350 Unit 9 Optoelectronic and Miscellaneous Devices PDF

Unit 9 Optoelectronic and Miscellaneous Devices – 1 Optoelectronics Light (photo)-emitters and Light-detectors Photo-emitters: LED, OLED, Laser diode Diodes can be made to emit light or sense light. - Light-Emitting Diode (LED) – these are light emitters. - Photodiodes – these are light detectors...

Unit 9 Optoelectronic and Miscellaneous Devices – 1 Optoelectronics Light (photo)-emitters and Light-detectors Photo-emitters: LED, OLED, Laser diode Diodes can be made to emit light or sense light. - Light-Emitting Diode (LED) – these are light emitters. - Photodiodes – these are light detectors. Light Emitting Diodes (LEDs): Introduced as a practical electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet and infrared wavelengths, with very high brightness. When a light-emitting diode is forward biased (switched on), the electrons cross the pn junction from the n-type material and recombine with holes in the p-type material releasing energy in the form of photons. These free electrons are in the conduction band and at a higher energy than the holes in the valence band. The difference in energy between the electrons and the holes corresponds to the energy of visible light. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. The emitted light tends to be monochromatic (one color) that depends on the band gap (and other factors). An LED is often small in area (less than 1 mm), and integrated optical components may be used to shape its radiation pattern. 350 Unit 9 F2013 IC 1 Advantages over incandescent light sources: Lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. Various impurities are added during the doping process to establish the wavelength of the emitted light. LEDs vary widely in size and brightness – from small indicating lights and displays to high-intensity LEDs that are used in traffic signals, outdoor signs, and general illumination. Typical LEDs The lens directs the light emitted from the LED to optimize visibility: Symbol for an LED: Basic operation of an LED: The graph shows that power of light output is directly proportional to the forward current IF. Reverse breakdown is lower than silicon rectifier diodes (3V to 10V). 350 Unit 9 F2013 IC 2 LEDs emit a specific range of wavelengths which depend on the construction and dye material used. The wavelength is given on the specification sheet. LEDs are available for visible light and infrared. Examples of typical spectral output curves for LEDs Example: the peak wavelength of a green LED is 540 nm V-I characteristic curves for visible-light LEDs Another characteristic shown in specification sheets is the radiation pattern for the LED. This plot is an example of a typical pattern in which light is concentrated in one direction. Radiation pattern is controlled using lens. The narrower the radiation pattern, the more the light is concentrated. General radiation pattern of a typical LED A wider viewing angle will show a wider pattern such as the TLDR5400: 350 Unit 9 F2013 IC 3 The forward voltage drop can vary from about 1.5 V to over 3 V depending on the type of diode, its color, and the amount of forward current. We need to take into account the specified maximum current allowed and the diode’s forward drop when choosing a limiting resistor. Example: A certain bright red LED drops 2.2 V at a maximum current of 20 mA. What series resistor is required to limit the current to 20 mA from a 5.0 V source? Other specifications, such as maximum power dissipation are given on the manufacturer’s specification sheet. To determine the power dissipated by the LED, multiply the forward voltage by the forward current. Example: A certain bright red LED drops 2.2 V at 20 mA. What power is dissipated by the LED? P = IV = (20 mA)(2.2 V) = 44 mW Applications of LEDs A transistor used to switch an LED on and off The 7-segment LED display LED’s are widely used in various applications like indicator lights, read out displays etc. - A very common application of LED’s is the seven-segment display. 350 Unit 9 F2013 IC 4 - Each segment is an LED. - By forward biasing selected combination of segments, any decimal digits can be formed. Basic concept and block diagram of a counting and control system Infrared LED’s are commonly used in remote controls. - Infrared LED emits beam of invisible light. - Each button corresponds to an electrical code which is converted to a light code and transmitted through the LED. - The receiver recognizes the code and takes the required action. 350 Unit 9 F2013 IC 5