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Inorganic Semiconductors for Light-emitting Diodes
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http://www.namipan.com/d/cd318a1 ... 96128bb4c110d245e00 Introduction 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 efficiently 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 105 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 opticalcavity resulting in a substantial modification of the LED emission characteristics. Theory and experimental results of such resonant-cavity LEDs (RCLEDs) are discussed in Section 1.3. The electrical characteristics of LEDs, to be discussed in Section 1.4, include parasitic voltage drops and current crowding phenomena that result in nonuniform light emission and shortened device lifetimes. Due to total internal reflection at the surfaces of an LED chip, the light-extraction efficiency in standard devices is well below 100 %. Section 1.5 discusses techniques such as chip shaping utilized to increase the extraction efficiency. A particular challenge in achieving efficient LEDs is the minimization of optical absorption processes inside the semiconductor. This can be achieved by covering absorbing regions, such as lower-bandgap substrates, with highly reflective mirrors. Such mirrors should have omnidirectional reflection characteristics and a high angleintegrated, TE-TM averaged reflectivity. A novel electrically conductive omnidirectional reflector is discussed in Section 1.6. Section 1.7 reviews the current state of the art in LED packaging including packages with low thermal resistance 一定要顶哟，记得回复！ |
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