LED, PHOTODIODE, PIN DIODE,APD,SOLAR CELL ENGINEERING PHYSICS , KTU , PHJYSICS

1. Engineering Physics
4/2/2020
Murali V S, Assoc. Prof. COLLEGE OF ENGINEERING ADOOR
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Murali V S
Assoc. Prof. in Physics
COLLEGE OF ENGINEERING
ADOOR, Kerala, India
murali7x@gmail.com

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3. PHOTONICS
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Topics
1. LED
2. Photodtetectors
a)Photodiode
b) PIN Diode
c) Avalanche Photo Diode (APD)
3) Solar Cell

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Light Emitting Diode (LED)
Definition: The LED is a PN-junction diode which emits light when
an electric current passes through it in the forward direction. In
the LED, the recombination of charge carrier takes place. The
electron from the N-side and the hole from the P-side are
combined and gives the energy in the form of heat and light. The
LED is made of semiconductor material which is colourless, and
the light is radiated through the junction of the diode.

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Construction of LED
The recombination of the charge carrier occurs in the P-type material, and hence
P-material is the surface of the LED. For the maximum emission of light, the
anode is deposited at the edge of the P-type material. The cathode is made of
gold film, and it is usually placed at the bottom of the N-region. This gold layer of
cathode helps in reflecting the light to the surface.

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8. PHOTO DETECTORS
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Working :
• The PIN photo diode operates with an applied reverse bias
voltage and when the reverse bias is applied, the space charge
region must cover the intrinsic region completely.
• Electron hole pairs are generated in the space charge region
by photon absorption.
• The switching speed can be enhanced by a small minority
carrier lifetime.
•For the photo detector applications where the speed of
response is important, the depletion region width should be
made as large as possible for small minority carrier lifetime as a
result the switch speed also increases.

17. Avalanche photo diode
• Avalanche photo diode is a photo detector which can convert
optical signals into electrical signals
• Avalanche photodiode structural configuration is very similar to
the PIN photodiode. But the width of depletion layer is large
A PIN photodiode consists of three regions-
p-region, intrinsic region & n-region.
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18. Working of APD
4/2/2020
Murali V S, Assoc. Prof. COLLEGE OF ENGINEERING
ADOOR
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*APD has three regions p-region, intrinsic region, n-region.
*A large reverse bias is applied which cause impact ionization.A silicon
diode need reverse bias between 100 to 200 volts.
*Firstly electron- hole pairs are generated by photon absorption in the
depletion region.
*These generate more electron hole pairs through impact ionization.
These are swept out of the depletion region quickly, i.e, the transit
time is very less.

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20. Solar Cell
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Murali V S, Assoc. Prof. COLLEGE OF ENGINEERING ADOOR
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24. Working of solar cell
• Sunlight falling on the cell the some photons
of the light are absorbed by solar cell.
• The absorbed photons will create a electron
hole pair if its energy is greater than energy
gap of semi conductor.
•
• These electrons and holes of e-h pairs are
called light-generated electrons and holes
•
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25. • The light-generated electrons near the p-n junction are
migrated to n-type side of the junction due to
electrostatic force of the field across the junction.
• Similarly the light-generated holes created near the
junction are migrated to p-type side of the junction
due to same electrostatic force.
• In this way a potential difference is established
between two sides of the cell and if these two sides are
connected by an external circuit current will start
flowing from positive to negative terminal of the solar
cell.
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26. V-I Characteristics of Solar cell
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Voc Open circuit voltage
Ioc - Short corcuit current
Vm - Voltage at maximum power
Im – Current at maximum power

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ENGINEERING ADOOR
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