Subject Name: Electronic Circuits Subject Code: 10CS32 Prepared By: Manju Khanna,Nimi P U

1. Subject Name: Electronic Circuits Subject Code: 10CS32 Prepared By: Manju Khanna,Nimi P U Department: CSE Date:30/8/2014

2. Agenda • Introduction • Photo sensors • Photoconductors • Photodiodes • Phototransistors • LED • LCD • CRT Displays • Emerging Display Technologies • Optocouplers

3. Introduction • Study of optics and electronics. • Optoelectronics is the study of electronic devices that emit, detect and control light ranging from ultraviolet to infrared. • Optoelectronic devices are used in various fields of instrumentation, measurement and diagnostics.

4. Optoelectronic devices Photosensors Optocouplers Photoemitters Displays photodiodes phototransistors Photoconductors LED CRT LCD Electrical to optical transducers Optical to Electrical transducers

5. Optoelectronic devices emit, sense and control light in the infrared ,visible and ultraviolet bands. • Wavelength spectrum of 1nm to 1mm. • Two approaches used to define units and quantities: • Radiometry • Photometry • Radiometry – is the set of techniques for measuring electromagnetic radiation • Photometry- is the science that deals with visible light and its perception to human eye.

6. Radiometric and Photometric Flux Flux it is a flow phenomenon occurring in space. Radiometeric flux ΦR Photometeric Flux ΦP K= ΦP / ΦR Where K is the Efficacy of a radiation source. K – measeured in lm/W(lumen per Watt) ΦR measured in Watt(W) ΦP measured in lumen(lm) I R - Radiometeric intensity(watts per steradian-W/sr) I R = ΦR /Ω (W/sr) Ω solid angle(sr) IP - Photometeric or luminos intensity( Candela(Cd)) IP = ΦP/ Ω (Cd)

7. E R - Radiant Incidence – Flux distribution on a surface. E R = ΦR / A A – area of flux distribution (m2 ) E P - Illuminance- photometeric flux distribution on a surface E P = ΦP / A (lux) 1 foot-candle = 10.764 lux Radiant Sterance – ratio of radiometric flux per unit solid angle per unit area (W/sr/m2 ) Luminance – ratio of photometric flux per unit solid angle per unit area (lm/sr/m2 )

8. Photosensors - electronic devices that detect the presence of light in the spectral band ranging from ultraviolet to infrared band.

9. Classification of Photo sensors Photoelectric sensors Thermal sensors Thermocouples Internal photo effect Thermopiles External photo effect

10. Characteristic Parameters • 1.Responsivity(R) : Ratio of electrical output to radiant light input. • measured in (A/W) • It is a function of wavelength. • 2.Noise Equivalent Power(NEP) : • NEP is radiant power applied to sensor that produces output signal equal to RMS noise output. • NEP= noise current • responsivity • Used for detection of weak signal

11. 3.Detectivity and Dee-star: • reciprocal of NEP • Higher detectivity value is more sensitive. • Depends on noise BW and sensor area. • To eliminate this normalised fig is used as DEE STAR • D*=D√A∆f A:area cm2; ∆f=bandwidth(Hz) 4. Quantum efficiency: Ratio of number of photoelectrons released to the no. of photons of incident light. ἠ=1240*R/ᴫ 5.Response time: Expressed as rise time Highest signal freq it can respond to is BW=0.35/tr

12. 6. Sensor noise Most critical factor in designing because of noise. • Johnson noise or Nyquist or thermal noise: Caused by thermal motion of charged particles in a resistive elements. VRMS=√4KRT∆f K-Boltzman constant b)Short noise: caused by discrete nature of photoelectrons generated. IRMS=√2eIav ∆f c)Generation –Recombination noise: Caused due to fluctuation in current generation and recombination rates. IGRMS= 2eG√ἠEA∆f G-photoconductive gain;n-quantam efficiency E-radiant incidence;A-sensor receiving area

13. d)Flicker noise or 1/f noise: Where conductor is not metal and in semiconductors that require bias. 7)Spectral response: Describe the wavelength rangeover which a sensor responds. Most photoelectric material have narrow spectral response while thermal sensors have wide spectral response.

