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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 !