Light-Emitting Diodes: Progress in Solid-State Lighting
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Light-Emitting Diodes: Progress in SolidState Lighting
Artu¯ras Zˇukauskas, Michael S. Shur, and Remis Gaska Introduction Until the beginning of the 19th century, flame produced by combustion was the only source of artificial light. Since then, physical phenomena other than pyroluminescence have been used to produce light.1 Limelight (incandescence of calcium oxide heated by the flame from an oxyhydrogen blowpipe), gas mantles (candoluminescence of gas-flame-heated rare-earth oxides), and the electrical Jablochkoff candle (an early type of carbon-arc lamp) were among the important milestones that led to modern lighting technology. In the 21st century, most of the residential lighting worldwide is provided by tungsten incandescent lamps. Compact fluorescent lamps are also actively promoted because of their higher performance—a broader spectrum for higher-quality white light and elimination of 100–120-Hz flickering, for example. Most work environments employ fluorescent tubes for general lighting, and street lighting is dominated by sodium lamps.2 Lighting consumes 2000 TWh of energy annually, about 21% of the global consumption of electricity.3 However, during the past 20 years, none of the conventional lighting technologies has exhibited a significant improvement in efficiency. The drive to save lighting energy and reduce its negative environmental impact (i.e., carbon emissions and the disposal of mercury contained in discharge lamps) stimulates the search for new, efficient sources of light. This search focused attention on lightemitting diodes (LEDs), which, prior to the last decade of the 20th century, were used only as indicator lamps and numerical displays in electronic devices. Today, mature methods for fabricating compoundsemiconductor materials, progress in LED design, and the emergence of blue AlInGaN-based LEDs have resolved the
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problems of low light output and limited color range that previously precluded LED applications in lighting. The brightness, efficiency, and color choices of LEDs have achieved a level that is leading to dramatic changes in lighting technology. In this article, we review the present status of solid-state lighting, including discussions of the concept of high-brightness LEDs, materials systems and chip design for monochrome LEDs, white LED lamps, and, finally, the emerging applications of solid-state lighting. A more detailed discussion of many issues related to solid-state lighting may be found in our upcoming book.4
High-Brightness LEDs The development of high-brightness LEDs relied on the introduction of new semiconductors with efficiencies of visible emission much higher than those of early LED materials, such as GaAsP (red), GaP (yellow-green), and SiC (blue). Semiconductors used for high-brightness LEDs must exhibit direct transitions with high rates of radiative recombination, have wide bandgaps to emit at visible (or, in certain cases, UV) wavelengths, and possess a low density of nonradiative recombination centers and high durability. Novel Group III–V direct-gap
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