It is based on electronics based on applied electronics.

1. Unit 05:
Optoelectronics Devices
By Mayur M.Sevak
Electronics and Communication Engg .Dept
B.V.M Engineering College
(An Autonomous Institution)

2. Introduction
● Luminescence: Light can be emitted from solid when it is stimulated by source of
incident energy.
● Examples are Photo- luminescence:,electro- luminescence, cathode-luminescence.
● The devices that resulted from emission of electrons from metallic surface with
vacuum and gasphoto tubes were commonly called as photoelectric devices.
● Modern solid state devices which include emitters,sensors and couplers are called
opto electronic devices.
Two categories of Optoelectronics devices
1) Photoconductive devices
2) Photovoltaic devices
2

3. Contd.
3

4. Photoconductive Devices
● Photoconductive Effect: When radiation is incident on semiconductor,some absorption of
light by the material take place and conductivity increases.
● In semiconductor material,the forbidden gap Eg is expressed as
● If frequency f is very low so that E<Eg,then energy is inadequate to transfer an electron
from valence band to conduction band hence light will pass through material and little
absorption take place.
● If E>=Eg then,electron in valence band absorb the incident photons and shift to conduction
band.
● Hence Conductivity of Semiconductor is given by
●
n=magnitude of free electron
concentration
p=magnitude of hole concentration
σ =conductivity
μn =mobility of electrons
μp=mobility of holes

5. Contd.
● Electron hole pairs generated by incident light plus
those created by thermally increases conductivity of
material result in increase in current into external
circuit.
● Hence material is called photoconductor or
photoresistor
● Therefore, photoconduction will only happen if
photon possess sufficient energy that is equal to
Energy Gap Eg or more.
● Minimum Freq fc for photoconduction is given by
Cut off or critical wavelength is given by
For Si, Eg=1.1 eV and λc=1.13 μm
For Ge,Eg=0.72 eV and λc=1.73 μm

6. Bulk Type Photoresistor or Photoconductive Cell
● It is two terminal device which is used as LDR
● It is made up of thin layer of semiconducting material such as
Cadmium sulphide(CdS),Lead sulphide(PbS),cadmium
selenide(CdSe).
● CdS is most popular because high dissipation capacity,excellent
sensitivity,low resistance when stimulated by light.
● But Cds has slower speed of response and PbS is faster.
● Look at the figure, in absolute darkness,the resistance is as high as
2 MΩ and in strong light it is less than 10Ω

7. Contd.
● Look at Simple circuit in which semiconductor layer
is enclosed in sealed housing.
● As light increases,the resistance of Photoconductive
cell decreases and hence current I increases and
vice-versa.
Applications:
1) On/Off control in automatic street lighting
2) It can be used as trigger level to control relays.
3) It is used in counting system as many time light is
interrupted counter is increased.
4) Used in twilight circuit where resistance falls below
threshold value.
5) Cameras to control shutter opening when flash is on

8. Junction Type Photoconductive Cell
Photodiode ● Silicon diodes converts the light signals into electrical signals.
● The diode is made from semiconductor PN junction which is
kept in sealed plastic or glass casing.
● Design of cover is such that light will be concentrated on one
surface of junction and other sides are painted.lens permits the
light to fall on junction
● When light falls on reverse bias junction then,electron-hole pairs
are created and magnitude of current is depending on number
of charge carriers generated and hence illumination of diode
element.
● Current is also affected frequency of light falling on the
junction.

9. Contd. ● From characteristics, reverse current increases in direct
proposition to level of illumination.
● Even when no light is applied,there is minimum reverse
leakage current flows that is called dark current.
● Applications:
● They are used as light detectors,demodulators,encoders.
● They are also used in optical communication system,high
speed counting and switching circuits.
● They are also used in computer card punching and
tapes,light operated switches,soundtrack films,electronic
control circuits.

10. Phototransistor
● It is more sensitive than photodiode
● The current produced by photo diode is very low which can’t be
directly used in control applications.
● Hence it combines Photodiode and Transistor Amplifier
● Figure shows NPN Transistor in CE Configurations with base open
● The lens is at base collector junction and the base current is
supplied when light is falling on that junction
● When there is no radiant excitation,the minority carriers are
generated thermally and electron moves from base to collector and
holes from collector to base that forms collector current Ico with
IB=0 the collector current is given by
When light is on then additional minority carriers are generated and total
collector current is given by

11. Contd.
● Look at the characteristics,the current in photo
transistor depends only on intensity of light falling on it
and it is less affected by voltage applied to external
circuits
● Applications:
● All the applications of Photodiode are supported by
Phototransistor

12. Photovoltaic Sensors
Photovoltaic Effect: If the PN Junction is open circuited,the light energy is used to create a potential
difference which is proportional to frequency and intensity of incident light.
● It produces voltage when illuminated by light which can be used to
provide direct supply of electrical power.
● In this device,without any applied voltage,the junction generate
voltage depending upon the illumination and the load.
● The Photovoltaic potential is voltage at which zero current is
obtained under open circuit condition. The Photovoltaic emf is 0.5
V for Si and selenium and 0.1 V for Ge and short circuit current is
1mA.
● The magnitude of current under large reverse bias is given by
The voltage Vmax corresponding to open circuit diode can be
obtained by putting I=0 and hence
As Is>>Io, Vmax increases as Is and hence illumination.