14. Photoconductors Referred as photo resistor or light dependent resistor and photocells. When light is incident on it, electrons jump from valence band to conduction band. Resistance decreases with increase in incident light intensity. It is divided into Intrinsic extrinsic

15. Photoconductors

16. Construction of photoconductive device

17. The four materials normally employed in photoconductive devices are: • Cadmium Sulphide (CdS) • Cadmium Selenide (CdSe) • lead sulphide (PbS) • Thallium Sulphide (TlSIn a typical construction of photoconductive device, thin film is deposited on an insulating substrate.

19. . • Desired characteristics of photoconductive materials They are • i) High spectral sensitivity in the wavelength region of interest • ii) Higher quantum efficiency • iii) Higher photoconductive gain • iv) Higher speed of response and • lesser noise

20. . APPLICATIONS OF PHOTOCONDUCTIVITY DEVICES • Light meters • Infrared detectors • TV cameras • Voltage regulator • Relays and • Detecting ships and air crafts

21. . • Photodiodes • Semiconductor light sensors that generate current or voltage when PN junction is illuminated by light . • The cut off wavelength is given by: • λc = 1240 / E g • E g - the band gap energy(eV) • Photodiodes are mostly constructed using Si,Ge,LeadSulphide. • Photodiode Types • PN Photodiode • PIN Photodiode • Schottky Photodiode • Avalanche Photodiode

22. . • Photodiodes • PN Photodiodes • Light incident generates electrons in the conduction band P-type material. • Holes in the valence band of N-type material. • Photodiode Reverse biased • Photoinduced electrons move down the potential hill from P to N side • Photoinduced holes add to current flow from P to N side.

23. . • Cross section of PN Photodiode

24. . • PIN Photodiode • High Resistance intrinsic layer added between P and N layer. • Reduces diffusion time of electron hole pairs • Improves response time. • Suitable for high speed photometery.

25. . • Schottky Photodiode • Thin gold coating is sputtered onto the N material . • Schottky photodiode have enhanced ultraviolet response. • Avalanche Photodiode • High speed, high sensitivity. • Constructed to provide a uniform junction that exhibits avalanche effect at reverse bias. • Electron-hole pairs generated by incident photons • Excellent SNR(Signal to Noise ) ratio.

26. .

27. . V - I Characteristics of Photodiode

28. . • Photodiode • Modes of Operation • Photovoltaic mode • No bias voltage is applied • A forward voltage is produced across the photodiode • Applications which require bandwidth less than 10kHz • Value of dark current is zero • Photoconductive mode • A reverse bias voltage is applied across the photodiode • Higher speed of response • For all applications it is operated in this mode. • Linearity is improved

29. . • Solar Cells • The operating principle of solar cells is based on photovoltaic effect • Solar cell when exposed to sunlight open circuit voltage is generated .

30. . • Solar Cells • Open circuit voltage leads to flow of current through a load resistor connected across. • Incident energy leads to generation of electron hole pair . • Electron-hole pairs recombine or drift. • Accumulation of positive and negative charge carriers constitute open circuit voltage.

31. Phototransistors • Phototransistors usually connected in the CE configuration. • Radiation concentrated in the CB region. Base Emitter P N Depletion Region(Active Region) N + Symbol Collector Crossection of phototransistor

32. When no radiation is incident the collector current Ic is given by I c = (β +1) ICO ICO : Reverse saturation current also called Dark current Light is incident then Ic is given by I c = (β +1) ICO + I λ I λ – is the current generated due to incident light photons.

33. Phototransistors V – I Characteristics

34. Phototransistor Application Circuits • Generally used in 2 configurations : CE and CC • CE : Output goes from high to low • CC: Output goes from low to high. • When light is incident on it. • Both configurations act in 2 modes: Active and switched • Active Mode: Transistor operates in active region ,output voltage is propositional to input light intensity. • Switched Mode: Transistor is switched between cut-off and saturation • Modes are controlled by value of resistor R.

35. Subject Name: Electronic Circuits Subject Code: 10CS32 Prepared By: Nimi P U Department: ISE UNIT – 3 (Second Half)

36. Light Emitting Diodes • LED is a semiconductor PN junction diode designed to emit light when forward-biased. • It is one of the most popular optoelectronic source. • LEDs consume very little power and are inexpensive. PN junction of an LED

37. Working • In the absence of an externally applied voltage, the N-type material contains electrons while the P-type material contains holes that can act as current carriers. When the diode is forward-biased, the energy levels shift and there is significant increase in the concentration of electrons in the conduction band on the N-side and that of holes in valance band on the P-side. The electrons and holes combine near the junction to release energy in the form of photons. The process of light emission in LED is spontaneous, i.e., the photons emitted are not in phase and travel in different directions. • Gallium Phosphide (GaP), Gallium Arsenide (GaAs) and Gallium arsenide Phosphide (GaAsP) are used in the construction of LEDs.

