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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(
240.1
m)(
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
h
Out
h
E2
E2
E1 E1
Absorption, spontaneous (random photon) emission and stimulated
emission.
© 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
E
1
h13
E
2
Metastable
state
E
1
E
3
E
2
h32
E
1
E
3
E
2
E
1
E
3
E
2
h21
h21
Coherent photons
OUT
(a) (b) (c) (d)
E
3
.
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 h3
• 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 Operation
p+ n+
E
Fn
(a)
E
g
E
v
E
c
E
v
Holes in VB
Electrons in CB
Junction
Electrons E
c
p+
E
g
V
n+
(b)
E
Fn
eV
E
Fp
Inversion
region
E
Fp
E
c
E
c
eV
o
•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 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 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 !

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LED & LASER sources of light

  • 1. Semiconductor Devices Presentation on Photonic sources of light By Rohit Singh M.Tech VST School of Engineering Shiv Nadar University
  • 2. 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
  • 4. What is an LED? • Light-emitting diode • Semiconductor • Has polarity
  • 5. LED: How It Works • When current flows across a diode • Negative electrons move one way and positive holes move the other way
  • 6. 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
  • 7. 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
  • 8. 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( 240.1 m)( gE  E=hc/
  • 9. Inside a Light Emitting Diode 1. Transparent Plastic Case 2. Terminal Pins 3. Diode
  • 11. Light Amplification by Stimulated Emission of Radiation : LASER
  • 13. Stimulated Emission E1 E2 h (a) Absorption h (b) Spontaneous emission h (c) Stimulated emission In h Out h E2 E2 E1 E1 Absorption, spontaneous (random photon) emission and stimulated emission. © 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.
  • 14. 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
  • 15. Principles of Laser E 1 h13 E 2 Metastable state E 1 E 3 E 2 h32 E 1 E 3 E 2 E 1 E 3 E 2 h21 h21 Coherent photons OUT (a) (b) (c) (d) E 3 . 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 h3 • 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.
  • 16. 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.
  • 17. 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.
  • 18. Diode Laser Operation p+ n+ E Fn (a) E g E v E c E v Holes in VB Electrons in CB Junction Electrons E c p+ E g V n+ (b) E Fn eV E Fp Inversion region E Fp E c E c eV o •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
  • 19. 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
  • 20. 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 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
  • 21. 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.
  • 22. 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
  • 23. 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
  • 24. Reflection of Photons Back and Forth, Higher Gain Fabry-Parrot Cavity The photons vibrates to and forth with resonant wavelength
  • 25. 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.
  • 26. Graph between optical power and diode current