1. Visvesvaraya National Institute of Technology
Nagpur
Presentation of the Photoconductivity of
CdS photo-resistor at constant irradiance
and constant voltag
3. Photoconductivity is an optical and electrical phenomenon in
which a material become more electrically conductive due to
the absorption of electromagnetic radiation such as visible
light, ultraviolet light, infrared light, or gamma radiations. It is
the effect of increasing electrical conductivity Ø in a solid due
to light absorption.
When the so-called internal photo effect takes place, the
energy absorbed enables the transition of activator
electrons into the conduction band and the charge
exchange of traps with holes being created in the valence
band. In the process, the number of charge carriers in the
crystal lattice increases and as a result, the conductivity
is enhanced.
Ravindra Pratap Meel –VNIT Nagpur
4. When light is absorbed by a material such as a
semiconductor, the number of free electrons and
electron holes changes 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.
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.
Ravindra Pratap Meel –VNIT Nagpur
6. Semiconductor resistors that depend on the irradiance
(photoconductive cells) are based on this principle.
They have opened up a wide field of applications and
are, among other things, employed in twilight switches
and light meters. The semiconductor materials most
commonly used are cadmium compounds, particularly
CdS.
The photocurrent is studied as a function of the
voltage applied to the photoresistor at a constant
irradiance (current-voltage characteristics) and as a
function of the irradiance at a constant voltage
(current-irradiance characteristics).
Ravindra Pratap Meel –VNIT Nagpur
8. The four materials normally employed in photoconductive
devices are: Cadmium Sulphide (CdS), Cadmium Selenide (CdSe),
lead sulphide (PbS) and Thallium Sulphide (TlS).In a typical
construction of photoconductive device, thin film is deposited on
an insulating substrate. The electrodes are formed by evaporating
metal such as gold through a mask to give comb -like pattern as
shown[above fig]. The geometry results in a relatively large area of
sensitive surface and a small inter electrode spacing. This helps
the device to provide high sensitivity.
Ravindra Pratap Meel –VNIT Nagpur
9. The photoconductive cell has very high
resistance in dark called dark resistance. When
illuminated the resistance falls. The spectral
response of CdS cell is similar to that of the
human eye. The illumination characteristics of
the cell is shown in below fig.
When the device under forward bias is
illuminated with light electron–hole pairs are
generated. The electron-hole pairs generated
move in opposite directions. This results in a
photocurrent.
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11. Let us suppose that electron-hole pairs are produced
uniformly throughout the volume of the crystal by
irradiation with an external light source. Recombination
occurs by direct combination of electrons with holes. It may
be that electrons leaving the crystal at one end are replaced
by electrons flowing in from the opposite electrode. The
mobility of holes may be neglected in comparison with the
mobility of the electrons.
Photoconductivity may be written as
σ= n0eμ ............(1)
Ravindra Pratap Meel –VNIT Nagpur
12. where μ is the electron mobility and n0 is the electron
concentration in the steady state. At a given voltage,the
photo-current varies with light intensity as L1/2,where L is
the number of photons absorbed per unit volume of the
specimen per unit time.the exponent varies between ½ and
1, with some ceystals having higher exponent. In case of CdS
crystal, the exponent between 0.92 al low level and 0.58 at
high level of illumination. The response time is given by
t0=n0/L
Ravindra Pratap Meel –VNIT Nagpur
13. form equation (1) we get
n0=σ/e μ
t0= σ/e μL ........... (2)
Response time is the time during which carrier concentration
should drop to 0.5 n0.
Response time should therefore be directly to the
photoconductivity at a given illumination level L.
Sensitive photo-conductors should have long response
time.
Ravindra Pratap Meel –VNIT Nagpur
14. A consequence of small band gap (Δ ) in semiconductors is that it is
possible to generate additional carriers by illuminating a sample of
semiconductor by a light of frequency greater than . This leads to an
increased conductivity in the sample and the phenomenon is known as
photoconductivity . The effect is not very pronounced at high
temperatures except when the illumination is by an intense beam of
light. At low temperatures, illumination results in excitation of
localized carriers to conduction or valence band.
Consider a thin slab of semiconductor which is illuminated
by a beam of light propagating along the direction of its
length (x-direction). Let be the radiation intensity (in
watts/m ) at a position from one end of the semiconductor.
If α=absorption coefficient per unit length, the power
absorbed per unit length is αI. The change in the intensity
with distance along the sample length is given by
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16. If we define η as the quantum efficiency , i.e. the fraction of
absorbed photons that produce electron-hole pairs, the
number of pairs produced per unit time is given by
In principle, the process of illumination will lead to a
continued increase in the number of carriers as the amount of
energy absorbed (and hence Δn and Δp ) will increase linearly
with time. However, the excited pairs have a finite life time
(≈10-7 to 10-3 s). This results in recombination of the pairs. The
relevant life time is that of minority carriers as a pair is required
in the process. Recombination ensures that the number of
excess carriers does not increase indefinitely but saturates.