38. The energy of the photon resulting from this recombination is equal to the bandgap energy of the semiconductor material and is expressed by: • λ= 1240/ΔΕ • Where λ is the wavelength (nm) and ΔE is the bandgap energy (eV).

39. LED Characteristic Curves As the LED is operated in the forward-biased mode, the VI characteristics in the forward-biased region are shown. VI characteristics of LEDs are similar to that of conventional PN junction diodes except that the cut-in voltage in the case of LEDs is in the range of 1.3-3V as compared to 0.7V for silicon diodes and 0.3V for germanium diodes.

40. V-I Characteristics: Figure shows the VI characteristics of different colors. As the LED is operated in forward biased mode, the VI characteristics in the forward biased region are shown.VI characteristics of LEDs are similar to that of conventional PN junction diode except that the cut in voltage in the case of LED is in range of 1.3-3V as compared to 0.7 for silicon diodes and 0.3 V for germanium. Spectral distribution Curve: Spectral distribution curve shows the variation of light intensity with wavelength. Figure shows typical spectral curves for yellow, green and red LED.

41. Light output versus input characters: Fig shows a typical light output versus input current curve depicting the dependence of emitted light on forward current flowing through the LED. Directional characteristics: Directional characteristics refer to the variation in the light output with change in the viewing angle.

42. LED Parameters • Forward Voltage (VF): It is the DC voltage across the LED when it is ON. • 2. Candle Power (CP): It is a measure of the luminous intensity or the brightness of the light emitted by the LED. It is a non-linear function of LED current and the value of CP increases with increase in the current flowing through the LED. • Radiant Power Output (Po): It is the light power of the LED.

43. Peak Spectral Emission (λP): It is the wavelength where the intensity of light emitted by the LED is maximum. • Spectral Bandwidth: It is the measure of concentration of color brightness around the LEDs nominal wavelength.

44. LEDs are operated in the forward-biased mode. As the current through the LED changes very rapidly with change in forward voltage above the threshold voltage, LEDs are current-driven devices. The resistor (R) is used limit the current flowing through the device. A silicon diode can be placed inversely parallel to the LED for reverse polarity voltage protection. • The current that will flow through the LED is given by • The value of the resistor (R) to be connected is given by

45. LCD – Liquid Crystal Displays • Liquid Crystals are materials that exhibit properties of both solids and liquids, that is, they are an intermediate phase of matter. • LCD can be classified into three different groups: nematic, smectic and cholestric. • Nematic liquid crystals are generally used in the fabrication of liquid crystal displays (LCDs) with the twisted nematic material being the most common.

46. Construction of an LCD • An LCD display consists of liquid-crystal fluid, conductive electrodes, a set of polarizers and a glass casing. • The outermost layers are the polarizers which are housed on the outer surface of the glass casing. The polarizer attached to the front glass is referred to as the front polarizer, while the one attached to the attached to the rear glass is the rear polarizer • On the inner surface of the glass casing, transparent electrodes are placed in the shape of desired image.

47. The electrode attached to the front glass is referred to as the segment electrode while the one attached to the rear glass is the backplane or the common electrode. The liquid crystal is sandwiched between the two electrodes.

48. Operation of LCD Display The basic principle of operation of LCD is to control the transmission of light by changing the polarization of the light passing through the liquid crystal with the help of an externally applied voltage. As LCDs do not emit their own light, backlighting is used to enhance the legibility of the display in dark conditions.

49. LCDs have the capability to produce both positive as well as negative images. • A positive image is defined as a dark image on a light background. • In a positive image display, the front and rear polarizers are perpendicular to each other. • Light entering the display is guided by the orientation of the liquid crystal molecules that are twisted by 90o from the front glass plate to the rear glass plate. • This twist allows the incoming light to pass through the second polarizer.

50. When a light is applied to the display, the liquid crystal molecules straighten out and stop redirecting the light. As a result light travels straight through and is filtered out by the second polarizer. Therefore, no light can pass through, making this region darker compared to the rest of the screen. Hence, in order to display characters or graphics, voltage is applied to the desired regions, making them dark and visible to the eye. • A negative image is a light image on a dark background. In negative image displays, the front and the rear polarizers are aligned parallel to each other.