Optoelectronics Devices PDF

Unit 05: Optoelectronics Devices By Mayur M.Sevak Electronics and Communication Engg.Dept B.V.M Engineering College (An Autonomous Institution) Introduction Luminescence: Light can be emitted from solid when it is stimulated by source of incident energy. Examples are Photo-...

Unit 05: Optoelectronics Devices By Mayur M.Sevak Electronics and Communication Engg.Dept B.V.M Engineering College (An Autonomous Institution) Introduction Luminescence: Light can be emitted from solid when it is stimulated by source of incident energy. Examples are Photo- luminescence:,electro- luminescence, cathode-luminescence. The devices that resulted from emission of electrons from metallic surface with vacuum and gasphoto tubes were commonly called as photoelectric devices. Modern solid state devices which include emitters,sensors and couplers are called opto electronic devices. Two categories of Optoelectronics devices 1) Photoconductive devices 2) Photovoltaic devices 2 Contd. 3 Photoconductive Devices Photoconductive Effect: When radiation is incident on semiconductor,some absorption of light by the material take place and conductivity increases. In semiconductor material,the forbidden gap Eg is expressed as If frequency f is very low so that E=Eg then,electron in valence band absorb the incident photons and shift to conduction band. Hence Conductivity of Semiconductor is given by n=magnitude of free electron concentration p=magnitude of hole concentration σ =conductivity μn =mobility of electrons μp=mobility of holes  Contd. Electron hole pairs generated by incident light plus those created by thermally increases conductivity of material result in increase in current into external circuit. Hence material is called photoconductor or photoresistor Therefore, photoconduction will only happen if photon possess sufficient energy that is equal to Energy Gap Eg or more. Minimum Freq fc for photoconduction is given by Cut off or critical wavelength is given by For Si, Eg=1.1 eV and λc=1.13 μm For Ge,Eg=0.72 eV and λc=1.73 μm Bulk Type Photoresistor or Photoconductive Cell It is two terminal device which is used as LDR It is made up of thin layer of semiconducting material such as Cadmium sulphide(CdS),Lead sulphide(PbS),cadmium selenide(CdSe). CdS is most popular because high dissipation capacity,excellent sensitivity,low resistance when stimulated by light. But Cds has slower speed of response and PbS is faster. Look at the figure, in absolute darkness,the resistance is as high as 2 MΩ and in strong light it is less than 10Ω Contd. Look at Simple circuit in which semiconductor layer is enclosed in sealed housing. As light increases,the resistance of Photoconductive cell decreases and hence current I increases and vice-versa. Applications: 1) On/Off control in automatic street lighting 2) It can be used as trigger level to control relays. 3) It is used in counting system as many time light is interrupted counter is increased. 4) Used in twilight circuit where resistance falls below threshold value. 5) Cameras to control shutter opening when flash is on  Junction Type Photoconductive Cell Photodiode Silicon diodes converts the light signals into electrical signals. The diode is made from semiconductor PN junction which is kept in sealed plastic or glass casing. Design of cover is such that light will be concentrated on one surface of junction and other sides are painted.lens permits the light to fall on junction When light falls on reverse bias junction then,electron-hole pairs are created and magnitude of current is depending on number of charge carriers generated and hence illumination of diode element. Current is also affected frequency of light falling on the junction. Contd. From characteristics, reverse current increases in direct proposition to level of illumination. Even when no light is applied,there is minimum reverse leakage current flows that is called dark current. Applications: They are used as light detectors,demodulators,encoders. They are also used in optical communication system,high speed counting and switching circuits. They are also used in computer card punching and tapes,light operated switches,soundtrack films,electronic control circuits. Phototransistor It is more sensitive than photodiode The current produced by photo diode is very low which can’t be directly used in control applications. Hence it combines Photodiode and Transistor Amplifier Figure shows NPN Transistor in CE Configurations with base open The lens is at base collector junction and the base current is supplied when light is falling on that junction When there is no radiant excitation,the minority carriers are generated thermally and electron moves from base to collector and holes from collector to base that forms collector current Ico with IB=0 the collector current is given by When light is on then additional minority carriers are generated and total collector current is given by Contd. Look at the characteristics,the current in photo transistor depends only on intensity of light falling on it and it is less affected by voltage applied to external circuits Applications: All the applications of Photodiode are supported by Phototransistor  Photovoltaic Sensors Photovoltaic Effect: If the PN Junction is open circuited,the light energy is used to create a potential difference which is proportional to frequency and intensity of incident light. It produces voltage