The seminar discussed luminescence and various light-emitting devices. It defined luminescence as light emission not resulting from heat and described different types including photoluminescence, electroluminescence, and cathodoluminescence. It also explained the working principles of light-emitting diodes (LEDs) and how direct bandgap semiconductors allow for light emission. Additional topics covered included solar cells and their use in applications such as toys, watches and water pumping.

1. SEMINAR On fed
TOPIC-LUMINESCENCE
AVNEET SINGH LAL
EC-2

2. OPTICAL ABSORPTION
 A technique used for measuring the band
gap energy of a semiconductor is the
absorption of incident photons by the
materials.
 This experiment gives an accurate measure
of the band gap energy.

3. DIRECT & INDIRECT BANDGAP
SEMICONDUCTORS
 Those materials for which maximum of valence band and
minimum of conduction band lie for same value of k,
called DIRECT BANDGAP MATERIALS (i.e. satisfies
the condition of energy and momentum conservation).
For example: GaAs, InP, CdS..etc

4.  Those materials for which maximum of valence
band and minimum of conduction band do not occur
at same value of k, called INDIRECT BANDGAP
MATERIALS.
 For example: Si and Ge

5. BAND GAPS OF SOME COMMON
SEMICONDUCTORS RELATIVE TO THE OPTICAL
SPECTRUM

6. LUMINESCENCE
 The term 'luminescence' was introduced in 1888 by
Eilhard Wiedemann.
 Luminescence is emission of light by a substance
not resulting from heat; it is thus a form of cold-
body radiation.
 It can be caused by chemical reactions, electrical
energy and many other processes.
 Many of the semiconductors are well suited for
light emission, particularly the compound
semiconductors with direct band gaps.
 This overall category can be subdivided according
to the excitation mechanism.

7. TYPES OF LUMINESCENCE
a) PHOTOLUMINESCENCE
b) ELECTROLUMINESCENCE
c) CATHODOLUMINESCENCE

8. PHOTOLUMINESCENCE
 Photoluminescence (abbreviated as PL) is light
emission from any form of matter after the
absorption of photons (electromagnetic radiation).
It is one of many forms of luminescence (light
emission) and is initiated by photoexcitation
(excitation by photons).

9.  The emission of photons stops within
approximately 10-8 seconds after the
excitation is turned off. Such fast
luminescent processes are often referred to
as FLUORESCENCE.
 In some materials, emission continues for
periods up to seconds or minutes after the
excitation is removed. These slow processes
are called PHOSPHORESCENCE.

10. ELECTROLUMINESCENCE
 Electroluminescence (EL) is an optical phenomenon
and electrical phenomenon in which a material emits
light in response to the passage of an electric
current or to a strong electric field.
 This is distinct from black body light emission
resulting from heat (incandescence), from a
chemical reaction (chemiluminescence), sound
(sonoluminescence), or other mechanical action
(mechanoluminescence).

11.  There are many ways by which electrical energy
can be used to generate photon emission in a solid.
In LEDs an electric current causes the injection of
minority carriers into regions of the crystal where
they can recombine with majority carriers,
resulting in the emission of recombination radiation.
This effect is called as INJECTION
ELECTROLUMINESCENCE.

12. CATHODOLUMINESCENCE
 Cathodoluminescence is an optical and
electromagnetic phenomenon in which electrons
impacting on a luminescent material such as a
phosphor, cause the emission of photons which may
have wavelengths in the visible spectrum.
 The most common example of the excitation of
luminescent materials by energetic electrons is the
CATHODE-RAY TUBE(CRT).
 This light emitting tube is the basis of the
oscilloscope, television set, and other visual-display
systems.
 The basic principle of CRT is the selective
excitation of a phosphorescent screen by a beam
of energetic electrons within a vacuum tube.

13. crt

14. LED

15. ABOUT LED
 LED is an opto-electronic device which emits a narrow
bandwidth of visible or invisible light when its internal diode
junction is stimulated by a forward electric current or
voltage.
 LEDs convert electrical energy into light energy.
 The material used for LEDs are the direct Band Gap
Semiconductors which are made from group 3rd and group 5th
elements. For eg.- GaAs

16. OPERATION OF LED
 The operation of LED is based on the phenomenon
of electro luminance.
 The recombination of charge carriers takes place in
a forward p-n junction as the electrons cross from
the n-region and recombine with holes existing in
p-region.

17. APPLICATIONS
 Medical Instrumentation
 Bar Code Readers
 Fibre Optic Communication
 Digital Cameras
 Laptops
 Traffic Signals
 Rail
 Tower Lights

18. Solar cell
A structure that converts solar energy
directly to DC electric energy.
It supplies a voltage and a current to a
resistive load (light, battery, motor).
It is like a battery because it supplies DC
power.
It is different from a battery in the sense
that the voltage supplied by the cell
changes with changes in the resistance of
the load.

19. WHY SOLAR CELLS?
Low maintenance, long lasting sources of
energy
Non-polluting and silent sources of
electricity
Renewable and sustainable power, as a
means to reduce global warming.

20. APPLICATIONS
 Toys, watches, calculators
 Water pumping
 Water treatment
 Emergency power
 Portable power supplies

21. CONCLUSION
 The International Year of Light was officially launched on 19
January 2015 with a two-day Opening Ceremony at UNESCO
Headquarters in Paris.
 The International Year of Light and Light-based Technologies 2015
or International Year of Light 2015 (IYL 2015) was a United
Nations observance that aimed to raise awareness of the
achievements of light science and its applications, and its
importance to humankind.

22. REFERENCES
 SOLID STATE ELECTRONIC DEVICE (BEN
G. STREETMAN)
 LECTURE IN PHYSICS (FENNMAN)
 https://en.wikipedia.org

23. THE END
THANK YOU FOR YOUR
ATTENTION