1 we must have

hv


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Optical cavity help to achieve the coherent emission output

by causing buildup of the optical intensity from positive feedback.

42

Optical cavity

Fig_15 shows the optical cavityfabricated by cleaving a galliumarsenide crystal along the (110)plane

The optical wave propagatesthrough the junction in the zdirection, bouncing back and forthbetween the end mirror.

Only partial optical wave is

transmitted out of the junction. For resonance, the length of the

cavity must be an integral numberof half wavelength

N(/2)=L Fig_15 a pn junction laser diode withcleaved(110) planes forming the Fabry-

Perot cavity



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There can be many resonant mode in the cavity, fig_16 showsdifferent resonant mode

43


Fig_16(a) shows resonant mode

as a function of wavelength

Fig_16(a) resonant mode cavity with length L (b) spontaneous curve (c) actual emission modes

Fig_16(b) shows spontaneous emissionwhen forward bias current is applied

Spontaneous emission is relativelybroadband and is superimposed on thepossible lasing mode

Fig_16(c) shows lasing mode which canoccurs at several specific wavelength.

Lasing will initiate when spontaneousemission gain become more than theoptical losses


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Radar/Laser Detectors

A radar/laser detector is a combination of a radar detector,

which senses radar in the air, and a laser detector, which looksfor laser beams directed at your car.

A laser beam is a very focused

beam of light that does not

separate out from its beampath.

Fortunately, there is a lot of

dust and fine particles in the air,

which causes the laser beam to

separate enough that the

beams can be seen by a proper

detector.

Light Emitting Device(uday).pptx

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Transcript of Light Emitting Device(uday).pptx