A basic presentation on Semiconductor and LEDs from NCERT Class XII Textbook prepared by me.

1. Semiconductors
& Leds
By Akash Ganguly
b.Tech. polymer science
Roll no. : 1308031

2. Index
 Semiconductors
 Concept of Energy bands
 intrinsic and extrinsic semiconductors
 Concept of electrons and holes
 N Type Semiconductors
 P type semiconductors
 P – N JUNCTION DIODE
 P-n junction in forward bias and reverse bias
 Graph for p-n junction at forward and reverse bias
 LIGHT EMITTING DIODE ( led)
 Major contributions of various scientists on LEDS
 Research on Use of LED in place of incandescent lamps!!!
 Basic Advantages of led light
 Disadvantages of leds

3. Semiconductors
 A semiconductor is a material which has electrical conductivity to a degree between that of a conductors
(such as copper) and that of an insulator (such as glass).
 They have resistivity or conductivity intermediate to metals and insulators.
ρ ~ 10-5 – 106Ohm m
~ 105 – 10 -6 Sm-1
 Semiconductors are the foundation of modern electronics, including transistors, solar cells, light-emitting
diodes (LEDs), quantum dots and digital and analog integrated circuits.
Concept of Energy bands
 The group of discrete but closely spaced energy levels for the orbital electrons in the atoms is called
energy band.
o The upper empty energy band is called conduction band and the lower completely filled energy band is
called valence band. In between there is a group of energy levels which cannot be occupied by the
electrons and hence called the forbidden band. Electrons can jump to the conduction band from the
valence band without stopping in the forbidden band.
o The difference of energy between the conduction band and the valence band is forbidden energy gap (Eg).

4.  For Silicon Eg = 1.1eV and for Germanium Eg = 0.7eV
Fermi Energy level
It is the maximum energy that an electron in the valence band
possesses at absolute zero is called Fermi energy and the level
corresponding to it is called Fermi Energy level.
Intrinsic semiconductor.
 intrinsic and extrinsic semiconductors
 A pure semiconductor is called
 To increase the electrical conductivity the semiconductor is mixed with either pentavalent impurity (
Group 15) or trivalent impurity ( Group 13). The process is called doping. The doped semiconductor is
called Extrinsic semiconductor.
 Generally one impurity atom is added for about 108 atoms of the semiconductor.
Concept of electrons and holes
 When temperature rises, an electrons jumps from the valence band and jumps to the conduction
band a vacancy is created in the valence band. This vacancy is called hole. For an intrinsic
semiconductor the sample is electrically neutral and the number of free electrons is equal to the
number of holes in the valence band.

5. N Type Semiconductors
 If a semiconductor is doped with a pentavalent
impurity ( Group 15 elements) such as
Arsenic(As),Antimony(Sb) , the extrinsic
semiconductor is called N Type semiconductors.
P type semiconductors
 If a semiconductor is doped with a trivalent
impurity ( Group 13 elements) such as Aluminium
(Al) , Indium (In) , the extrinsic semiconductor is
called P Type semiconductor.

6. P – N JUNCTION DIODE
 When a p –type semiconductor is suitably joined to an n – type semiconductor, their contact surface is
called p-n junction.
 The electronic device consisting of a p-n junction is called diode.
 Potential barrier: When a p-n junction is formed, the electrons diffuse across the junction from n-type to p-
type crystal du to concentration difference( of electrons.) Similarly holes diffuse from p-type to n-type
crystal. As a result the n-type crystal acquires a positive potential and p-type crystal acquires a negative
potential. The potential difference created across the p-n junction due to the diffusion of holes and
electrons is called barrier potential.
 The potential difference blocks the diffusion of holes and a depletion layer is developed at the junction.
 The thickness of depletion layer is about 10-6m.

7. P-n junction in forward bias and reverse bias
Forward Bias
 Some concepts
 Effective barrier height becomes VO - V.
 Depletion layer decreases.
i. This is the principle on which LED works.
Reverse bias
 Some Concepts
 Effective barrier height becomes V + VO .
 Depletion layer increases.
Where VO is the barrier potential.

8. (Coutesy:http://www.tmi.vu.lt/legacy/pfk/funkc_dariniai/diod/index.html)
Graph for p-n junction at forward and reverse bias
 Beyond knee voltage, there is a steep rise.
 In forward bias, there is knee voltage and in reverse bias, there is zener voltage.

