PHOTONICS

PHOTONICS
• Photonics is the branch of science
which deals with the production,
control and detection of photons.
• Photons are particles representing
quanta of light.
• Energy of 1 photon, E = h𝜗
• Photonics is a technology that
combines optics and electronics.
LIGHT EMITTING DIODE (LED)

• LED is a heavily doped PN

Junction of suitable material that
emits light when it is forward
biased.
• In forward bias state, electrons
move from n-region to p-region
and holes move from p region to
n region.
• Electron hole recombination
takes place on either side of the
pn junction.
• The loss of energy in these
recombination appears as light.
• Energy of the light emitted will be equal to bandgap of the
semiconductor material used.
• Let Eg be the bandgap energy of the material, then
Eg = h𝜗
𝑐
Eg = ℎ 𝜆
ℎ𝑐
Then wavelength of light emitted, 𝜆 =
𝐸𝑔

• Where h - plancks const, c - speed of light in free space, Eg is

bandgap energy.
• All semiconductor diodes emit energy during electron-hole
recombination, but the emitted energy is in the form of heat (IR
radiation).
• In an LED band gap is wide and the junction is constructed such a
way that the energy emitted is in the range of visible light.
• Semiconductor materials used for LED should have a bandgap of
2eV.
• Indium Gallium Nitride - Violet, Blue, Green
• Aluminium Gallium Indium Phosphide - Green, Yellow, Orange,
Red.
LED - Advantages and Disadvantages

Advantages Disadvantage
• Helps to save energy. • A slight excess in voltage or
• Reduction in cost. current can damage the
device.
• Miniature in size and less
weight. • Efficiency of LED decreases
as electric current increases.
• Low voltage and current
required.
• Response time is less,
around 10ns.
Applications of LED

• LEDs in conjunction with photodiode or other photosensitive

devices - used as light source in optic fibre communication
systems.
• Used in digital dispalys in all modern day electronic devices.
• Used as bulb in homes and industries.
• Used in motor cycles and cars.
• Used at traffic light signals.
 PHOTODETECTORS

• Photodetector is a device used to convert light signals

that hit the junction into a voltage or current.
• They convert light energy to electrical energy.
 Photodetectors must have the following properties:
• Photodetectors should exhibit wide bandwidth.
• Response time of the photodetectors must be low.
• It must have long operating life.
• Cost of photodetectors must be low.
• size of photodetectors must be low.
• Sensitivity of photodetectors should be high.
• Degree of tolerance of photodetectors with variation in
temperature should be high.
PHOTODIODE
• Semiconductor device that converts
light into current.
• Predominant type of Photodetector.
• They use amplifiers to detect even
small light falling on them.
• They are similar to regular
semiconductor diodes, but they may
be transparent enough to let light
reach the delicate part of the device.
• It works on reverse bias.
• In the absence of light, very small reverse current flows due to
minority charge carriers, known as dark current or leakage current.
• When the light falls on the depletion region, photons of the light is
absorbed by the bound electrons of the immobalized ions in the
depletion region, new electron hole pair is generated.
• Electrons starts moving towards the positive terminal of the battery
due to the electric field present in the depletion region.
• This newly generated electrons contribute to more current.
• As a result, large reverse current flows through the junction diode,
known as photo-current.
• Photo-current is propotional to intensity of light.
PHOTODIODE : JUNCTION DIODE

• It is a reverse biased pn
junction, which is embedded
in clear plastic medium.
• When it is exposed to light,
the current varies linearly with
intensity of light.
Construction
• Formed by diffusion of lightly
doped P region into heavly
doped n region.
• Space between p region and n region is called depletion
region.
• Here some portion of the front area is called active area which
is coated with antireflection coating.
• Remaining portion is non active area which is coated with thick
layer of SiO2.
• We can control/slightly vary the response time of the
photodiode by adjusting the thickness of the non active area.
Working
• Connected in reverse bias condition.
• A constant current flows through the circuit when reverse bias
voltage is very low - dark current - due to minority carriers.
• Does not allow majority carriers to cross the junction.
• Light is allowed to fall on the depletion
region.
• When a photon is absorbed from the
incident light, an electron is released and
moves towards the positive terminal of
battery.
• This create holes which seems to be moving
towards negative terminal of the battery.
• Thus incident light produce a large number
of electron-hole pairs.
• These electron-hole pairs are called photo
carriers and produced current is known as
photo current.
• Dark current should be minimized to
increase the sensitivity of the device.
• Larger the depletion region, better the
photodiode.
Applications of Photodiode