13. Contd.
● Figure shows output voltage vs light intensity and op current vs light
intensity
● Applications:
● They are used in low power devices such as light meters
● More power is produced using solar cells
● When operated in short circuit mode ,it is used for direct reading foot
candle meter.

14. SOLAR CELL
● When sunlight is incident on photovoltaic cell,it is converted into
electrical energy.Such converter is solar cell or solar battery
● The cell consists of semiconductor crystal with PN junction by
adding P-type and N-type impurities
● When sunlight incident on glass plate G, it reaches to the junction as
a result incident light photon at junction collide with valence electron
and make it transition to conduction band
● As a result electron hole-pairs are generated and minority carriers
cross the junction from P and N side
● With this accumulation of majority charge carriers occurs that
develops photovoltaic voltage across junction in open circuit.This
voltage is logarithmic function of illumination.
● Single solar cell exhibits 0.6 V.
● Average cell produce 30 mW per square inch and operating load is
4-ohm
● Efficiency range is 10 to 40 %
● Silicon and Selenium is used for excellent temperature
characteristics.

15. Photoemissive Sensors
● Photo tube is radiant energy device that control
electron emission when exposed to incident light.
● As from figure anode and cathode are placed in high
vacuum glass envelope
● When sufficient voltage is applied between anode and
cathode than collector current is directly proportional
to amount of incident light.
● Look at the characteristics.
Vacuum Photo tube

16. Gas filled Phototube
● The current handled in a vacuum phototube may be increased if small amount of gas is introduced
resulting in characteristics shown
● At voltage above saturation level of 10 V the electron may acquire sufficient energy in passing
between cathode and anode to ionise some of the gas atoms which add more charges to current.
● The process must be limited to prevent secondary emission or bombardment of cathode by positive
ions as bombardment will destroy the emitting surface.Safe limit to operate is 90V .
●

17. Photomultiplier
● As the emitted currents from photoelectric surfaces are very small,especially with low light levels.This
currents can be directly amplified in this device.
● It consists of evacuated glass envelope containing a photocathode,anode and several additional
electrodes called dynodes( Each at higher potential then previous)
● As from figure it consists of cathode,six dynodes and anode.When light falls on cathode,electrons are
emitted and directed towards d1.Which again has secondary emission with coefficient δ which is
higher than unity.Again same procedure follow for next dynodes .
● This process is repeated for n times where n are the dynodes. δ has the range between 5 to 10.
● If the electrons are deflected from their normal path between stages due to magnetic field and miss a
dynode then gain falls.For this magnetic shields are placed around the photomultiplier tube

18. Contd.
● The characteristics of photomultiplier tube depend upon
voltage/dynode and potential between last dynode and
collector.
● Applications:
● Space Exploration
● Laser Communications
● Scintillators and Radiation detectors of X-Rays,Gamma rays
● Energetic particle found in nuclear physics.

19. LIGHT EMITTERS
● LED is PN Junction device which emits light when forward bias by
phenomenon called electroluminescence.
● In semiconductor PN junctions ,some of the energy may be radiated as
heat or some as light.In material such as Gallium
Phosphide(GaP),Gallium Arsenide Phosphide(GaAsP), emitted energy is
in the form of visible light.
● When LED is forward biased,the electrons and holes moves towards the
junction and recombination take place.As result of
recombination,electrons in conduction band of N-region fall into the
holes lying in the valence band of P -region.Light is generated due to
recombination and brightness of LED light is proportional to forward
bias current.
● Figure shows the structure of LED.A metal(gold) film is applied at the
bottom of substrate for reflecting light as much as possible.
● GaAs-(Infrared Radiation)
● GaP-(Red or Green)
● GaAsP-(Red or Yellow) 19
LED

