Photosensitive diodes

Chapter Photosensitive diodes

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • LEDs and IREDsDepending on the exact mixture of semiconductors used in manufacture, visible light ofalmost any color can be produced. Infrared-emitting devices also exist. The most com-mon color for a light-emitting diode (LED) is bright red. An infrared-emitting diode(IRED) produces wavelengths too long to see.The intensity of the light or infrared from an LED or IRED depends to some extenton the forward current. As the current rises, the brightness increases up to a certainpoint. If the current continues to rise, no further increase in brilliance takes place. TheLED or IRED is then said to be in a state of saturation.Digital displaysBecause LEDs can be made in various different shapes and sizes, they are ideal for usein digital displays. You’ve probably seen digital clock radios that use them. They arecommon in car radios. They make good indicators for “on/off,” “a. m. /p. m.,” “batterylow,” and other conditions.In recent years, LED displays have been largely replaced by liquid-crystal dis-plays (LCDs). This technology has advantages over LEDs, including much lower powerconsumption and better visibility in direct sunlight.CommunicationsBoth LEDs and IREDs are useful in communications because their intensity can bemodulated to carry information. When the current through the device is sufficient toproduce output, but not enough to cause saturation, the LED or IRED output willfollow along with rapid current changes. Voices, music, and digital signals can beconveyed over light beams in this way. Some modern telephone systems make use ofmodulated light, transmitted through clear fibers. This is known as fiberoptic tech-nology.Special LEDs and IREDs produce coherent radiation; these are called laserdiodes. The rays from these diodes aren’t the intense, parallel beams that you probablyimagine when you think about lasers. A laser LED or IRED generates a cone-shapedbeam of low intensity. But it can be focused, and the resulting rays have some of thesame advantages found in larger lasers.Photosensitive diodesVirtually all P-N junctions exhibit characteristics that change when electromagneticrays strike them. The reason that conventional diodes are not affected by these rays isthat most diodes are enclosed in opaque packages.Some photosensitive diodes have variable resistance that depends on light inten-sity. Others actually generate dc voltages in the presence of electromagnetic radiation.Silicon photodiodesA silicon diode, housed in a transparent case and constructed in such a way that vis-ible light can strike the barrier between the P-type and N-type materials, forms aphotodiode.378 Some uses of diodes