General Principle Of Operation Engineering Essay


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In this area of research we have studied the behaviour of commercial diodes and tried to prepare photodiodes in lab. One can say that bottom-up approach is easily applicable to produce layered nanostructures on solid surface. In this course work applications of photodiodes are studied in photo and electrochemical devices. Layer by layer pattern is adopted to produce self assembled multilayered diode with metallic aid. Target of production of nanostructures is to construct inter connection between terminals of device. A diode is prepared which use the low voltage (9 volts) to provide light as the aim of nanoscience is to produce more output by using less energy. [1] Photodiode of CdS (440nm) are prepared by thin film coating and thermally evaporated. The properties like current-voltage (VI) and capacitance-voltage ere studied for this nano layered photodiode. Properties of vacuum evaporated diode are studied along with fabrication. Sample prepared shows that the thermionic emission theory is followed to some extent. [2]

A brief introduction


This process is used widely to produce silicon chips which we use in manufacture of different devices especially electric devices. Different methods are used for this purpose. Most common of these are deposition (physical vapour deposition, chemical vapour deposition, electrochemical deposition, molecular beam epitaxy, atomic layer deposition, etc), removal processes (wet etching, dry etching, chemical mechanical planarization), patterning covers (lithography, plasma ashing).


"Photodiodes are the photo detectors which can transform light into electrical energy".

Photodiodes are different from ordinary diodes as they are packed with optical fibres that can connect them to the sensitive part of device. Photodiodes can also be exposed and in exposed state they are used to detect UV light and X-rays.

General principle of operation:

Operation of photodiode is generally based on simple light absorption. When a photon of suitable energy is absorbed, electrons are excited and as a result free electrons and positive holes are created. Electrons start moving towards cathode and holes move towards anode. This movement of charges produces current. Photodiodes have PN junction or in specification PIN junction. This is the depletion region where electrons are exited and holes are created. Photodiodes can work in different modes but most common are;

Photovoltaic mode

Photoconductive mode

Avalanche Photodiodes

Photoconductive diode is faster than the other two.

Preparation of photodiodes:

Photodiodes are made of different materials. Selection of material to make photodiode is critical as the only requirement is to absorption of specific photon. Different materials have different band gaps so absorb photon of different energies. Some materials with their wave length ranges are as follows;


Wavelength range





Indium gallium arsenide


Led(ll) sulphate


Performance of photodiodes is terms of responsivity and in responsivity ratio between responses (A/W) and wavelength absorbed are used to describe the behaviour of photo diodes.

General uses of photodiodes:

Generally diodes are used in



Charge coupled devices

Photomultiplier tubes

Compact disc player

Smoke detectors

Receivers for remote controls in TV and VCRs

Camera light meter

Street cameras

Various medical applications like tomography, sample analysers and pulse oximeters etc.

Optical communication

Lighting regulation


Laser finding devices

Light intensity meters

Night vision devices


Photodiodes at nano scale:

The universe is developing towards smaller instrumentations in the field of science. There is the need to increase the capacity in minimum ranges of power, expenses, size, and to get maximum output of efficiency, beneficial aspects and reliability.

Same is the case in the field of electronics and such devices are being prepared which have very small size but incredible output. Photodiodes are the constituents of all instruments used in daily life either in general or in specific research works. Nano scale photodiodes show high sensitivity. These are used in integrating devices and key success is achieved by using semiconductors at nano scale like nanowires and nanotubes which produce electrically conducting detectors.

If we take the example of avalanche photodiodes which are highly sensitive semiconducting devices which work according to photoelectric effect and convert light to the electrical energy. These photodiodes have more electrons in their valance band than conduction band. Electrons from valence band are needed to be excited and promoted to the conduction band because when they return to the ground state, they provide more energy than that energy what they consume to jump from valence band to the conduction band. Energy required is more than that of the band gap. In this way they are used as photomultipliers. Avalanche photodiodes are used in laser rangefinders and long range telecommunications where they are proved to be very fruitful. Their working capability is dependent on different parameters like quantum efficiency. Quantum efficiency describes the ability or extent of absorbing photons of light and producing electrical energy.

