The photoelectric transducer converts the light energy into electrical energy. It is made of semiconductor material. The photoelectric transducer uses a photosensitive element, which ejects the electrons when the beam of light absorbs through it.

1. KAROLINEKERSIN. E
ASSISTANT PROFESSOR
PHOTOELECTRIC TRANSDUCERS

2. PHOTOELECTRIC TRANSDUCER
• The photoelectric transducer converts the light energy into electrical
energy.
• It is made of semiconductor material.
• The photoelectric transducer uses a photosensitive element, which
ejects the electrons when the beam of light absorbs through it.

3. • The discharges of electrons vary the property of the photosensitive
element.
• Hence the current induces in the devices.
• The magnitude of the current is equal to the total light absorbed by
the photosensitive element.

4. BASIC WORKING PRINCIPLE OF PHOTOELCTRIC TRANSDUCER
• The photoelectric transducer absorbs the
radiation of light which falls on their
semiconductor material.
• The absorption of light energises the
electrons of the material, and hence the
electrons start moving.

5. EFFECTS OF MOBILITY OF ELECTRONS
The mobility of electrons produces one of the three effects.
• The resistance of the material changes.
• The output current of the semiconductor changes.
• The output voltage of the semiconductor changes.

6. CLASSIFICATION OF PHOTOELECTRIC TRANSDUCERS
Photo
emissive
cells or
photo tube
Photo diode
Photoconductive
cell
Photovoltaic
cell
Photo
transistor

7. PHOTO TUBES
• A phototube or photoelectric cell is a type of gas-filled or vacuum tube
that is sensitive to light.
• Such a tube is more correctly called a 'photoemissive cell' to
distinguish it from photovoltaic or photoconductive cells.
• Phototubes were previously more widely used but are now replaced in
many applications by solid state photodetectors.

8. PRINCIPLE
• Phototubes operate according to the photoelectric effect: Incoming
photons strike a photocathode, knocking electrons out of its surface,
which are attracted to an anode.
• Thus current is dependent on the frequency and intensity of incoming
photons.

9. CONSTRUCTION AND WORKING
• A PT consists of an evacuated glass or quartz
chamber containing an anode and a cathode.
• Cathode surfaces are composed of materials that
readily give up electrons; Group I metals such as
Cs work well of this purpose.
• A relatively large potential is placed across the
anode and cathode, usually 90V, and the gap is
referred to as a dynode.
• Electrons contained in the cathode are released
as photons with a sufficient energy strike the
surface.

10. • This causes electrons to move through the low-pressure gap to the
anode, which produces a current.
• For a PT, a single photon causes only a single electron to be
measured.
• For emission spectroscopy, the magnitude of current produced by the
cascade of electrons in the detector is directly proportional to the
concentration of analyte in the sample.

11. APPLICATIONS
Reading of optical sound tracks for
projected films
Detector in spectrophotometer

12. PHOTO MULTIPLIER TUBE
• Photomultiplier tubes (PMT) are an extension of the phototube where
numerous dynodes are aligned in a circular or in a linear manner.
• A photomultiplier tube, useful for light detection of very weak signals,
is a photoemissive device in which the absorption of a photon results
in the emission of an electron.
• These detectors work by amplifying the electrons generated by a
photocathode exposed to a photon flux.

13. CONSTRUCTION AND WORKING
• Photomultipliers acquire light through a glass or
quartz window that covers a photosensitive
surface, called a photocathode, which then
releases electrons that are multiplied by
electrodes known as metal channel dynodes.
• At the end of the dynode chain is an anode or
collection electrode.
• Over a very large range, the current flowing
from the anode to ground is directly
proportional to the photoelectron flux
generated by the photocathode.

14. • The spectral response, quantum efficiency, sensitivity, and dark current of a
photomultiplier tube are determined by the composition of the photocathode.
• The best photocathodes capable of responding to visible light are less than
30 percent quantum efficient, meaning that 70 percent of the photons
impacting on the photocathode do not produce a photoelectron and are
therefore not detected.
• Photocathode thickness is an important variable that must be monitored to
ensure the proper response from absorbed photons.
• If the photocathode is too thick, more photons will be absorbed but fewer
electrons will be emitted from the back surface, but if it is too thin, too
many photons will pass through without being absorbed.

15. • Electrons emitted by the photocathode are
accelerated toward the dynode chain, which may
contain up to 14 elements.
• Focusing electrodes are usually present to ensure
that photoelectrons emitted near the edges of the
photocathode will be likely to land on the first
dynode.
• Upon impacting the first dynode, a photoelectron
will invoke the release of additional electron that
are accelerated toward the next dynode, and so
on.