when illuminated by light which can be used to provide direct supply of electrical power. In this device,without any applied voltage,the junction generate voltage depending upon the illumination and the load. The Photovoltaic potential is voltage at which zero current is obtained under open circuit condition. The Photovoltaic emf is 0.5 V for Si and selenium and 0.1 V for Ge and short circuit current is 1mA. The magnitude of current under large reverse bias is given by The voltage Vmax corresponding to open circuit diode can be obtained by putting I=0 and hence As Is>>Io, Vmax increases as Is and hence illumination. Contd. Figure shows output voltage vs light intensity and op current vs light intensity Applications: They are used in low power devices such as light meters More power is produced using solar cells When operated in short circuit mode ,it is used for direct reading foot candle meter. SOLAR CELL When sunlight is incident on photovoltaic cell,it is converted into electrical energy.Such converter is solar cell or solar battery The cell consists of semiconductor crystal with PN junction by adding P-type and N-type impurities When sunlight incident on glass plate G, it reaches to the junction as a result incident light photon at junction collide with valence electron and make it transition to conduction band As a result electron hole-pairs are generated and minority carriers cross the junction from P and N side With this accumulation of majority charge carriers occurs that develops photovoltaic voltage across junction in open circuit.This voltage is logarithmic function of illumination. Single solar cell exhibits 0.6 V. Average cell produce 30 mW per square inch and operating load is 4-ohm Efficiency range is 10 to 40 % Silicon and Selenium is used for excellent temperature characteristics. Photoemissive Sensors Vacuum Photo tube Photo tube is radiant energy device that control electron emission when exposed to incident light. As from figure anode and cathode are placed in high vacuum glass envelope When sufficient voltage is applied between anode and cathode than collector current is directly proportional to amount of incident light. Look at the characteristics. Gas filled Phototube The current handled in a vacuum phototube may be increased if small amount of gas is introduced resulting in characteristics shown At voltage above saturation level of 10 V the electron may acquire sufficient energy in passing between cathode and anode to ionise some of the gas atoms which add more charges to current. The process must be limited to prevent secondary emission or bombardment of cathode by positive ions as bombardment will destroy the emitting surface.Safe limit to operate is 90V.  Photomultiplier As the emitted currents from photoelectric surfaces are very small,especially with low light levels.This currents can be directly amplified in this device. It consists of evacuated glass envelope containing a photocathode,anode and several additional electrodes called dynodes( Each at higher potential then previous) As from figure it consists of cathode,six dynodes and anode.When light falls on cathode,electrons are emitted and directed towards d1.Which again has secondary emission with coefficient δ which is higher than unity.Again same procedure follow for next dynodes. This process is repeated for n times where n are the dynodes. δ has the range between 5 to 10. If the electrons are deflected from their normal path between stages due to magnetic field and miss a dynode then gain falls.For this magnetic shields are placed around the photomultiplier tube Contd. The characteristics of photomultiplier tube depend upon voltage/dynode and potential between last dynode and collector. Applications: Space Exploration Laser Communications Scintillators and Radiation detectors of X-Rays,Gamma rays Energetic particle found in nuclear physics. LIGHT EMITTERS LED LED is PN Junction device which emits light when forward bias by phenomenon called electroluminescence. In semiconductor PN junctions ,some of the energy may be radiated as heat or some as light.In material such as Gallium Phosphide(GaP),Gallium Arsenide Phosphide(GaAsP), emitted energy is in the form of visible light. When LED is forward biased,the electrons and holes moves towards the junction and recombination take place.As result of recombination,electrons in conduction band of N-region fall into the holes lying in the valence band of P -region.Light is generated due to recombination and brightness of LED light is proportional to forward bias current. Figure shows the structure of LED.A metal(gold) film is applied at the bottom of substrate for reflecting light as much as possible. GaAs-(Infrared Radiation) GaP-(Red or Green) GaAsP-(Red or Yellow) 19 Contd. To protect the LED , 1KΩ or 1.5 KΩ must be connected in series with LED. They operate at voltage levels from 1.5 V to 3.3 V. Power requirements are 10 to 150 mW. Life time: 1,00,000+ Hours LED can be switched ON and OFF at speed of 1ns. Applications Burglar Alarm Systems Picture Phones Multimeters Calculators Digital Computers and Microprocessors. Electronic Panels,Digital Watches,Intercoms and Telephone Exchanges. Solid state video displays Optical Communications Systems. 20 Liquid Crystal Display(LCD) A Liquid Crystal Display (LCD) is a thin , flat panel display device used for electronically displaying information such as text ,images and moving picture. LCD is used in Computer monitors, Televisions , Instrument panels, Gaming devices etc. Polarization of lights is used here to display objects. 