Most of the photodiodes of this type use silicon as a substrate but other materials can also be used for this purpose. Some of materials with their ranges of light are as follows;


Germanium Infrared range

Indium gallium arsenide 0.9-1.7µm

Gallium nitride Ultraviolet range

HgCdTe 14µm

Silicon Visible and near IR

Performance of device is limitized or enhanced by different factors which are mostly


Time of transmission

Avalanche multiplication [4], [5], [6], [7]


The aim of this course work is to create an understanding of using different photodiodes for the production of electric current with more efficiency than any other sources or methods used. This is because photodiodes are being used in almost all instruments used in daily life. This work also creates a sense of applications of fabrication of nanostructures in an appropriate way in all types of electronic devices like photodiodes, photovoltaic cells, LED (light emitting diodes), etc. It also provides us the ways how these can be helpful in optical communication.


Objectives of this research work are to create a sense and interest of readers and observers about uses and beneficial aspects of nano technology. It is also helpful to establish and standardize an analytical tumble for nanomaterials categorization. Nanotechnology is to facilitate the clinical development and regulatory review of nanomaterials for cancer clinical trials. Medically nanomaterials are applicable in identifying critical parameters related to nanomaterial absorption, distribution, metabolism, and toxicity profile.

To introduce new nanotechnology products in market that one can start investment in small business.

Another objective of nanotechnology is to improve measurements and standardization in research and development. [8]

To produce photodiodes with less energy consumption and more output.

Project outline:

"Fabrication of nanostructures and their application in photodiodes of CdS (440nm)"

Semiconductor compounds of elements from group 2 and group 6 have been proved to be very effective in photovoltaic devices, electroluminescent layers and many more. Photodiodes of cadmium, zinc and lead have shown great efficiency towards absorption of electromagnetic waves. [9] This property has revealed their importance in making solar cells and photodiodes and phototransistors. Solar cells with expended bandgap window and narrow bandgap absorption are focusing issue of nanoscience. CdS is proved to be a super material for this purpose with a band gap of 2.43eV in the production of solar cells. [10]. Different methods of deposition of CdS, as thin film, are applied which is helpful in describing barrier height. Metal semiconductors of such type are frequently used in solar cells.

According to our knowledge some of the properties and applications of metal-semiconductor devices are reported in this coursework. This coursework is description of CdS usage in electro-phonetic devices and discussing about Cds based devices used for producing photo voltaic electricity. In present work photodiodes were prepared by vacuum evaporation of Al and their analytic report was prepared. Report shows the current-voltage and capacitance-voltage properties analyzed by IVT.

Work description and Experimental details:

Al/n-CdS was prepared in lab by passing through following steps

1: Cutting and washing of ITO(Indium Tin Oxide):

First of all ITO was cut according to the size of copper mask sample compartment. ITO should be cut carefully and goggles should be used while cutting. When the sample was appropriate to the size of compartment, it was washed with propanone to remove all dust and impure particles. Propanone leaves a greasy layer on ITO like the natural grease of human skin. To remove this ITO was washed in alcohol (isopropyl alcohol). With both washings material was placed in a small glass container and sonnicated for three minutes. When completed sample was dried in air or by heating. Don't touch the ITO with hands, it may cause contamination.

2: Conductivity check:

Conductive side of ITO was checked with the help of multimeter as shown in the figure. In figure a device is shown to be checked by meter.

3: Spin Coating:

After the check of conductivity side, ITO was placed in spin coater and fitted tightly with conductive side placed upward. One drop of p-dot was applied to it and spinned for three minutes at a speed of 180rps (about 1800 rpm). Then Ito was taken out and dried for 30 minutes on hot plate at a temperature of about 200-250oC (pre adjusted to this temperature). Then sample was again fitted in spin cotter and 2-3 drops of CdS(440nm) quantum DOT were applied and spinned for 3 minutes at a speed of 100rps (about 1000rps) for 30 seconds. The sample was dried again and cooled at room temperature.