16. • The surface composition and geometry of the dynodes determines their ability to
serve as electron multipliers
• Because gain varies with the voltage across the dynodes and the total number of
dynodes, electron gains of 10 million are possible if 12-14 dynode stages are
employed.
• Photomultipliers produce a signal even in the absence of light due to dark
current arising from thermal emissions of electrons from the photocathode,
leakage current between dynodes, as well as stray high-energy radiation.
• Electronic noise also contributes to the dark current and is often included in the
dark-current value.

17. APPLICATIONS
• Photomultipliers are used in research laboratories to measure the intensity
and spectrum of light-emitting materials such as compound semiconductors
and quantum dots.
• Photomultipliers are used as the detector in many spectrophotometers.

18. • A special type of PN junction device that
generates current when exposed to light is
known as Photodiode.
• It is also known as photodetector or
photosensor.
• It operates in reverse biased mode and
converts light energy into electrical energy.
PHOTO DIODE

19. CONSTRUCTION OF PHOTODIODE
• The PN junction of the device placed inside a glass material.
• This is done to order to allow the light energy to pass through
it. As only the junction is exposed to radiation, thus, the other
portion of the glass material is painted black or is metallised.
• The overall unit is of very small dimension nearly about 2.5 mm.
• It is noteworthy that the current flowing through the device is in
micro-ampere and is measured through an ammeter.

20. OPERATIONAL MODES OF PHOTODIODE
Photodiode basically operates in two modes:
Photovoltaic mode: It is also known as zero-bias mode because no
external reverse potential is provided to the device. However, the flow of
minority carrier will take place when the device is exposed to light.
Photoconductive mode: When a certain reverse potential is applied to the
device then it behaves as a photoconductive device. Here, an increase in
depletion width is seen with the corresponding change in reverse voltage.

21. WORKING OF PHOTODIODE
21

22. WORKING OF PHOTODIODE
• In the photodiode, a very small reverse current flows through the device
that is termed as dark current.
• It is called so because this current is totally the result of the flow of
minority carriers and is thus flows when the device is not exposed to
radiation.
• The electrons present in the p side and holes present in n side are the
minority carriers.
• When a certain reverse-biased voltage is applied then minority carrier, holes
from n-side experiences repulsive force from the positive potential of the
battery

23. Contd..
• Similarly, the electrons present in the p side experience repulsion
from the negative potential of the battery.
• Due to this movement electron and hole recombine at the junction
resultantly generating depletion region at the junction.
• Due to this movement, a very small reverse current flows through the
device known as dark current.
• The combination of electron and hole at the junction generates neutral
atom at the depletion. Due to which any further flow of current is
restricted.

24. Contd..
• Now, the junction of the device is illuminated with light. As the
light falls on the surface of the junction, then the temperature
of the junction gets increased. This causes the electron and hole
to get separated from each other.
• At the two gets separated then electrons from n side gets
attracted towards the positive potential of the battery. Similarly,
holes present in the p side get attracted to the negative
potential of the battery.

25. Contd..
• This movement then generates high reverse current
through the device.
• With the rise in the light intensity, more charge carriers are
generated and flow through the device. Thereby, producing
a large electric current through the device.
• This current is then used to drive other circuits of the
system

26. Contd..
• The intensity of light energy is directly
proportional to the current through the
device.
• Only positive biased potential can put
the device in no current condition in
case of the photodiode

27. Contd..
• Here, the vertical line represents the
reverse current flowing through the
device and the horizontal line represents
the reverse-biased potential.
• The first curve represents the dark
current that generates due to minority
carriers in the absence of light.
• all the curve shows almost equal spacing
in between them. This is so because
current proportionally increases with the
luminous flux.

28. ADVANTAGES OF PHOTODIODE
It shows a
quick response
when exposed
to light.
High
operational
speed.
It provides a
linear
response.
Low-cost
device

29. DISADVANTAGES OF PHOTODIODE
It is a temperature-dependent device. And shows poor temperature
stability.
When low illumination is provided, then amplification is necessary.

30. APPLICATION OF PHOTODIODE
Counters and
switching
circuits.
optical
communication
system.
Logic circuits
and encoders.
Burglar alarm
systems.

31. PHOTOVOLTAIC CELLS
• A photovoltaic (PV) cell, also known as a solar cell, is an electronic
component that generates electricity when exposed to photons, or
particles of light.
• This conversion is called the photovoltaic effect, which was
discovered in 1839 by French physicist Edmond Becquerel

32. • A photovoltaic (PV) cell is an energy harvesting technology,
that converts solar energy into useful electricity through a
process called the photovoltaic effect.
• There are several different types of PV cells which all use
semiconductors to interact with incoming photons from the
Sun in order to generate an electric current.