9. LIGHT EMITTING DIODE ( led)
 These are specially designed diodes which give out light radiations when forward biased.
 It is heavily doped p-n junction.
 On recombination, energy released in the form of photons.
 Photons with energy equal to or slightly less than the band gap are emitted.
 When the forward current of the diode is small, the intensity of light emitted is small.
 As the forward current increases the intensity of light increases and reaches a maximum.
 Further increases in the forward current results in decrease of light intensity.
 The reverse breakdown voltages of LED are very low, typically around 5V.
 LEDs that can emit red, yellow, orange, green and blue light.
 LEDs are made of GaAs1-xPx ( Eg ~ 1.9eV for red LED) , GaAs(Eg~1.4eV) is used for making infrared LED

10. Major contributions by Various Scientists on LEDS
 1920 -Oleg V. Losev studied the phenomena of light emitting diodes in radio sets. His first work on 'LEDs' involved a
report on light emission from SiC. In 1927 he published a detailed report but his work was not well known until the
1950s when his papers resurfaced.
 1961 - Bob Biard and Gary Pittman developed the Infrared LED at Texas instruments. This was the first modern LED.
It was discovered by 'accident' while TI tried to make a laser diode. The discovery was made during a test of a tunnel
diode using a zinc diffused area of a GaAs (Gallium Arsenide) semi-insulating substrate.
 1962 - Nick Holonyack Jr. develops the red LED, the first LED of visible light. He used GaAsP (Gallium Arsenide
Phosphide) on a GaAs substrate. General Electric.

11. Research on Use of LED in place of incandescent lamps!!!
 Advantages of LEDs over conventional incandescent lamp are :
 Low operational voltage and less power.
 Fast action and no warm- up time required.
 The bandwidth of emitted light is 100 A to 500 A or in other words it is nearly but
exactly monochromatic.
 Long life and ruggedness
 Fast on-off switching capability

12. Basic advantages of LED Light
 Energy efficient - LED’s are now capable of outputting 135 lumens/watt
 Long Lifetime - 50,000 hours or more if properly engineered
 Rugged - LED’s are also called “Solid State Lighting (SSL) as they are made of solid material with no filament
or tube or bulb to break
 No warm-up period - LED’s light instantly – in nanoseconds
 Not affected by cold temperatures - LED’s “like” low temperatures and will start-up even in sub-zero weather
 Directional - With LED’s you can direct the light where you want it, thus no light is wasted
 Excellent Colour Rendering - LED’s do not wash out colours like other light sources such as fluorescents,
making them perfect for displays and retail applications
 Environmentally friendly - LED’s contain no mercury or other hazardous substances
 Controllable - LED’s can be controlled for brightness and colour.
OLED organic leds
These are LEDs in which the electroluminescent layer is made up of a film of organic compounds.Eg- poly(p-phenylene vinylene).

13. Disadvantages of leds
 There are concerns that LEDs do not provide the warmth that ordinary bulbs give when lit.
 LEDs are currently more expensive, on an initial capital cost basis, than more conventional lighting technologies.
However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass
incandescent or halogen sources and begin to threaten compact fluorescent lamps.
 The Chart Below compares different light sources based upon the life of the bulb and the electrical cost at 10 cents per
kWh (kilowatt hour). Note: fixture costs and installation costs are not included.
 LED performance largely depends on correctly engineering the fixture to manage the heat generated by the LED,
which causes deterioration of the LED chip itself. Over-driving the LED or not engineering the product to manage
heat in high ambient temperatures may result in overheating of the LED package, eventually leading to device
failure. Adequate heat-sinking is required to maintain long life. The most common design of a heat sink is a metal
device with many fins, which conducts the heat away from the LED.
 LEDs must be supplied with the correct voltage and current at a constant flow. This requires some electronics
expertise to design the electronic drivers.
 LED’s can shift colour due to age and temperature. Also two different white LED will have two different color
characteristics, which affect how the light is perceived.

14. Thank You
Special Thanks to the Foundation Course : Science and Life
As I could avail this great opportunity of making this presentation
“Since every opportunity is like a heartbeat so we should not miss it.”