4: Deposition:

Physical vapour deposition chamber was turned on and a pressure of 1*10-5 was achieved. A boat of molybdenum was made and placed inside the chamber. Chamber was heated to avoid contamination. Continued heating until colour of molybdenum became red. Sample was then placed in copper mask with conductive side outwards. The mask was hung up in the physical vapour deposition chamber. Al coil was placed into molybdenum boat. A pressure of 1*10-5 was achieved and then heating chamber was started. Continued heating until Al started bubbling. Stopped heating and released the pressure. Sample was taken out. Sample clearly showed the signs of deposition of Al.

5: Making Connections:

One side of ITO was scratched with toothpick and a conductive wire was attached to that site with the help of zinc paint. This site was anode. Scratching was done to get direct contact of conductive surface by removing p-dot and active material. Cathodes were made by attaching conductive wire on each of the deposited layer. All the connections were strengthened by applying gum.

6: Analyzing the sample:

At the end sample was analysed. For this purpose, first of all connections were checked with multimetre. Then sample was checked on oscilloscope by connecting anode to one terminal and cathodes to the other one by one. Data attained by oscilloscope is as follows;


(curve showing that there is some conduction of current)


The curve shown is not the actual form of LED result.

In actual it should be a graph with right angle. Reason of difference is the change of procedure and difference of precautions. All this work is done in controlled environment in the presence of inert gas which was not possible in lab. Secondly high pressure was required which was 2*10-5but we could achieve only 1*10-5 .

After that, sample was analysed on IV tester. IV tester showed some peaks which proved that electric current passed through the sample and there was some light sensitive affect. Current was shown in light line and potential was shown by red line. Data obtained by manipulation is as follows;