33. PRINCIPLE
• The photovoltaic effect is a process that generates voltage or
electric current in a photovoltaic cell when it is exposed to
sunlight.
• The photovoltaic effect can be defined as being the appearance
of a potential difference (voltage) between two layers of a
semiconductor slice in which the conductivities are opposite, or
between a semiconductor and a metal, under the effect of a
light stream.

34. CONSTRUCTION OF PHOTOVOLTAIC CELLS
• A photovoltaic cell is made of
semiconductor materials that absorb the
photons emitted by the sun and generate a
flow of electrons.
• Photons are elementary particles that
carry solar radiation at a speed of 300,000
kilometers per second.
• In the 1920s, Albert Einstein referred to
them as “grains of light”. When the photons
strike a semiconductor material like silicon
, they release the electrons from its atoms,
leaving behind a vacant space

35. • The stray electrons move around randomly looking for another “hole”
to fill.
• To produce an electric current, however, the electrons need to flow in
the same direction.
• This is achieved using two types of silicon.
• The silicon layer that is exposed to the sun is doped with atoms of
phosphorus, which has one more electron than silicon, while the other
side is doped with atoms of boron , which has one less electron.

36. • On either side of the semiconductor is a layer of conducting
material which "collects" the electricity produced.
• Note that the backside or shaded side of the cell can afford to
be completely covered in the conductor, whereas the front or
illuminated side must use the conductors sparingly to avoid
blocking too much of the Sun's radiation from reaching the
semiconductor.
• The final layer which is applied only to the illuminated side of
the cell is the anti-reflection coating.

37. • Since all semiconductors are naturally reflective, reflection loss can be
significant.
• The solution is to use one or several layers of an anti-reflection
coating (similar to those used for eyeglasses and cameras) to reduce
the amount of solar radiation that is reflected off the surface of the
cell

38. WORKING OF PHOTOVOLTAIC CELLS
• These solar cells are composed of two
different types of semiconductors—a p-
type and an n-type—that are joined
together to create a p-n junction.
• By joining these two types of
semiconductors, an electric field is
formed in the region of the junction as
electrons move to the positive p-side
and holes move to the negative n-side.
• This field causes negatively charged
particles to move in one direction and
positively charged particles in the other
direction.

39. • Light is composed of photons, which are simply small bundles of
electromagnetic radiation or energy.
• When light of a suitable wavelength is incident on these cells, energy from
the photon is transferred to an electron of the semiconducting material,
causing it to jump to a higher energy state known as the conduction band.
• In their excited state in the conduction band, these electrons are free to
move through the material, and it is this motion of the electron that creates
an electric current in the cell.

40. • Efficiency is a design concern for photovoltaic cells, as there are many
factors that limit their efficiency.
• The main factor is that 1/4 of the solar energy to the Earth cannot be
converted into electricity by a silicon semiconductor.
• The physics of semiconductors requires a minimum photon energy to remove
an electron from a crystal structure, known as the band-gap energy.
• If a photon has less energy than the band-gap, the photon gets absorbed as
thermal energy. For silicon, the band-gap energy is 1.12 electron volts.

41. • Since the energy in the photons from the sun cover a wide range of
energies, some of the incoming energy from the Sun does not have
enough energy to knock off an electron in a silicon PV cell.
• Even from the light that can be absorbed, there is still a problem.
Any energy above the band-gap energy will be transformed into heat.
• This also cuts the efficiency because that heat energy is not being
used for any useful task.
•

42. • Of the electrons that are made available, not all of them will
actually make it to the metal contact and generate electricity.
• This is because some of them will not be accelerated
sufficiently by the voltage inside the semiconductor.
• Because of the reasons listed, the theoretical efficiency of
silicon PV cells is about 33%
• .

43. • There are ways to improve the efficiency of PV cells, all of which come
with an increased cost.
• Some of these methods include increasing the purity of the
semiconductor, using a more efficient semiconducting material such as
Gallium Arsenide, by adding additional layers or p-n junctions to the cell,
or by concentrating the Sun's energy using concentrated photovoltaics.
• On the other hand, PV cells will also degrade, outputting less energy
over time, due to a variety of factors including UV exposure and weather
cycles.

44. DIFFERENT TYPES OF PHOTOVOLTAIC CELLS
Crystalline
Silicon Cells
Thin-Film
Cells
Organic Cells Perovskites

45. PHOTOCONDUCTIVE CELLS
• The photoconductive cell is a two terminal
semiconductor device whose terminal resistance
will vary (linearly) with the intensity of the inci-
dent light.
• It is frequently called as a photoresistive device.
• Its resistance will vary (linearly) with the
intensity of the incident light.

