Semiconductor lasers operate based on stimulated emission of radiation from a semiconductor material. When a semiconductor is forward biased, electrons from the n-type region combine with holes in the p-type region and release energy in the form of photons. These photons stimulate additional electrons to release photons of the same frequency, resulting in coherent laser emission. Semiconductor lasers can be homojunction lasers made of the same semiconductor material on both sides or heterojunction lasers made of different materials on each side. They have applications in optical storage, laser printing, barcode scanners, and fiber optic communication due to their small size, efficiency and ability to be integrated with other devices.

1. By Vidhya Moorthy

2. • Semiconductor materials
• P N Junction Diode
• History Of Semiconductor Lasers
• Types Of Semiconductor Based On Emission
• Principle Of Semiconductor Laser
• Classification Of Semiconductor Laser
• Homojunction Semiconductor Laser
• Heterojunction Semiconductor Laser
• Characteristics
• Advantages
• Applications

3. • Semiconductor materials are
a combination of third group
elements of the Periodic
Table.
• They have a resistance of
10^-2.
• These materials usually have
a ‘valence band’ and
‘conduction band’ of
electrons.
• Normally there is no electrons
in the conduction band ; the

4. When a small
amount of energy
is given the
electrons jump
from valence
band to
conduction band
to conduct
current.
Conduction
band
Valence band
ENERGY
BAND
DIAGRAM
ENERG
Y

5. • It consist of a junction of
a P-type semiconductor
and N-type
semiconductor.
• P-type semiconductor :
conduction through
positive holes; doped
with trivalent impurity.
• N-type semiconductor :
conduction through
negative electrons;
doped with pentavalent
impurity.

6. Semiconductor materials are of two types based on
emission ;
• Direct Band Gap Semiconductor
• Indirect Band Gap Semiconductor
Direct Band Gap Semiconductor
When the holes and electrons combine during the action of
the diode; Photons are emitted.
Example : Ga-As diode
Indirect Band Gap Semiconductor
When the holes and electrons combine during the action of
the diode; Heat Energy is emitted.
Example : Ge, Si diodes

7. DIRECT BAND GAP
DIODE
INDIRECT BAND GAP DIODE

8. • The first laser diodes were
developed in the early 1960s
• The device shown is an early
example. It would require very
high current flow to maintain a
population inversion, and due
to the heat generated by the
steady-state current, the
device would be destroyed
quickly.
• Nowadays technological
innovations have made Diode
lasers more powerful and
strong.

9. Basov, Vul, Popov, Krokhin:
1957 first semiconductor laser
proposal and development
1961 first injection laser
proposal (also Dumke 1962)
Basov: Nobel prize 1964 (with
Prokhorov and Townes

10. Laser operatimg at
room temperature

11. When the P-N Junction diode is Forward Biased (i.e) the
P end of the diode is connected to the positive terminal of
the battery and the N end is connected to the negative
terminal of the battery. The poles and electrons diffuse
through the junction and combine with each other;
meanwhile light radiations or photons are radiated. This is
called Recombination Radiation
These emitted photons stimulate Other
electrons & holes to recombine which
Results in stimulated emission required for
Lasing Action.

12. Semiconductor Laser
Homojunction
Diode Laser
Heterojunction
Diode Laser
Double
Heterojunction
Diode Laser
Single
Heterojunction
Diode Laser

13. Homojunction diode lasers are those in which P end and N end
of the diode are made of the same semiconductor material.
Example : Ga As laser
• They use Direct Band Gap Semi-
conductor material.
• P-N Junction act as the active
medium.
• The crystal is cut at a thickness of
0.5 mm
• Applied voltage is given through
metal contacts on both surfaces of
the diode.
• Pulse beam of laser of 8400 Å is produced

14. FORWARD BIASED DIODE LASER
metal contact
Ga –As material on both ends
P end
N end
Laser beam
+

15. FREE ELECTRONS
Conduction Band
energy barrier
stimulated emission Band gap
(Eg)
Energy
laser (8400 Å )
FREE HOLES
Valence Band

16. Heterojunction Semiconductor lasers are those in which P end is
made of one type of semiconductor material and the N end is
made of another type of semiconductor material
Example : GaAIAs diode laser
Use Direct Band Gap Semiconductor
Consist of five layers namely
• GaAs – p type
• GaAIAs – p type
• GaAs – p type (Active Medium)
• GaAIAs – n type
• GaAs – n type
The end faces of the third layer is highly polished and perfectly
paralell to each other to reflect the laser beam ; one end is
partially polished to release the continious beam.

17. metal contact
P end
N end
Laser Beam
Ga As
GaAIAs

19. • Most SC lasers operate in 0.8 – 0.9 µm or 1 – 1.7 µm
spectral region
• Wavelength of emission determined by the band gap
• Different SC materials used for different spectral regions
• 0.8 – 0.9 µm : Based on Gallium Arsenide
• 1 – 1.7 µm : Based on Indium Phosphide (InP)
• Pumping method : Direct Conversion
• High power lasers usually (1 mV )

20. HOMOJUNCTION DIODE
LASER
• P and N regions are made
of the same diode
material
• Active medium : Single
crystal of PN Diode
• Pulse beam
• Wavelength : 8300Å-
8500Å
• Example : GaAs,InP.
HETEROJUNCTION DIODE
LASER
• P and N regions are made
of different diode material
• Active Medium : Third
layer of p type material
among the five layers
• Continuous beam
• Wavelength : 8400 Å
• Example : GaAs/GaAIAs,
InP/InAIP .

21. • They are light weighted and portable.
• Battery supported ; easily replaceable
• Capability of direct modulation into Gigahertz region
• Small size and low cost
• Capability of Monolithic integration with electronic
circuitry
• Direct Pumping with electronic circuitry
• Compatibility with optical fibres

26. • The semiconductor materials have valence band V and conduction band C, the
energy level of conduction band is Eg (Eg>0) higher than that of valence band.
To make things simple, we start our analysis supposing the temperature to be 0
K. It can be proved that the conclusions we draw under 0 K applies to normal
temperatures.
• Under this assumption for nondegenerate semiconductor, initially the conduction
band is completely empty and the valence band is completely filled. Now we
excite some electrons from valence band to conduction band, after about 1 ps,
electrons in the conduction band drop to the lowest unoccupied levels of this
band, we name the upper boundary of the electron energy levels in the
conduction band the quasi-Fermi level Efc. Meanwhile holes appear in the
valence band and electrons near the top of the valence band drop to the lowest
energy levels of the unoccupied valence energy levels, leave on the top of the
valence band an empty part. We call the new upper boundary energy level of the
valence band quasi-Fermi level Efv. When electrons in the conduction band run
into the valence band, they will combine with the holes, in the same time they
emit photons. This is the recombination radiation. Our task is to make this
recombination radiation to laser
• Read more:
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