Cds photo resistor

Visvesvaraya National Institute of Technology Nagpur

Presentation of the Photoconductivity of CdS photo-resistor at constant irradiance and constant voltag

Submitted To - Dr.B.R.SankapalSubmitted By- Ravindra Pratap Meel

M.Sc.-1st

year

PhotoconductivityCdS photo-resistor Experiment CdS photo-resistor

Ravindra Pratap Meel –VNIT Nagpur

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.

Photoconductivity

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

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.

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.


Sketch of a photoconductive device

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hv

PhotoconductivitySemiconductor 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

Construction of photoconductive device

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Geometry of the photoconductive cellRavindra Pratap Meel –VNIT Nagpur

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.



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.



Photoconductor in circuit


Variation of photoconductivity with illumination

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)


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

Variation of photoconductivity with illumination

t0=n0/L


form equation (1) we get

n0=σ/e μt0= 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.


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.

Photoconductivity Vs Current

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


which has solution


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

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


Desired characteristics of photoconductive materials

i) High spectral sensitivity in the wavelength region of interest

ii) Higher quantum efficiencyiii) Higher photoconductive gainiv) Higher speed of response and v) lesser noise


APPLICATIONS OFPHOTOCONDUCTIVITY DEVICES

•Light meters

•Infrared detectors

•TV cameras

•Voltage regulator

•Relays and

•Detecting ships and air crafts


CdS photo-resistor


CdS photo-resistor CdS nanowires were synthesized in the Lauhon

laboratory in a chemical vapor deposition reactor via the vapor-liquid solidgrowth 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.Ravindra Pratap Meel –VNIT

Nagpur

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 photoresponsecorresponds 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 itsoriginal value. This process was repeated two more times.

Current-over-time graph used to measure the photoresponse of the CdS nanowires to visible light.


Current-over-time graph used to measure the photoresponse of the CdS nanowires to visible light.


Spectral response of CdS cell VNIT,Nagpur


CONSTRUCTION OF CdS PHOTORESISTOR


CdS photo-resistor


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.


Variations in deposition rate of the CdS films deposited on glass substrates at room temperature as a function of working pressure.

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The photocurrent IPh as a function of voltage V at a constant irradiance ØThe 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. Ravindra Pratap Meel –VNIT

Nagpur

Current–Voltage characteristics of CdS photoresistor

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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


Current-irradiance characteristics of the CdS

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photoresistor


CdS photo-resistor image


CdS Experiment

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Lamp housing, adjustable slit,

polarizer, analyzer, two convex lens,

photoresistor, multimeter, an optical

bench with fixing mounts.

EXPERIMENT-APPARATUS


Block diagram:


OBSERVATION : a) Measuring photocurrent IPh as a function of voltage V at a constant irradiance Ø1. 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° .


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)

161412108642


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


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 .


RESULT:THE CdS PHOTORESISTOR BEHAVES LIKE AN OHMIC RESISTANCE THAT DEPENDS ON THE IRRADIANCE…


Thank you for your attention !

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Cds photo resistor