Chapter 26

Optoelectric Devices

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• 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.

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• 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.

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• 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.

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– 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.

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• Photoconductive cell (photo cell)– The oldest optoelectric device.– The internal resistance changes with a change

in light intensity.

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– 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.

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– 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.

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• 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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– 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.

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• 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.

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– Low output compared to other photosensitive devices.

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• Phototransistor– Constructed with two PN junctions.– Resembles a standard NPN transistor.– Used and packaged like a photodiode, except:

• Has three leads:– Emitter

– Base

– Collector

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– Produces higher output current than photodiodes.

– Response to light changes is not as fast as the photodiode.

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• Light-emitting diode (LED)– The most common semiconductor light-

emitting device.– Unlimited life span due to the absence of a

filament.

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– 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.

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• 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.

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• 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.

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– 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

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

device with a light-emitting device.