Development of Multiple LED Colors
The first commercial light emitting diode, developed in the 1960s, utilized the primary constituents gallium, arsenic, and phosphorus to produce red light (655-nanometer wavelength). An additional red light-emitting material, gallium phosphide, was later used to produce diodes emitting 700-nanometer light. The latter version has seen limited application, in spite of high efficiency, due to the low apparent brightness resulting from relative insensitivity of the human eye in that spectral region. Throughout the 1970s, technological developments enabled additional diode colors to be introduced, and production improvements increased the quality control and reliability of the devices.
Changes in the elemental proportions, doping, and substrate materials resulted in development of gallium-arsenide-phosphorus (GaAsP) diodes producing orange and yellow emission, as well as a higher-efficiency red emitter. Green diodes based on GaP chips were also developed. The introduction and refinement of the use of gallium-aluminum-arsenide (GaAlAs), during the 1980s, resulted in a rapid growth in the number of applications for light emitting diodes, largely due to an order-of-magnitude improvement in brightness compared to previous devices. This gain in performance was achieved by the use of multilayer heterojunction structures in the chip fabrication, and although these GaAlAs diodes are limited to emission in the red (660 nanometers), they began to be used in outdoor signs, bar code scanners, medical equipment, and fiber optic data transmission.
