Inorganic Semiconductors for Light-emitting Diodes [chapter]

E. Fred Schubert, Thomas Gessmann, Jong Kyu Kim
2006 Organic Light Emitting Devices  
During the past 40 years, light-emitting diodes (LEDs) have undergone a significant development. The first LEDs emitting in the visible wavelength region were based on GaAsP compound semiconductors with external efficiencies of only 0.2 %. Today, the external efficiencies of red LEDs based on AlGaInP exceed 50 %. AlGaInP semiconductors are also capable of emitting at orange, amber, and yellow wavelengths, albeit with lower efficiency. Semiconductors based on AlGaInN compounds can emit
more » ... y in the UV, violet, blue, cyan, and green wavelength range. Thus, all colors of the visible spectrum are now covered by materials with reasonably high efficiencies. This opens the possibility to use LEDs in areas beyond conventional signage and indicator applications. In particular, LEDs can now be used in high-power applications thereby enabling the replacement of incandescent and fluorescent sources. LED lifetimes exceeding i 10 5 h compare favorably with incandescent sources (Z 500 h) and fluorescent sources (Z 5000 h), thereby contributing to the attractiveness of LEDs. Inorganic LEDs are generally based on p-n junctions. However, in order to achieve high internal quantum efficiencies, free carriers need to be spatially confined. This requirement has led to the development of heterojunction LEDs consisting of different semiconductor alloys and multiple quantum wells embedded in the light-emitting active region. The light-extraction efficiency, which measures the fraction of photons leaving the semiconductor chip, is strongly affected by the device shape and surface structure. For high internal-efficiency active regions, the maximization of the light-extraction efficiency has proven to be the key to highpower LEDs. This chapter reviews important aspects of inorganic LED structures. Section 1.2 introduces the basic concepts of optical emission. Band diagrams of direct and indirect semiconductors and the spectral shape of spontaneous emission will be discussed along with radiative and nonradiative recombination processes. Spontaneous emission can be controlled by placing the active region in an optical
doi:10.1002/3527607986.ch1 fatcat:lhchjhaus5eg5gfq3luqwpsnga