Ravindra Pratap Meel –VNIT Nagpur
17. Consider an n-type semiconductor. If the recombination
life time for the minority carriers is τp , the rate of change
of carrier concentration is given by
Under steady state condition dδp/dt=0 , which gives
This excess hole density leads to an additional conductivity
Ravindra Pratap Meel –VNIT Nagpur
18. Desired characteristics of
photoconductive materials
i) High spectral sensitivity in the
wavelength region of interest
ii) Higher quantum efficiency
iii) Higher photoconductive gain
iv) Higher speed of response and
v) lesser noise
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21. CdS nanowires were synthesized in the Lauhon laboratory in a
chemical vapor deposition reactor via the vapor-liquid solid
growth mechanism. In this process,an Au particle is used to
catalyze onedimensional growth, resulting in nanowires.
The Cd and S atoms from the vapor precursor (Cd
Diethyldithiocarbamate) form a liquid alloy with the Au
particle.
The alloy becomes supersaturated, and the Cd and S atoms
diffuse to the deposition surface, combining to form CdS.
The resulting wires typically ranged from 5 to 15 μm in
length, and the distribution in wire diameters was
determined by the distribution in gold particle size. The
synthesized CdS nanowires were suspended in isopropanol
and pipetted onto each grid.
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22. As seen in Figure , the current through the CdS nanowire
was observed as the intensity of light from the incandescent
lamp was modified in the presence of a 3V bias across the
wire. When the light was turned on, conductivity
immediately increased. This observed photoresponse
corresponds to a 25nA increase in current, which is
approximately 18 percent of the initial (i.e., light off ) value.
After the light was turned off, the current returned to its
original value. This process was repeated two more times.
Ravindra Pratap Meel –VNIT Nagpur
27. CdS photo-resistor
Figure shows the variations in deposition rate of CdS
films deposited on glass substrates at room temperature
and at rf power of 50 W as a function of working
pressure. As shown in Figure, the deposition rate of the
films decreases monotonously with increasing working
pressure.
In this experimental condition, a decrease in deposition
rate with increasing working pressure was attributed to the
scattering effect of the particles detached from the target
by argon plasma. An available thickness of CdS films can
be chosen from these data in terms of variations in
working pressure.
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28. Variations in deposition rate of the CdS films
deposited on glass substrates at room temperature as
a function of working pressure.
VNIT,Nagpur
29. The photocurrent IPh as a function
of voltage V at a constant irradiance Ø
The relation between the photocurrent IPh and the
voltage U applied at a constant irradiance (constant
angle α between the polarization planes of the
filters), i.e. the current-voltage characteristics, is
shown in Figure. Even at an angle α = 90° between
the polarization planes of the filters a photocurrent
flows, since in this position the polarization filters
do not extinguish the ray of light completely.
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31. Measuring photocurrent IPh as a
function of irradiance Ø at a constant
voltage V:
The relation between the photocurrent IPh and the
irradiance at a constant voltage, the
currentirradiance characteristics, is shown in Fig. 4.
The term cos2α is a relative measure for the
irradiance (α angle between the polarization planes
of the filters). As expected, the photocurrent
increases with increasing irradiance. However, the
characteristics are not perfectly linear. The slope
rather decreases with increasing irradiance
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37. OBSERVATION :
a) Measuring photocurrent IPh as a function of
voltage V at a constant irradiance Ø
1. Set the analyzer at 0° mark provided on the analyzer
scale.
2. Interrupt the path of the ray of light, and determine
the photocurrent I(0°) due to the residual lightness.
3. Starting from 16V, reduce the voltage V to 0V in steps
of 2V. Measure the photocurrent IPh, each time and
record it.
4. Repeat the series of measurements with α at 30°, 60°,
and 90° .
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38. Table 1: The photocurrent IPh as a function of the voltage
V at a constant irradiance Ø
V (Volt) IPh at 0°
(mA)
IPh at 30°
(mA)
IPh at 60°
(mA)
IPh at 90°
(mA)
16
14
12
10
8
6
4
2
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39. b) Measuring photocurrent IPh as a function of
irradiance Ø at a constant voltage V:
1. Set the voltage 16 V, interrupt the path of the ray of light
and measure the photocurrent I˳ due to the residual
lightness.
2. In order to vary the irradiance Ø, increase the angle α
between the polarization planes of the filters in steps of
10° from 0° to 90°. Measure the photocurrent IPh,
each time and record it.
3. Repeat the series of measurements at V =8V and V =1V
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40. α cos2α IPh at 16V
(mA)
IPh at 8V
(mA)
IPh at 1V
(mA)
00
100
200
300
400
500
600
700
800
900
Table 2: The photocurrent IPh as a function of irradiance Ø
at a constant voltage .
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41. THE CdS PHOTORESISTOR
BEHAVES LIKE AN OHMIC
RESISTANCE THAT DEPENDS
ON THE IRRADIANCE…
Ravindra Pratap Meel –VNIT Nagpur
