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Optoelectronics
Optoelectronics
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Optoelectronics is the study and application of
electronic devices that source, detect and control light,
usually considered a sub-field of photonics.
Optoelectronic devices are electrical-to-optical or opticalto-electrical transducers, or instruments that use such
devices in their operation.
Electro-optics is often erroneously used as a synonym,
but is in fact a wider branch of physics that deals with all
interactions between light and electric fields, whether or
not they form part of an electronic device.
What is Light?
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Light or visible light is electromagnetic radiation that is visible to the
human eye, and is responsible for the sense of sight.
Visible light has wavelength in a range from about 380 to about 740
nm, with a frequency range of about 405 THz to 790 THz.
Electromagnetic wave
EM Spectrum
Light interaction with solids
Optical
classification:
1. Transparent
2. Transluscent
3. Opaque
Semiconductor
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A semiconductor is a solid material that
has electrical conductivity in between a
conductor and that of an insulator.
Silicon (Si) is the most semiconductor
material, but dozens of other materials are
used as well.
Why semiconductor materials are so
useful?
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main reason is that the
behaviour of a semiconductor can
be easily manipulated by the
addition of impurities, known as
doping.
Why semiconductor materials are so
useful?
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Semiconductor conductivity can be controlled
by introduction of an electric field, by
exposure to light, and even pressure and
heat;
thus, they can make excellent sensors.
Current conduction in a semiconductor
occurs via mobile or "free" electrons and
holes, collectively known as charge carriers.
Diode
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Diode is a simplest semiconductor devices.
A diode has a low resistance in one direction
and a high resistance to it in the reverse
direction.
This property makes a diode useful as a
rectifier, which can convert AC into DC.
Real diode (p-n junction)
A typical p-n junction diode characteristic curve
What is LED?
LEDs are semiconductor p-n junctions that under forward bias conditions can emit
radiation by electroluminescence in the UV, visible or IR spectrum regions. The quanta
of light energy released is approximately proportional to the band gap of the
semiconductor.
The pn Junction LED
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Electron-hole recombination is the process
that occurs in diodes.
In a regular diode: recombinations release
energy thermal (heat) – nonradiative
recombination.
In an LED: recombinations release the light –
radiative recombination.
In reality, both types of recombination occur in
a diode, when a majority of recombinations are
radiative, we have an LED.
LEDs
LEDs
Red LED
LED for displays
White LED
Blue LED
LED for traffic light
Photodiodes
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The photodiode is a p-n junction under reverse bias.
Exposing a semiconductor to light can generate
electron-hole pairs, which increases the number of free
carriers and its conductivity.
Only those that have correct wavelength can be
absorbed by the semiconductor.
Separation of charge can be collected and measured as
current or voltage.
 If device is left open circuit  voltage detected 
photovoltaic effect
 If device is short-circuited (or under reverse bias) 
photoconductive mode
Photodetectors
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When a photon/light strikes a semiconductor, it can
promote an electron from the valence band to the
conduction band creating an electron-hole (e-h) pair.
The concentration of these e-h pairs is dependent on the
amount of light striking the semiconductor, making the
semiconductor suitable as an optical detector.
There are two ways to monitor the concentration of e-h
pairs:
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In photodiodes, a voltage bias is present and the concentration of
light-induced e-h pairs determines the current through
semiconductor.
Photovoltaic detectors contain a p-n junction, that causes the e-h
pairs to separate to produce a voltage that can be measured.
Solar Cell/Photovoltaic Device
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Photovoltaic devices or solar cells are semiconductor p-n
junction that can convert solar radiation into electrical
energy.
Diagram of a PV cell.
Photovoltaic cells, modules, panels and
arrays.
Major photovoltaic system components.
Converting Sunlight to Electricity
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A typical PV cell consists of semiconductor p-n junction.
Sunlight striking the cell raises the energy level of electrons and frees them
from their atomic shells.
The electric field at the p-n junction drives the electrons into the n region
while positive charges are driven to the p region.
A metal grid on the surface of the cell collects the electrons while a metal
back-plate collects the positive charges.
Converting Sunlight to Electricity
Solar Cells
Laser
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For atomic systems in thermal equilibrium,
emission of light is the result of two main
processes:
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ABSORPTION of energy
SPONTANEOUS EMISSION of energy (a random
photon is emitted)
A third mechanism is crucial to the
formation of LASER action, which is
“STIMULATED EMISSION”.
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Light Amplification of Stimulated Emission Radiation
Laser
Basic optical transitions
Diode Laser
Diode lasers have been used for cutting, surgery,
communication (optical fibre), CD writing and
reading etc
The power-current curve of a laser diode. Below threshold, the
diode is an LED. Above threshold, the population is inverted and
the light output increases rapidly
Boltzmann distribution vs Population inversion
How to create a population
inversion?
Laser