21 Why to select LCD? Smaller size —LCDs occupy approximately 60 percent less space than CRT displays an important feature when office space is limited. Lower power consumption—LCDs typically consume about half the power and emit much less heat than CRT displays. Lighter weight —LCDs weigh approximately 70 percent less than CRT displays of comparable size. No electromagnetic fields —LCDs do not emit electromagnetic fields and are not susceptible to them. Thus, they are suitable for use in areas where CRTs cannot be used. Longer life —LCDs have a longer useful life than CRTs. 22 Liquid Crystals Liquid crystals are liquid chemicals in a state that has properties between those conventional liquid and solid crystals. That is a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal like way. Liquid crystals molecules can be aligned precisely when subjected to electric fields, as like as in the way metal nematic shavings line up in the field of a magnet. When properly aligned, the liquid crystals allow light to pass through. Two liquid crystal materials which are important in display technology are nematic and smectic. smectic. 23 Contd. The most popular liquid crystal structure is the nematic liquid crystal(NLC). When they are in a nematic phase, liquid crystals are a bit like a liquid: their molecules can move around and shuffle past one another, but they all point in broadly the same direction. The liquid is normally transparent, but if it is subjected to a strong electric field, ions move through it and disrupt the well ordered crystal structure, causing the liquid to polarise and hence turn opaque. The removal of the applied field allows the crystals structure to reform and the material regains its transparency. 24 How it works? Liquid crystals can adopt a twisted up structure and when we apply electricity to them, they straighten out again. This is the key how LCD displays turn pixels on and off. The polarization property of light is used in LCD screen to switch its colored pixels on or off. At the back of the screen, there is a bright light that shines out towards the viewer. In front of this, there are the millions of pixels, each one made up of smaller areas called sub-pixels, that are colored Red, Green, or Blue 25  Each pixel has a polarizing glass filter behind it and another in front of it at 90 degrees. Normally the pixels looks dark. In between the two polarizing filters there is a tiny twisted, nematic liquid crystal that can be switched on or off electronically. When it is switched on, it rotates the light passing through it through 90 degrees, effectively not allowing light to flow through the two polarizing filters and making the pixel look dark. Each pixel is controlled by a separate transistor that can switch it on or off many times each second. 26 Types of LCD Direct Address Display When the display include limited variable components such as Watches Calculators Simple electronics is used to control the components 27 Passive Matrix Display Passive matrix display has Rows of electrodes on one piece of glass. Columns of electrodes on the opposing piece of glass. Complex electrical waveform control the voltage differential at the intersection of the electrodes. The intersection of the columns and rows are the pixels 28 Active Matrix Display Allow very high resolution Each sub-pixel is individually controlled by an isolated thin-film transistor (TFT). It allows the electrical signal for each sub-pixel to avoid influencing adjacent elements. The TFT is patterned into the glass layer 29 Twisted Nematic Display It is the most common LCD Display. The two alignments layer for the liquid crystal material are orthogonal. The light entering the polarize panel rotates by the twist in the liquid crystal and allowing it to pass through the second polarize 30 Optocoupler It is solid state component in which the light emitter,light path and light detector are enclosed in component. As it provides isolation and hence it is also called opto isolator.It allows the signal to transfer without coupling wires,capacitors or transformers. It can couple digital or analog signals. Figure shows infrared LED and Photodetector.LED is used to transduce voltage into light intensity and light is transduced back in voltage by photo sensors. There is encapsulation to permit the passage of light to pass and data can be transmitted in the range of Mhz 31 Contd. Significant advantage of opto isolator is that it provides high isolation resistance of the order of 10^11 -Ohm and isolation voltages of the order of 2500 V between input and output terminals. Applications are in different type of logic circuits and level positioning sensing circuits. The power dissipation of LED and phototransistor is almost equal and Iceo is in nano-amperes. The relative output current is almost constant when temperature varies from 25 to 75 C The switching time of an opto isolator decreases with increased current ,while for many devices it is exactly reverse. 32  Thank You Find more maps at slidescarnival.com/extra-free-resources-icons-and-maps 33  Free templates for all your presentation needs For PowerPoint and 100% free for personal Ready to use, Blow your audience Google Slides or commercial use professional and away with attractive customizable visuals