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The Solid-state Lamp

The LED is sometimes referred to as a solid-state lamp, especially in older scientific publications. Henry Round was the first to place a crystal of silicon carbide between two wires in 1907. He found that, at ten volts, it gave off a yellow light. A Russian experimenter-alternately referred to as Lossew or Lossyev-first contributed a basic understanding of the electroluminescent properties of silicon carbide, which he coupled with zinc oxide in point-contact diodes in 1923. Georges Destriau in 1936 first conceived of an AC solid-state light source, but efficiencies improved dramatically in lab research during the 1950s.

In 1990 at Cambridge University in England, Richard Friend made the first LED utilizing polymers (chemical compounds consisting of combinations of molecules that form crystals in repeating structural units), which are more flexible as a medium than silicon. The advantage of a polymer hinges on its chemical redundancies, artificially induced at the molecular level. Such a network of redundant material can be compared to a floating tangle of cooked noodles, but their reactions are more uniform than their appearance may suggest.

This diagram of a typical LED indicator light shows how small the actual LED is. Although the average lifetime of a small light bulb is 5-10 years, a modern LED should last 100 years or more before it fails. Illustration by Hans & Cassidy. Courtesy of Gale Group.

When an electrical current is run through an electrically unstable polymer, its electrons all react in unison, as if a single switch flips on all the lights in an apartment building at once. Another advantage is that the wavelength of the emitted photons can easily be adjusted by altering the polymer. Some polymer chains last longer than others before being exhausted, however, and the reason for this phenomenon remains elusive.

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Science EncyclopediaScience & Philosophy: Laser - Background And History to Linear equationLED - The Solid-state Lamp, Convenient Uses And Ambitious Plans