This presentation describes about the working of LED & LASER.

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

3. Light Emitting Diode: LED

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

10. Kinds of LEDs

11. Light Amplification by Stimulated
Emission of Radiation :
LASER

12. Principle of Laser
Diode

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

27. Thank You !