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[资源] Inorganic Semiconductors for Light-emitting Diodes

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
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