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Photoelectric transducers and its classification

The document discusses photoelectric transducers, which convert light energy into electrical energy, highlighting their components, working principles, and types, such as phototubes, photomultipliers, and photodiodes. It explains the basic operation of these devices, focusing on how light interaction with semiconductor materials leads to the generation of current. Furthermore, it covers the applications of these devices in various fields, including optical communication, detection systems, and solar energy conversion.

KAROLINEKERSIN. E ASSISTANT PROFESSOR PHOTOELECTRIC TRANSDUCERS
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.
• 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.
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.
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.
CLASSIFICATION OF PHOTOELECTRIC TRANSDUCERS Photo emissive cells or photo tube Photo diode Photoconductive cell Photovoltaic cell Photo transistor
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.
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.
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.
• 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.
APPLICATIONS Reading of optical sound tracks for projected films Detector in spectrophotometer
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.
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.
• 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.
• 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.
• 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.
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.
• 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
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.
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.
WORKING OF PHOTODIODE 21
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
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.
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.
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
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
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.
ADVANTAGES OF PHOTODIODE It shows a quick response when exposed to light. High operational speed. It provides a linear response. Low-cost device
DISADVANTAGES OF PHOTODIODE It is a temperature-dependent device. And shows poor temperature stability. When low illumination is provided, then amplification is necessary.
APPLICATION OF PHOTODIODE Counters and switching circuits. optical communication system. Logic circuits and encoders. Burglar alarm systems.
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
• 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.
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.
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
• 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.
• 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.
• 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
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.
• 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.
• 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.
• 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. •
• 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% • .
• 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.
DIFFERENT TYPES OF PHOTOVOLTAIC CELLS Crystalline Silicon Cells Thin-Film Cells Organic Cells Perovskites
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.
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.
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
• 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
• 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.
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
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.
DRAWBACKS • Temperature variations cause substantial variations in resistance for a particular light intensity. • Unsuitable for analog applications.
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
• 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.
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.
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..
• 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
• 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.
• 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
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.
• 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.
• 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
• 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.
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.
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.
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
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.
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
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.
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.
• 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.
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In this document
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Introduction to the speaker, Karoline Kersin, focusing on photoelectric transducers.

Discusses how photoelectric transducers convert light energy into electrical energy, using semiconductor materials.

Explains the absorption of light by semiconductor material and how it leads to changes in resistance and current.

Lists the classification of photoelectric transducers including photoemissive cells, photodiodes, etc.

Details about phototubes, their application, construction, and functioning based on the photoelectric effect.

Mentions specific applications of phototubes, such as in optical sound track reading and spectrophotometers.

Introduces photomultiplier tubes (PMT), their construction, working principle, and key factors affecting performance.

Lists applications of PMT including their role in research laboratories and spectrophotometers.

Introduces photodiodes, their operation modes, and their ability to convert light energy into electrical energy.

Explains the working of photodiodes, current flow, effects of light exposure, and the significance of dark current.

Lists advantages such as quick responses and disadvantages like temperature dependence in photodiodes.

Describes various applications for photodiodes including their use in optical communication and security systems.

Introduces photovoltaic cells (solar cells), their function, construction, and the photovoltaic effect.

Explains how photovoltaic cells generate electricity through photon interaction and factors limiting efficiency.

Discusses methods to improve the efficiency of photovoltaic cells and mentions different types of PV cells.

Describes photoconductive cells, their principle of operation, and construction details.

Examines the characteristics of photoconductive cells including their resistance changes due to light.

Details on how photoconductive cells are used in relay control applications.

Introduction to phototransistors, their operation via light exposure, and key characteristics and advantages.

Details the operation of phototransistors, how they respond to light and their current amplification properties.

Lists the various applications of phototransistors in environments like security systems and logic circuitry.

Describes how photoelectric transducers are used in biomedical applications, including blood volume and pressure measurement.

Photoelectric transducers and its classification

  • 1.
  • 2.
    PHOTOELECTRIC TRANSDUCER • Thephotoelectric 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 dischargesof 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 PRINCIPLEOF 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 MOBILITYOF 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 PHOTOELECTRICTRANSDUCERS Photo emissive cells or photo tube Photo diode Photoconductive cell Photovoltaic cell Photo transistor
  • 7.
    PHOTO TUBES • Aphototube 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 operateaccording 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 causeselectrons 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 opticalsound 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 spectralresponse, 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 emittedby 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 surfacecomposition 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 areused 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 specialtype 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 OFPHOTODIODE 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.
  • 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, theelectrons 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, thejunction 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 movementthen 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 intensityof 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, thevertical 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 Itshows a quick response when exposed to light. High operational speed. It provides a linear response. Low-cost device
  • 29.
    DISADVANTAGES OF PHOTODIODE Itis a temperature-dependent device. And shows poor temperature stability. When low illumination is provided, then amplification is necessary.
  • 30.
    APPLICATION OF PHOTODIODE Countersand switching circuits. optical communication system. Logic circuits and encoders. Burglar alarm systems.
  • 31.
    PHOTOVOLTAIC CELLS • Aphotovoltaic (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 photovoltaiceffect 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 PHOTOVOLTAICCELLS • 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 strayelectrons 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 eitherside 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 allsemiconductors 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 PHOTOVOLTAICCELLS • 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 iscomposed 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 isa 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 theenergy 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 theelectrons 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 areways 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 OFPHOTOVOLTAIC CELLS Crystalline Silicon Cells Thin-Film Cells Organic Cells Perovskites
  • 45.
    PHOTOCONDUCTIVE CELLS • Thephotoconductive 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 strikingthe 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 isa 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 PHOTOCONDUCTIVECELL • 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 PHOTOCONDUCTIVECELL • 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 variationscause substantial variations in resistance for a particular light intensity. • Unsuitable for analog applications.
  • 53.
    PHOTO TRANSISTOR • Phototransistoris 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 iscapable 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 andnear-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 phototransistorsused 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 nolight 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-basejunction 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 • Aphototransistor 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 thesimple 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 devicesrespond 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 availabledifferent 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 • Tomeasure 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 measurechanges 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 • Bloodpressure 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 • Todetermine 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 firstdetector 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.