The document provides an overview of photoconductivity, detailing its definition, types, effects, and applications, particularly focusing on photoconductive cells and photoresistors. It explains the principles behind photoconductivity, the use of different materials in photoconductive cell construction, and outlines their characteristics, working mechanisms, and applications in various circuits. Additionally, it discusses the limitations and performance indicators of these devices.

1. MARINE CONTROL
ENGINEERING AND
AUTOMATION
Mohamed Shaz Jaleel | S-8 B 03

2. PHOTOCONDUCTIVE
CELLS

3. Table of Contents
Index No.
1.
2.
3.
4.
5.
6.
7.
8.
Topic
Introduction – Photoconductivity Definition
Photoelectric Effect – Deviation from Photovoltaic Effect
Various types of Photoconductivity – Negative & Magnetic
Applications – Examples of Photoconductive materials
Photoconductive Cells - Design, Construction & Working
Photoresistor – The photoconductive cell or LDR
Application of Photoconductive Cells
Parameters of Photoconductive Cells – Terminologies, Drawbacks
Slide No.
4
7
9
12
15
22
26
30
3

4. 1. Introduction
Photoconductivity - Definition

5. Photoconductive Effect
▪ 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
▪ Photoconductivity is an optical and electrical phenomenon in which a
material becomes more electrically conductive due to the absorption
of electromagnetic radiation such as visible
light, ultraviolet light, infrared light, or gamma radiation.
5

6. Photoconductive Effect
▪ When light is absorbed by a material such as a semiconductor, the
number of free electrons and electron holes increases and raises its
electrical conductivity. To cause excitation, the light that strikes the
semiconductor must have enough energy to raise electrons across
the band gap, or to excite the impurities within the band gap. When
a bias voltage and a load resistor are used in series with the
semiconductor, a voltage drop across the load resistors can be
measured when the change in electrical conductivity of the material
varies the current through the circuit.
6

7. 2. Photoelectric Effect
Deviation from Photovoltaic Efect

8. Photoelectric Effect
The emission of electrons or
other free
carriers when electromagnetic
radiation, like light, hits a
material. Electrons emitted in
this manner can be
called photoelectrons.
Differentiating Photoelectric and Photovoltaic Effect
Photovoltaic Effect
The generation
of voltage and electric
current in a material upon
exposure to light. It is
a physical and chemical pheno
menon.
8

9. 3. Various types of
Photoconductivity
Negative and Magnetic Photoconductivity

10. Negative Photoconductivity
▪ Some materials exhibit deterioration in photoconductivity upon
exposure to illumination. One prominent example is hydrogenated
amorphous silicon (a-Si:H) in which a metastable reduction in
photoconductivity is observable. Other materials that were reported to
exhibit negative photoconductivity include molybdenum
disulfide, graphene, indium arsenide nanowires, and
metal nanoparticles.
10

11. Magnetic Photoconductivity
▪ It was demonstrated that in some photoconductive material a
magnetic order can exist. One prominent example is CH3NH3(Mn:Pb)I3.
In this material a light induced magnetization melting was also
demonstrated thus could be used in magneto optical devices and data
storage.
11

12. 4. Applications
Examples of Photoconductive materials

13. Basic application of Photoconductivity
▪ When a photoconductive material is connected as part of a circuit, it
functions as a resistor whose resistance depends on the light
intensity. In this context, the material is called a photoresistor (also
called light-dependent resistor or photoconductor). The most common
application of photoresistors is as photodetectors, i.e. devices that
measure light intensity. Photoresistors are not the only type of
photodetector—other types include charge-coupled
devices (CCDs), photodiodes and phototransistors. Some
photodetector applications in which photoresistors are often used
include camera light meters, street lights, clock radios, infrared
detectors, nanophotonic systems and low-dimensional photo-sensors
devices 13

14. Common Photoconductive materials
▪ the conductive polymer polyvinylcarbazole, used extensively
in photocopying (xerography)
▪ lead sulfide, used in infrared detection applications, such as the
U.S. Sidewinder and Soviet (now Russian) Atoll heat-seeking missiles;
▪ selenium, employed in early television and xerography.
14

15. 5. Photoconductive Cells
Design, Construction and Working

16. Photoconductive Cell –
Design & Construction
The Photoconductive Cell Construction and
Working is illustrated in fig., and the graphic
symbol is shown. 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.
16

17. Photoconductive Cell –
Design & Construction
▪ 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.
17

18. Photoconductive Cell –
Design & Construction
▪ 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.
18

19. Working of Photoconductive Cells
▪ An photoconductive cell is made of any semiconductor material with a high
resistance. It has a high resistance because there are very few electrons that are
free and able to move - the vast majority of the electrons are locked into the
crystal lattice and unable to move. Therefore in this state there is a high LDR
resistance.
▪ As light falls on the semiconductor, the light photons are absorbed by the
semiconductor lattice and some of their energy is transferred to the electrons.
This gives some of them sufficient energy to break free from the crystal lattice so
that they can then conduct electricity. This results in a lowering of the resistance
of the semiconductor and hence the overall LDR resistance.
▪ The process is progressive, and as more light shines on the LDR semiconductor,
so more electrons are released to conduct electricity and the resistance falls
further
19

