Semiconductor Devices

Presentation on Photonic sources of light

By Rohit Singh

M.Tech VST

School of Engineering

Shiv Nadar University

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

Light Emitting Diode: LED

What is an LED?

• Light-emitting diode• Semiconductor• Has polarity

LED: How It Works

• When current flows across a diode

• Negative electrons move one way and positive holes move the other way

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

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

Calculation of wavelength emitted by LED

• where E is energy h is planks constant and c is velocity of light λ is wavelength of light.

• Rearranging the term we get final equation. )eV(

240.1m)(

gE

E=hc/

Inside a Light Emitting Diode

1. Transparent Plastic Case

2. Terminal Pins

3. Diode

                                                                  

                                                                                                                                                                     

Kinds of LEDs

Light Amplification by Stimulated Emission of Radiation :

LASER

Principle of Laser Diode

Stimulated Emission

E1

E2

h

(a) Absorption

h

(b) Spontaneous emission

h

(c) Stimulated emission

In hOut

h

E2 E2

E1 E1

Absorption, spontaneous (random photon) emission and stimulatedemission.

© 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 amplified – one incoming photon resulting in two photons coming out.

Population Inversion• Non equilibrium distribution of

atoms among the various energy level atomic system

• To induce more atoms in E2, i.e. to create population inversion, a large 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

Principles of Laser

E1

hu13

E2

Metastablestate

E1

E3

E2

hu32

E1

E3

E2

E1

E3

E2

hu21

hu21

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 E2 population inversion achieved!

i.e. between E2 and E1.

Coherent Photons Production

• When one atom in E2 decays spontaneously, a random photon resulted which will induce stimulated photon from the neighbouring atoms

• The photons from the neighbouring atoms will stimulate their neighbours and form avalanche of photons.

• Large collection of coherent photons resulted.

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 be degenerately doped

By degenerated doping, the Fermi level of the n-side will lies in the conduction band whereas the Fermi level in the p-region will lie in the valance band.

Diode Laser Operationp+ n+

EFn

(a)

Eg

Ev

Ec

Ev

Holes in VBElectrons in CB

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

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 the applied 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

Population Inversion in Diode Laser

Electrons in CBEFn

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 conduction band 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

The Lasing Action

• The population inversion region is a layer along the junction also call inversion layer or active region

• Now consider a photon with E = Eg

• Obviously this photon can not excite electrons from EV since there is NO electrons there

• However the photon CAN STIMULATE electron to fall down from CB to VB.

• Therefore, the incoming photon stimulates emission than absorption

• The active region is then said to have ‘optical gain’ since the incoming photon has the ability to cause emission rather than being absorbed.

Pumping Mechanism in Laser Diode

• It is obvious that the population inversion between energies near EC and those near EV occurs by injection of large charge carrier across the junction by forward biasing the junction.

• Therefore the pumping mechanism is FORWARD DIODE CURRENT Injection pumping

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 by making photons pass through the gain region multiple times

Insert 2 mirrors at each end of laser

This is term an oscillator cavity or Fabry Perot cavity

Mirrors are partly transmitted and party reflected

Reflection of Photons Back and Forth, Higher Gain

Fabry-Parrot Cavity

The photons vibrates to and forth with resonant wavelength

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 of wavelengths (multiple).

• The peak output power is measured in milliwatt.

Graph between optical power and diode current

Thank You !