Test No Time Current Light Intensity Voltage

0.000000 0.000000 0.000010 0.400000 0.003335

1.000000 300.000000 0.000050 0.300000 0.016576

2.000000 600.000000 0.000090 0.400000 0.029802

3.000000 900.000000 0.000130 0.200000 0.043099

4.000000 1200.000000 0.000170 0.400000 0.056391

5.000000 1500.000000 0.000210 0.400000 0.069580

Test No Time Current Light Intensity Voltage

6.000000 1800.000000 0.000249 0.300000 0.082269

7.000000 2100.000000 0.000289 0.300000 0.095563

8.000000 2400.000000 0.000329 0.300000 0.108758

9.000000 2700.000000 0.000369 0.300000 0.121643

10.000000 3000.000000 0.000409 0.400000 0.134336

11.000000 3300.000000 0.000449 0.300000 0.147395

12.000000 3600.000000 0.000489 0.400000 0.160180

13.000000 3900.000000 0.000529 0.300000 0.172827

14.000000 4200.000000 0.000569 0.300000 0.185448

15.000000 4500.000000 0.000609 0.300000 0.198387

16.000000 4800.000000 0.000648 0.300000 0.210150

17.000000 5100.000000 0.000688 0.400000 0.222863

18.000000 5400.000000 0.000728 0.400000 0.235527

19.000000 5700.000000 0.000768 0.400000 0.247647

20.000000 6000.000000 0.000808 0.300000 0.259784

21.000000 6300.000000 0.000848 0.300000 0.272108

22.000000 6600.000000 0.000888 0.300000 0.284153

23.000000 6900.000000 0.000928 0.300000 0.296652

24.000000 7200.000000 0.000968 0.400000 0.308059

25.000000 7500.000000 0.001008 0.600000 0.322112

26.000000 7800.000000 0.001047 0.300000 0.333534

27.000000 8100.000000 0.001087 0.300000 0.345298

Test No Time Current Light Intensity Voltage

28.000000 8400.000000 0.001127 0.300000 0.356256

29.000000 8700.000000 0.001167 0.300000 0.367922

30.000000 9000.000000 0.001207 0.400000 0.379138

31.000000 9300.000000 0.001247 0.400000 0.390060

32.000000 9600.000000 0.001287 0.300000 0.401587

33.000000 9900.000000 0.001327 0.300000 0.412595

34.000000 10200.000000 0.001367 0.300000 0.423884

35.000000 10500.000000 0.001407 0.300000 0.435126

36.000000 10800.000000 0.001446 0.400000 0.445974

37.000000 11100.000000 0.001486 0.300000 0.456800

38.000000 11400.000000 0.001526 0.400000 0.466650

39.000000 11700.000000 0.001566 0.400000 0.477471

40.000000 12000.000000 0.001606 0.300000 0.487830

41.000000 12300.000000 0.001646 0.300000 0.498539

42.000000 12600.000000 0.001686 0.400000 0.507784

43.000000 12900.000000 0.001726 0.400000 0.518622

44.000000 13200.000000 0.001766 0.400000 0.529020

45.000000 13500.000000 0.001806 0.300000 0.539399

46.000000 13800.000000 0.001845 0.300000 0.548780

47.000000 14100.000000 0.001885 0.400000 0.558973

48.000000 14400.000000 0.001925 0.300000 0.569182

49.000000 14700.000000 0.001965 0.400000 0.579862

Test No Time Current Light Intensity Voltage

50.000000 15000.000000 0.002005 0.400000 0.589503

51.000000 15300.000000 0.002045 0.300000 0.598606

52.000000 15600.000000 0.002085 0.400000 0.608178

53.000000 15900.000000 0.002125 0.400000 0.617133

54.000000 16200.000000 0.002165 0.300000 0.626887

55.000000 16500.000000 0.002205 0.400000 0.635845

56.000000 16800.000000 0.002244 0.300000 0.645378

57.000000 17100.000000 0.002284 0.400000 0.654863

58.000000 17400.000000 0.002324 0.300000 0.664966

59.000000 17700.000000 0.002364 0.300000 0.672957

60.000000 18000.000000 0.002404 0.300000 0.682114

61.000000 18300.000000 0.002444 0.300000 0.691736

62.000000 18600.000000 0.002484 0.300000 0.699378

63.000000 18900.000000 0.002524 0.300000 0.709309

64.000000 19200.000000 0.002564 0.300000 0.718126

65.000000 19500.000000 0.002604 0.300000 0.727096

66.000000 19800.000000 0.002643 0.300000 0.736190

67.000000 20100.000000 0.002683 0.300000 0.744213

68.000000 20400.000000 0.002723 0.400000 0.753169

69.000000 20700.000000 0.002763 0.300000 0.760588

70.000000 21000.000000 0.002803 0.300000 0.769660

71.000000 21300.000000 0.002843 0.300000 0.778067

Test No Time Current Light Intensity Voltage

72.000000 21600.000000 0.002883 0.300000 0.786468

73.000000 21900.000000 0.002923 0.300000 0.795394

74.000000 22200.000000 0.002963 0.300000 0.804422

75.000000 22500.000000 0.003003 0.300000 0.812535

76.000000 22800.000000 0.003042 0.300000 0.820232

77.000000 23100.000000 0.003082 0.300000 0.828543

78.000000 23400.000000 0.003122 0.300000 0.836039

79.000000 23700.000000 0.003162 0.400000 0.843351

80.000000 24000.000000 0.003202 0.400000 0.851528

81.000000 24300.000000 0.003242 0.400000 0.860238

82.000000 24600.000000 0.003282 0.300000 0.868126

83.000000 24900.000000 0.003322 0.300000 0.876651

84.000000 25200.000000 0.003362 0.300000 0.884186

85.000000 25500.000000 0.003402 0.300000 0.892358

86.000000 25800.000000 0.003441 0.300000 0.900622

87.000000 26100.000000 0.003481 0.400000 0.909302

88.000000 26400.000000 0.003521 0.300000 0.917622

89.000000 26700.000000 0.003561 0.400000 0.926494

90.000000 27000.000000 0.003601 0.300000 0.933883

91.000000 27300.000000 0.003641 0.400000 0.941391

92.000000 27600.000000 0.003681 0.400000 0.948165

93.000000 27900.000000 0.003721 0.400000 0.955981

Test No Time Current Light Intensity Voltage

94.000000 28200.000000 0.003761 0.400000 0.963728

95.000000 28500.000000 0.003801 0.300000 0.971390

96.000000 28800.000000 0.003840 0.400000 0.978813

97.000000 29100.000000 0.003880 0.400000 0.987773

98.000000 29400.000000 0.003920 0.400000 0.995842

99.000000 29700.000000 0.003960 0.300000 1.004300

100.000000 30000.000000 0.004000 0.300000 1.013250

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