Light Emitting Diodes
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Light Emitting Diodes Light emitting diodes in use today are cheap, efficient and bright sources of light and are used in displays, communication and, increasingly, mass market consumer products such as automobile tail lights. The development of LEDs, however, was far from straightforward. Indeed, toV day's accomplishments could be considered as fortuitous lights at the end of a very long, dark and tortuous tunnel. Electroluminescence — the production of light due to the passage of electrical current through a material and the basis for the light emitting diodes — was first seen 80 years ago by radio researcher H.J. Round. He noticed that a current through a silicon carbide "cat's whisker" crystal at times produced a yellowish light. While the phenomenon was not central to his work, Round was intrigued enough to write a "Note on Carborundum" to the journal Electrical World: "During an investigation of the unsymmetrical passage of current through a contact of carborundum and other substances a curious phenomenon was noted. On applying a potential of 10 volts between two points on a crystal of carborundum, the crystal gave out a yellowish light." Round noted that only a few whiskers did this at such low voltages, but many exhibited the behavior when the voltage was raised over 110 volts. "The writer would be glad of references to any published account of an investigation of this or any allied phenomena." Neither Round — nor anyone else apparently— pursued this effect further for at least 15 years. Indeed, Round's observations were forgotten until 1968 when this note was republished in IEEE Spectrum by Henry F. Ivey. Round's lack of followup can be explained by his and colleagues' preoccupation with developing radio receivers. yThe next chapter in LED history is also not well known. It deals with the accomplishments of Oleg V. Losev, a brilliant Russian experimentalist w h o led an inspired but tragic life. Losev packed many modern electronic
discoveries into his 20 years of research. The story of Losev's role in LED development is included in a detailed history of the development of the device written by Egon E. Loebner in the July 1976 (Bicentennial) issue of IEEE Transactions on Electron Devices. At the age of 19, Losev rediscovered in 1922 what Round had noticed — electroluminescence in silicon carbide. He later discovered zinc oxide radio frequency diode oscillators and amplifying detectors, and developed battery-powered "crystadyne" solid-state radio receivers that foreshadowed modern transistor radios. Though his radio receivers were advanced for their day, tubes became the dominant amplification device technology. That turn of events, as well as trouble with an American vendor of the zinc oxide crystals he used, led Losev to change the thrust of his research. In 1927, Losev concentrated his efforts on the study of light emitting and light sensing diodes — a decision that would continue to keep him decades ahead of his contemporaries. Losev correctly deduced that LED emission represents an inverse case of Eins
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