46. PRINCIPLE
• Light striking the surface of a material can provide sufficient energy
to cause electrons within the material to break away from their
atoms.
• Thus, free electrons and holes (charge carriers) are created within
the material, and consequently its resistance is reduced.
• This is known as the Photoconductive effect.

47. CONSTRUCTION AND WORKING
• Light-sensitive material is arranged in the form of a long strip
zigzagged across a disc-shaped base.
• The connecting terminals are fitted to the conducting material
on each side of the strip; they are not at the ends of the strip.
• Thus, the light sensitive material is actually a short, wide strip
between the two conductors.
• For added protection, a transparent plastic cover is usually
included

48. • Cadmium sulfide (CdS) and cadmium selenide (CdSe) are the two
materials normally used in photoconductive cell manufacture.
• Both respond rather slowly to changes in light intensity.
• For cadmium selenide, the response time (tres) is around 10 ms,
while for cadmium sulfide it may be as long as 100 ms.
• Temperature sensitivity is another important difference between the
two materials

49. • There is a large change in the resistance of a cadmium selenide cell with
changes in ambient temperature, but the resistance of cadmium sulfide
remains relatively stable.
• As with all other devices, care must be taken to ensure that the power
dissipation is not excessive.
• The spectral response of a cadmium sulfide cell is similar to that of the
human eye; it responds to visible light.
• For a cadmium selenide cell, the spectral response is at the longer
wavelength end of the visible spectrum and extends into the infrared region.

50. CHARACTERISTICS OF PHOTOCONDUCTIVE CELL
• The illumination characteristics of a typical
photoconductive cell are in figure.
• Initially the cell is not lit up.
• At that time its resistance can be more than 10
kilo-ohm.
• This resistance is the dark resistance.
• When the cell is lit up. Now the resistance may
fall to few hundred ohms.
• Cell sensitivity is expressed in terms of cell
current, input voltage and input level of
illumination

51. APPLICATIONS OF PHOTOCONDUCTIVE CELL
• The photoconductive cell used for relay
control. When the cell is lit up.
• Then its resistance is less and the relay
current is at its maximum.
• When the cell is dark, its high resistance
reduces the current down to a level too low
to energize the relay.
• Resistance R is to limit the relay current to
desired level when the resistance of the cell
is low.

52. DRAWBACKS
• Temperature variations cause substantial variations in resistance for a
particular light intensity.
• Unsuitable for analog applications.

53. PHOTO TRANSISTOR
• Phototransistor is an electronic switching and current amplification
component which relies on exposure to light to operate.
• When light falls on the junction, reverse current flows which are proportional
to the luminance.
• Phototransistors are used extensively to detect light pulses and convert
them into digital electrical signals.
• These are operated by light rather than electric current.
• Providing a large amount of gain, low cost and these phototransistors might
be used in numerous applications

54. • It is capable of converting light energy into electric energy.
• Phototransistors work in a similar way to photoresistors commonly known as LDR
(light dependent resistor) but are able to produce both current and voltage while
photoresistors are only capable of producing current due to change in resistance.
• Phototransistors are transistors with the base terminal exposed.
• Instead of sending current into the base, the photons from striking light activate
the transistor.
• This is because a phototransistor is made of a bipolar semiconductor and focuses the
energy that is passed through it.
• These are activated by light particles and are used in virtually all electronic devices
that depend on light in some way.

55. CHARACTERISTICS
Low-cost visible and near-IR photodetection.
Available with gains from 100 to over 1500.
Moderately fast response times.
Available in a wide range of packages including epoxy-coated, transfer-molded and surface
mounting technology.
Electrical characteristics were similar to that of signal transistors.

56. CONSTRUCTION OF PHOTOTRANSISTOR
• A phototransistor is nothing but an ordinary bi-polar
transistor in which the base region is exposed to the
illumination.
• It is available in both the P-N-P and N-P-N types
having different configurations like common emitter,
common collector and common base.
• Common emitter configuration is generally used.
• It can also work while the base is made open.
Compared to the conventional transistor it has more
base and collector areas..

57. • Ancient phototransistors used single semiconductor materials like silicon and
germanium but now a day’s modern components use materials like gallium
and arsenide for high-efficiency levels.
• The base is the lead responsible for activating the transistor. It is the gate
controller device for the larger electrical supply.
• The collector is the positive lead and the larger electrical supply.
• The emitter is the negative lead and the outlet for the larger electrical
supply

58. • With no light falling on the device there will be a small current flow due to
thermally generated hole-electron pairs and the output voltage from the
circuit will be slightly less than the supply value due to the voltage drop
across the load resistor R.
• With light falling on the collector-base junction the current flow increases.
• With the base connection open circuit, the collector-base current must flow
in the base-emitter circuit and hence the current flowing is amplified by
normal transistor action.

