led laser presentation

7/29/2019 LED Laser Presentation

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Semiconductor Devices

Presentation on Photonic sources of light

Dr. Atul Vir Singh

Assistant Professor

Shiv Nadar University


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Student Presenter

Rohit Singh

M.Tech (VST)

Shiv Nadar University


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Photonic sources of light

Photonic sources of light are those source of

light in which the basic particle of light-the

photon, plays a major role.

LED

LASER


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Light Emitting Diode: LED


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What is an LED?

Light-emitting diode

Semiconductor

Has polarity


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LED: How It Works

When current flows

across a diode

Negative electrons move one way and

positive holes move the other way


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LED: How It Works

The holes exist at a

lower energy level than

the free electrons

Therefore when a free electrons falls it

losses energy


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LED: How It Works

This energy is emitted in

a form of a photon,

which causes light

The color of the light is determined by the

fall of the electron and hence energy level

of the photon


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Calculation of wavelength emitted

by LED

where E is energy h is

planks constant and c is

velocity of light is

wavelength of light.

Rearranging the termwe get final equation. )eV(

240.1m)(

gE

E=hc/


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Inside a Light Emitting Diode

1. Transparent Plastic

Case

2. Terminal Pins

3. Diode


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Kinds of LEDs


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Light Amplification by Stimulated

Emission of Radiation :

LASER


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Principle of Laser

Diode


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Stimulated Emission

E1

E2

h

(a) Absorption

h

(b) Spontaneous emission

h

(c) Stimulated emission

Inh

Out

h

E2

E2

E1 E1

Absorption, spontaneous (random photon) emission and s timulatedemission.

1999 S. O. Kasap,Optoelectronics (Prentice Hall)

In stimulated emission, an incoming photon with energy h

stimulates the emission process by inducing electrons in E2 to transit

down to E1.

While moving down to E1, photon of the same energy h will be

emitted

Resulting in 2 photons coming out of the system

Photons are amplifiedone incoming photon resulting in two

photons coming out.


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Population Inversion

Non equilibrium distribution ofatoms among the various energy

level atomic system

To induce more atoms in E2, i.e. to

create population inversion, alarge amount of energy is required

to excite atoms to E2

The excitation process of atoms so

N2 > N1 is called pumping

It is difficult to attain pumping

when using two-level-system.

Require 3-level system instead

E2

E1

More atoms

here

N2

N1

N2> N1

E2

E1

E3

There level

system


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Principles of Laser

E1

h13

E2

Metastable

state

E1

E3

E2

h32

E1

E3

E2

E1

E3

E2

h21

h21

Coherent photons

OUT

(a) (b) (c) (d)

E3

.

IN

In actual case, excite atoms from E1 to E3.

Exciting atoms from E1 to E3 optical pumping

Atoms from E3 decays rapidly to E2 emitting h3

If E2 is a long lived state, atoms from E2 will not decay to E1 rapidly

Condition where there are a lot of atoms in E2population inversion achieved!i.e. between E2 and E1.


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Coherent Photons Production

When one atom in E2 decaysspontaneously, a random photon resulted

which will induce stimulated photon fromthe neighbouring atoms

The photons from the neighbouring atomswill stimulate their neighbours and form

avalanche of photons.

Large collection of coherent photonsresulted.


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Laser Diode Principle

Consider a p-n junction

In order to design a laser diode, the p-n junction

must be heavily doped.

In other word, the p and n materials must bedegenerately doped

By degenerated doping, the Fermi level of the

n-side will lies in the conduction band whereasthe Fermi level in the p-region will lie in the

valance band.


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Diode Laser Operationp+ n+

EFn

(a)

Eg

Ev

Ec

Ev

HolesinVB

ElectronsinCB

Junction

Electrons Ec

p+

Eg

V

n+

(b)

EFn

eV

EFp

Inversionregion

EFp

Ec

Ec

eVo

P-n junction must be degenerately doped.

Fermi level in valance band (p) and

conduction band (n).

No bias, built n potential; eVo barrier to

stop electron and holes movement

Forward bias, eV> Eg

Built in potential diminished to zero

Electrons and holes can diffuse to the

space charge layer


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Application of Forward Bias

Suppose that the degenerately doped p-n

junction is forward biased by a voltage greater

than the band gap; eV > Eg

The separation between EFn and EFp is now theapplied potential energy

The applied voltage diminished the built-in

potential barrier, eVo

to almost zero.

Electrons can now flow to the p-side

Holes can now flow to the n-side


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Population Inversion in Diode Laser

Electrons in CB

EFn

EFp

CB

VB

Eg

Holes in VB

eV

EFn-EfP = eV

eV > Eg

eV = forward bias voltage

Fwd Diode current pumping

injection pumping

More electrons in

the conductionband near EC

Than electrons in

the valance band

near EV

There is therefore a population inversion

between energies near EC and near EV around the

junction.

This only achieved when degenerately doped p-n

junction is forward bias with energy > Egap


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The Lasing Action

The population inversion region is a layer along thejunction also call inversion layeroractive region

Now consider a photon with E = Eg

Obviously this photon can not excite electrons from EV

since there isNOelectrons there However the photon CAN STIMULATE electron to fall

down from CB to VB.

Therefore, the incoming photon stimulates emission thanabsorption

The active region is then said to have opticalgain sincethe incoming photon has the ability to cause emissionrather than being absorbed.


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Pumping Mechanism in Laser Diode

It is obvious that the population inversionbetween energies near EC and those near

EV occurs by injection of large chargecarrier across the junction by forwardbiasing the junction.

Therefore the pumping mechanism is

FORWARD DIODE CURRENT Injection pumping


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For Successful Lasing Action:

1. Optical Gain (not absorb)Achieved by population inversion

2. Optical Feedback

Achieved by device configuration

Needed to increase the total optical amplification bymaking photons pass through the gain region

multiple times

Insert 2 mirrors at each end of laser

This is term an oscillator cavity or Fabry Perotcavity

Mirrors are partly transmitted and party reflected


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Reflection of Photons Back and Forth,

Higher Gain

Fabry-Parrot Cavity

The photons vibrates to

and forth with resonant

wavelength


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Difference between LASER and LED

LASER

Lasers are monochromatic

(single color wavelength),

collimated (non-divergent)and coherent (wavelengths

in- phase)

The peak output power is

measured in watt

LED

LED's are neither coherent

nor collimated and generate

a broader band ofwavelengths (multiple).

The peak output power is

measured in milliwatt.


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Graph between optical power and

diode current


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Thank You !

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