20. Contd. ● To protect the LED , 1KΩ or 1.5 KΩ must be
connected in series with LED.
● They operate at voltage levels from 1.5 V to 3.3 V.
● Power requirements are 10 to 150 mW.
● Life time: 1,00,000+ Hours
● LED can be switched ON and OFF at speed of 1ns.
● Applications
● Burglar Alarm Systems
● Picture Phones
● Multimeters
● Calculators
● Digital Computers and Microprocessors.
● Electronic Panels,Digital Watches,Intercoms and
Telephone Exchanges.
● Solid state video displays
● Optical Communications Systems.
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21. Liquid Crystal Display(LCD)
● A Liquid Crystal Display (LCD) is a thin , flat panel display device
used for electronically displaying information such as text ,images
and moving picture.
● LCD is used in Computer monitors, Televisions , Instrument panels,
Gaming devices etc.
● Polarization of lights is used here to display objects.
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22. 22
Why to select LCD?
● Smaller size —LCDs occupy approximately 60 percent less space than CRT displays an
important feature when office space is limited.
● Lower power consumption—LCDs typically consume about half the power and emit much
less heat than CRT displays.
● Lighter weight —LCDs weigh approximately 70 percent less than CRT displays of
comparable size.
● No electromagnetic fields —LCDs do not emit electromagnetic fields and are not
susceptible to them. Thus, they are suitable for use in areas where CRTs cannot be used.
● Longer life —LCDs have a longer useful life than CRTs.

23. 23
Liquid Crystals
● Liquid crystals are liquid chemicals in a state that has
properties between those conventional liquid and solid
crystals. That is a liquid crystal may flow like a liquid,
but its molecules may be oriented in a crystal like way.
● Liquid crystals molecules can be aligned precisely when
subjected to electric fields, as like as in the way metal
shavings line up in the field of a magnet. When properly
aligned, the liquid crystals allow light to pass through.
● Two liquid crystal materials which are important in
display technology are nematic and smectic.
● nematic
● smectic.

24. 24
Contd.
● The most popular liquid crystal structure is the nematic liquid crystal(NLC). When
they are in a nematic phase, liquid crystals are a bit like a liquid: their molecules can
move around and shuffle past one another, but they all point in broadly the same
direction.
● The liquid is normally transparent, but if it is subjected to a strong electric field, ions
move through it and disrupt the well ordered crystal structure, causing the liquid to
polarise and hence turn opaque. The removal of the applied field allows the crystals
structure to reform and the material regains its transparency.

25. 25
How it works?
● Liquid crystals can adopt a twisted up structure and when we apply electricity to
them, they straighten out again. This is the key how LCD displays turn pixels on and
off.
● The polarization property of light is used in LCD screen to switch its colored pixels
on or off. At the back of the screen, there is a bright light that shines out towards
the viewer. In front of this, there are the millions of pixels, each one made up of
smaller areas called sub-pixels, that are colored Red, Green, or Blue

26. 26
● Each pixel has a polarizing glass filter behind it and
another in front of it at 90 degrees. Normally the pixels
looks dark.
● In between the two polarizing filters there is a tiny
twisted, nematic liquid crystal that can be switched on
or off electronically.
● When it is switched on, it rotates the light passing
through it through 90 degrees, effectively not allowing
light to flow through the two polarizing filters and
making the pixel look dark.
● Each pixel is controlled by a separate transistor that can
switch it on or off many times each second.

27. 27
Types of LCD
● When the display include
limited variable components
such as
● Watches
● Calculators
● Simple electronics is used to
control the components
Direct Address Display

28. 28
● Passive matrix display has Rows of
electrodes on one piece of glass.
● Columns of electrodes on the opposing
piece of glass.
● Complex electrical waveform control the
voltage differential at the intersection of
the electrodes.
● The intersection of the columns and rows
are the pixels
Passive Matrix Display

29. 29
● Allow very high resolution
● Each sub-pixel is individually controlled by an
isolated thin-film transistor (TFT).
● It allows the electrical signal for each sub-pixel
to avoid influencing adjacent elements.
● The TFT is patterned into the glass layer
Active Matrix Display

30. 30
Twisted Nematic Display
● It is the most common LCD Display.
● The two alignments layer for the liquid
crystal material are orthogonal.
● The light entering the polarize panel
rotates by the twist in the liquid crystal
and allowing it to pass through the
second polarize

31. 31
Optocoupler
● It is solid state component in which the
light emitter,light path and light detector are
enclosed in component.
● As it provides isolation and hence it is also
called opto isolator.It allows the signal to
transfer without coupling wires,capacitors
or transformers.
● It can couple digital or analog signals.
● Figure shows infrared LED and
Photodetector.LED is used to transduce
voltage into light intensity and light is
transduced back in voltage by photo
sensors.
● There is encapsulation to permit the
passage of light to pass and data can be
transmitted in the range of Mhz

32. 32
Contd.
● Significant advantage of opto isolator is that it provides high isolation
resistance of the order of 10^11 -Ohm and isolation voltages of the order of
2500 V between input and output terminals.
● Applications are in different type of logic circuits and level positioning sensing
circuits.
● The power dissipation of LED and phototransistor is almost equal and Iceo is in
nano-amperes.
● The relative output current is almost constant when temperature varies from 25
to 75 C
● The switching time of an opto isolator decreases with increased current ,while
for many devices it is exactly reverse.

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