20. Working of Photoconductive Cells
▪ The active semiconductor region is normally deposited onto a semi-insulating
substrate and the active region is normally lightly doped.
▪ In many discrete photoconductive devices, an interdigital pattern is used to
increase the area of the photoresistor that is exposed to light. The pattern is cut in
the metallisation on the surface of the active area and this lets the light through.
The two metallise areas act as the two contacts for the resistor. This area has to be
made relatively large because the resistance of the contact to the active area
needs to be minimised. This type of structure is widely used for many small
photoresists or light dependent resistors that are seen. The interdigital pattern is
quite recognisable.
▪ The materials used for photoresists are semiconductors and include materials
such as CdSe, CdS, CdTe, InSb, InP, PbS, PbSe, Ge, Is, GaAs. Each material gives
different properties in terms of the wavelength of sensitivity, etc.
20

21. Photoconductive Cell –
Characteristics
▪ Typical illumination characteristic for a
photoccnductive cell are shown in fig. It is
seen that, when the cell is not illuminated
its resistance can be greater than 100 kΩ.
This is known as the dark resistance of the
cell. When the cell is illuminated, its
resistance might fall to a few hundred
ohms. Note that the scales on the
illumination characteristic are
logarithmic to cover a wide ranges of
resistance and illumination that are
possible.
21

22. 6. Photoresistor
The photoconductive cell or the Light
Dependent Resistance

23. Photo
resistor
An active component that decreases
resistance with respect to receiving
luminosity (light) on the component's
sensitive surface.
23

24. Photoresistors or LDRs
▪ The resistance of a photoresistor
decreases with increase in incident light
intensity; in other words, it
exhibits photoconductivity. A photoresistor
can be applied in light-sensitive detector
circuits and light-activated and dark-
activated switching circuits acting as a
resistance semiconductor. In the dark, a
photoresistor can have a resistance as high
as several megaohms (MΩ), while in the
light, a photoresistor can have a resistance
as low as a few hundred ohms.
24

25. Photoresistors or LDRs
▪ If incident light on a photoresistor exceeds
a certain frequency, photons absorbed by
the semiconductor give
bound electrons enough energy to jump
into the conduction band. The resulting
free electrons (and their hole partners)
conduct electricity, thereby
lowering resistance. The resistance range
and sensitivity of a photoresistor can
substantially differ among dissimilar
devices. Moreover, unique photoresistors
may react substantially differently to
photons within certain wavelength bands.
25

26. 7. Applications of
Photoconductive Cells
Examples of LDRs in various circuits

27. Applications
▪ Figure shows a photoconductive cell used
for relay control. When the cell is
illuminated, its resistance is low and the
relay current is at its maximum. Thus, the
relay is energized. When the cell is dark, its
high resistance keeps the current down to a
level too low to energize the relay.
Resistance R1 is included to limit the relay
current to the desired level when the cell
resistance is low.
27

28. Applications
▪ Photoconductive Cell Construction and
Working employed to switch transistors on
and off are shown in figure. When cell is
dark, the cell resistance (RC) is high.
Consequently, the transistor base is biased
above its emitter voltage level, and Q1 is
turned on. When the cell is illuminated, its
resistance is reduced, and the lower cell
resistance in series with R1 biases the
transistor base below its emitter voltage
level. Thus Q1 is turned off when the cell is
illuminated.
28

29. Applications
▪ In figure, Q1 is biased off
when the cell is dark,
because RC is high. When
illuminated, the reduced
cell resistance causes Q1 to
be biased on
29

30. 8. Parameters of
Photoconductive Cells
Terminologies | Drawbacks

31. Some Important Terminologies of LDR
Spectral Response
The resistors are only
light dependent over a
limited range of wave-
lengths. Light Dependent
resistor (LDRs) have their
maximum response at
about 680 nm.
Temperature
Dependency
The electrons are excited
not only by photons but also
by thermal agitation.
Therefore the dark
resistance is not infinite at
normal temperatures. It
increases with increase in
ambient temperature. And it
can be reduced by cooling
the device.
Recovery Rate
When an Light
Dependent Resistor (LDR)
is brought from a certain
illuminating level into
total darkness, then the
resistance does not
increase immediately to
the dark value. The
recovery rate is specified
in k-ohm/second.
31

32. Drawbacks of Photoconductive cells
▪ LDRs are inexpensive, sensitive, and they are readily available devices.
They have very good power and voltage handling capabilities. It is very
similar to those of a conventional resistor. Their only significant defect
is that they are fairly low acting, taking tens or hundreds of milli-
seconds to respond to sudden changes in light level.
▪ Temperature variations cause substantial variations in resistance for a
particular light intensity. Therefore such cell is unsuitable for analog
applications. Such cells find wide use in industrial and laboratory
control applications.
32

33. 33
Thanks!
Presented by:
Mohamed Shaz Jaleel | S-8 B 03
▪ @mohamed.shaz
▪ shazfred@gmail.com

34. Credits
The content of this presentation has been obtained from the following
sources:
▪ https://www.eeeguide.com/photoconductive-cell-construction-and-
working/
▪ http://www.circuitstoday.com/photoconductive-cells
▪ https://electronicslesson.com/transducer/photoconductive-cell/
▪ https://en.wikipedia.org/wiki/Photoconductivity
▪ https://en.wikipedia.org/wiki/Photoelectric_effect
▪ https://en.wikipedia.org/wiki/Photovoltaic_effect
▪ https://en.wikipedia.org/wiki/Photoconductivity 34

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