LIGHT emitting diodes (LEDs) -- devices that emit intense light of particular colours when electricity is passed through them -- are set to revolutionise the world of artificial illumination. With the recent finding that gallium nitrides and allied compounds can produce intense blue-green light, LEDs now span the entire range of visible colours with luminous intensity high enough for use in displays and lighting (Science, Vol 267, No 5194).
LEDs are efficient converters of electricity to light and consume only 10-20 per cent of the power that incandescent light bulbs need to produce the same luminous intensity. They are compact and have working lives at least 5 times that of ordinary bulbs.
LEDs can be used in displays, illumination, turn indicators, billboard advertisements, light sources for accelerated photosynthesis, and in medical diagnosis and treatment. In fact, brake lights in the latest automobiles and running lights in trucks are red LEDs.
Before the current breakthrough, powerful LEDs were limited to red light; LEDs of any other colour were of dismally low luminous intensities, which restricted their applications. Research in LEDs concentrated on 2 things: the right semiconductor materials and an efficient technology to manufacture high-quality crystals and films for making the LEDs.
High luminosity LEDs emitting red, orange, amber and green are pretty commonplace but bright blue and blue-green emitters have eluded researchers for years -- till the recent introduction of gallium nitrides that emit ultraviolet light. To produce blue light, a research team led by H Morkoc at the University of Illinois in Urbana-Champaign made an alloy from indium and gallium nitrides.
Scientists at the Japanese companies, Nichia and Toyota Gosei, have also produced bright blue and blue-green LEDs from alloys of gallium, indium and aluminium nitrides. Both teams have also successfully developed technology to grow high-quality crystals and films of indium-gallium nitride.