Chapter 26 Optoelectric Devices

2 Objectives –After completing this chapter, the student should be able to: Identify the three categories of semiconductor devices that react to light. Classify the major frequency ranges of light. Identify major light-sensitive devices and describe their operation and applications.

3 Identify major light-emitting devices and describe their operation and applications. Draw and label the schematic symbols associated with optoelectric devices. Identify packages used for optoelectric devices.

4 Basic principles of light –Light is electromagnetic radiation that is visible to the human eye. –Thought to travel in a form similar to radio waves. –Measured in wavelengths. –Travels at 186,000 miles per second or 30,000,000,000 centimeters per second through a vacuum.

5 –The frequency range of light is 300 to 300,000,000 gigahertz. Visible range is from 400,000 to 750,000 gigahertz. Infrared light range is below 400,000 gigahertz Ultraviolet light range is above 750,000 gigahertz –Light waves at the upper end have more energy that those at the lower end.

6 Photoconductive cell (photo cell) –The oldest optoelectric device. –The internal resistance changes with a change in light intensity.

7 –Made from light-sensitive material Cadmium sulfide (CdS). Cadmium selenide (CdSe). –More sensitive to light than any other device. Useful for low-light applications. –It can stand high voltages of 200 to 300 volts with a low power consumption of 300 milliwatts.

8 –Slow response to light change is a disadvantage. –Used in light meters for: Photographic equipment. Instruction detectors. Automatic door openers. Test equipment used to measure light intensity.

9 Photovoltaic cell (solar cell) –Converts light energy directly into electrical energy. –A PN junction made from semiconductor materials. Most commonly made from silicon. P and N layers form the PN junction. The metal support and contact act as the contacts. Designed with a large surface area.

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11 –Highly inefficient device, with a top efficiency of 15 to 20%. –Has a low voltage output. –Applications include: Light meters for photographic equipment. Motion picture projector soundtrack decoders. Battery chargers for satellites.

12 Photodiode –Uses a PN junction. –Construction similar to the solar cell. –Used as a light variable resistor. –Made primarily from silicon. –Constructed in two ways. –Used to control current flow. –Responds to light changes quickly.

13 –Low output compared to other photosensitive devices.

14 Phototransistor –Constructed with two PN junctions. –Resembles a standard NPN transistor. –Used and packaged like a photodiode, except: Has three leads: –Emitter –Base –Collector

15 –Produces higher output current than photodiodes. –Response to light changes is not as fast as the photodiode.

16 Light-emitting diode (LED) –The most common semiconductor light- emitting device. –Unlimited life span due to the absence of a filament.

17 –Packaged for optimum emission of light. –Most LEDs contain a lens that gathers and intensifies the light. –In a circuit, it is connected with a forward bias to emit light.

18 Optical coupler –Consists of a LED and a phototransistor in the same package. –Allows one circuit to pass a signal to another circuit while providing a high degree of electrical insulation.

19 In Summary –Semiconductor devices that interact with light can be classified as: Light-detection devices. Light-conversion devices. Light-emitting devices. –Light is electromagnetic radiation that is visible to the human eye.

20 –The frequency range of light is: Infrared light--less than 400,000 gigahertz. Visible light--400,000 to 750,000 gigahertz. Ultraviolet light--greater than 750,000 gigahertz. –Light-sensitive devices: Photo cells Solar cells Photodiodes Phototransistors

21 –Light-emitting devices include the LED. –An optical coupler combines a light-sensitive device with a light-emitting device.