• Frequently used for exact measurement of the

intensity of light.
• Widely used in medical field. For example, it is used
in instruments to analyze samples.
• Used in cameras as photosensors.
SOLAR CELL

• Solar cells are devices making use of photovoltaic effect to

convert solar energy into electrical energy.
• Generates electrical energy when irradiated with optical
radiation.
• Large area photodiode, made to work in photovoltaic mode and
zero bias.
• So there is no flow of current out of the diode and hence a
voltage develops inside the device.
• i.e., sunlight is trapped inside the device to produce electric
effect in the form of voltage. This is known as photovoltaic
effect.
• A single solar cell produces an
output voltage of about 0.6V.
• Total output voltage can be
increased by connecting a
number of solar cells in series.
• A solar panel consist of large
number of solar cells
connected together.
 CONSTRUCTION
• A solar cell is basically a junction
diode, although its construction it is
little bit different from conventional
p-n junction diodes.
• A very thin layer of n-type
semiconductor is grown on a
relatively thicker p-type
semiconductor.
• Anode and cathode connections is
made from the bottom and cathode
from the top.
• Antireflection coating done on the
top layer to avoid light reflections.
 WORKING
• When light reaches the pn junction of the solar cell, electrons are
knocked out generating electron-hole pair.
• Electrons in the depletion region moves towards the n side of the
junction and holes moves towards p side due to the electric field
present there.
• As the concentration of electrons becomes higher in one side, i.e. n-
type side of the junction and concentration of holes becomes more in
another side, i.e. the p-type side of the junction, the p-n junction will
behave like a small battery cell.
• A voltage is set up which is known as photo voltage.
• If we connect a metal wire across the junction, there will be a tiny
current flowing through it.
 I-V Characteristics of solar cell
• I-V measurements are done to
characterize solar cells.
• Solar cell parameters can be
obtained from I-V Characteristics.
• Inorder to plot I-V characteristics
of solar cell, it is connected as
shown in figure.
• If no load connected(∞ resistance)
- Voc obtained without current.
• If terminals of solar cells shorted
together(0 resistance) - Isc flows
without voltage.
• When load connected-voltage is developed-current flows-there will
be an output power also.
• There will be a maximum power for a specific load resistance.
• Vmax and Imax - maximum V and I corresponding to maximum power
point.
• Maximum power point - condition under which solar cell provides
maximum power output.
• Its good if solar cell working at maximum power point.
• Design of solar cell - such that it works at maximum power point.
Efficiency of a solar cell
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑜𝑢𝑡𝑝𝑢𝑡 𝑝𝑜𝑤𝑒𝑟 𝑃𝑚𝑎𝑥
𝜂= =
𝐼𝑛𝑝𝑢𝑡 𝑜𝑝𝑡𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝐿𝑖𝑔ℎ𝑡 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 𝑥 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑠𝑜𝑙𝑎𝑟 𝑐𝑒𝑙𝑙

Fill factor
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑝𝑜𝑤𝑒𝑟 𝑜𝑢𝑡𝑝𝑢𝑡 𝑃𝑜𝑤𝑒𝑟(𝑃𝑟𝑎𝑐𝑡𝑖𝑐𝑎𝑙)
𝑓𝑓 =
𝑉𝑜𝑐 𝑥 𝐼𝑠𝑐 𝑃𝑜𝑤𝑒𝑟(𝑡ℎ𝑒𝑜𝑟𝑜𝑡𝑖𝑐𝑎𝑙)

Fill factor ranges - 0.65 to 0.8.

Advantages and Disadvantages of Solar cell

Advantages Disadvantages
• Donot use fuels - • Very delicate - They are brittle
Environmental friendly. and may be broken easily.
• Long lasting without • Large space consumption.
maintanance • Panels must be cleaned time
• Donot produce atmospheric to time to get better results.
or noise pollution while • Slow response - cannot be
working. used as photodetectors.
Applications of Solar cell

• Used for rural electrification, water pumping,

domestic supply etc.
• Used in pocket calculators, watches, torches, garden
lights etc.
• Used in defence equipments like remote
instrumentations, remote radars etc.
• Used in satallites and rockets as source of power.