59. • The collector-base junction is very sensitive to light.
• Its working condition depends upon the intensity of light.
• The base current from the incident photons is amplified by the gain of
the transistor, resulting in current gains that range from hundreds to
several thousand.
• A phototransistor is 50 to 100 times more sensitive than a photodiode
with a lower level of noise

60. PHOTOTRANSISTOR CIRCUIT
• A phototransistor works just like a normal transistor, where the base current is multiplied
to give the collector current, except that in a phototransistor, the base current is
controlled by the amount of visible or infrared light where the device only needs 2 pins.

61. • In the simple circuit, assuming that nothing is connected to Vout, the
base current controlled by the amount of light will determine the
collector current, which is the current going through the resistor.
• Therefore, the voltage at Vout will move high and low based on the
amount of light.
• The output of a phototransistor is dependent upon the wavelength of
the incident light.

62. • These devices respond to light over a broad range of wavelengths
from the near UV, through the visible and into the near IR part of the
spectrum.
• For a given light source illumination level, the output of a
phototransistor is defined by the area of the exposed collector-base
junction and the dc current gain of the transistor

63. • Phototransistors available different configurations like optoisolator, optical
switch, retro sensor.
• Optoisolator is similar to a transformer in that the output is electrically
isolated from the input.
• An object is detected when it enters the gap of the optical switch and blocks
the light path between the emitter and detector.
• The retro sensor detects the presence of an object by generating light and
then looking for its reflectance off of the object to be sensed.

64. ADVANTAGES OF PHOTOTRANSISTOR
• Phototransistors produce higher current than photodiodes.
• Phototransistors are relatively inexpensive, simple, and small enough
to fit several of them onto a single integrated computer chip.
• Phototransistors are very fast and are capable of providing nearly
instantaneous output.
• Phototransistors produce a voltage, that photo-resistors cannot do so.

65. DISADVANTAGES OF PHOTOTRANSISTOR
• Phototransistors that are made of silicon are not capable of
handling voltages over 1,000 Volts.
• Phototransistors are also more vulnerable to surges and spikes of
electricity as well as electromagnetic energy.
• Phototransistors also do not allow electrons to move as freely as other
devices do, such as electron tubes.

66. APPLICATIONS
Punch-card readers.
Security systems
Encoders – measure speed and direction
IR detectors photo
electric controls
Computer logic circuitry.
Relays
Lighting control (highways etc)
Level indication
Counting systems

67. BIOMEDICAL APPLICATION
• To measure pulsatile blood volume change
with a Photodetector.
• To detect the pulse, we can either use
Transmittance or Reflectance techniques
• In transmittance technique, pulsating blood
flow modifies the optical density. In
reflectance technique, blood flow changes the
intensity of reflected light.
• The changes in blood flow are seen
immediately with these methods.

68. PNEUMOGRAPH
• To measure changes around the circumference of the
chest with a pneumograph that has a photodiode.
• Wrap the chest with a rubber bellow.
• Inside the bellows, the movable metal bar is
attached.
• When the chest expands during breathing, the
amount of light that falls on the photodiode varies
due to the metal bar.
• Calibrate the obtained result to get the respiratory
volume

69. BLOOD PRESSURE
• Blood pressure can be measured with Photodetectors as
shown in the figure below. At the free end of a bourdon
tube between lamp and photodiode, a shade is attached.
• Bourdon tube is filled using a saline solution.
• Pressure is created inside the tube due to blood
pressure.
• As the blood pressure increases, the pressure inside the
tube displaces the shade.
• This displacement is proportional to the output from the
phototube.

70. PULSE OXIMETRY
• To determine the oxygen saturation in
the blood (oximetry) photoelectric
transducers are used.
• Measurement of oxygen content is
important during open-heart surgery.
• In the human body, earlobes are rich
with vascular beds.
• So, earlobes are illuminated by a light
source. The reflected light is detected
with two photovoltaic detectors.

71. • The first detector detects the emitted radiation in the red region (640mµ),
and another detector detects in the IR region (800mµ).
• The output from the red channel is related to the oxygen content in blood
and the presence of blood and tissue along the optical path.
• However, the output from the IR spectrum is not proportional to the oxygen
saturation.
• Finally, the difference between the two outputs is proportional to the
amount of oxygen present in the blood.

72. 72