Light-Emitting Diodes , E. Fred Schubert Cambridge U. Press, New York, 2003. $110.00, $60.00 paper (313 pp.). ISBN 0-521-82330-7, ISBN 0-521-53351-1 paper
Light-emitting diodes have become an integral part of our daily lives. Since their introduction in the 1960s, their light-emitting efficiency has doubled about every 2 years, and their range of colors has greatly expanded. Today, LEDs serve as bright and colorful indicators, interior and exterior automobile lights, display elements, and backlights for nearly all electronic appliances. And IR LEDs are used for communications and remote control.
Most recently, the efficiency of LEDs has grown substantially. Their emission spectra have been expanded to include white light and the entire visible range; thus they can be used for traffic signals and outdoor, full-color displays. Researchers expect that LEDs will ultimately be applied to general illumination, which could lead to tremendous energy savings. In Light-Emitting Diodes , E. Fred Schubert provides an excellent review of the physics and technology of semiconductor LEDs.
The first LEDs were made of Carborundum (abrasive silicon carbide crystals). The material’s properties were revealed when a curious someone touched an electrical probe to a nugget of SiC made for sandpaper grit, which caused it to emit visible light. Interesting anecdotes like this one, clearly written by someone with broad perspective and expertise, appear throughout the book, making it enjoyable to read. The first few chapters summarize the physics of carrier injection and recombination in LED operation. Although the descriptions are rather terse, they provide a good introduction to the basic mechanisms underlying the light-emission process.
Chapter 2 on radiative and nonradiative recombination processes is well referenced for those who want to dig deeper into the device physics. The sections on nonradiative recombination are especially notable, because the phenomenon represents a critical limitation to LED efficiency. Schubert provides an excellent description of the undesirable recombination pathway. LEDs have an operating current density that is one to two orders of magnitude lower than that of semiconductor laser diodes, and nonradiative processes caused by defects can be dominant at such low operating current densities.
The materials and processes required for fabrication of modern, high-brightness LEDs demand perfection. Despite their ubiquity, LEDs are difficult to make because the underlying technology is enormously sophisticated. Schubert’s book explains how LEDs are made, starting from raw semiconductor materials that make up a layered heterostructure, which is processed into a die and then packaged into lamps. The text also discusses advanced designs for high-efficiency LEDs.
The recent introduction of high-brightness blue and green LEDs made from nitride semiconductors has led to a family of better full-color displays. However, understanding the benefits of visible LEDs requires some knowledge of how humans perceive colors, and Schubert has included a chapter on human vision to explain how LEDs produce a wider color gamut than that of displays based on phosphors or filtered whitelight sources. Likewise, he offers a self-contained chapter on optical communication, which serves as background for the description of communications LEDs and their modulation characteristics.
The considerable attention paid to resonant-cavity LEDs in chapter 10 seems disproportionate because such LEDs are relatively rare. Still, Schubert has made pioneering contributions to those devices, and this variety of LED may become more widely used in the future. The chapter on visible LEDs summarizes the evolution of the semiconductor materials, from aluminum gallium arsenide and nitrogen-doped gallium arsenide phosphide to the nitride and phosphide alloys currently used to span the visible spectrum.
The book, however, provides relatively few details about the significant differences between nitride and phosphide semiconductors and how they impact visible-LED behavior. For example, only a brief mention is made of the apparent benefit of alloy segregation in the structurally imperfect nitride semiconductors and the role alloy segregation plays in the peculiar efficiency of nitride LEDs. Furthermore, the large polarization fields present in the nitride structures are not described. However, nitride LEDs are undergoing rapid development. Compared with more traditional materials, they are still not well explained, so such shortcomings in the book are understandable.
Overall, Light-Emitting Diodes is an excellent examination of the physics and technology of semiconductor LEDs. The narration is simple and direct, and the book is well referenced for those seeking a deeper understanding of the topic. Written for the graduate level, the text will appeal to a broad audience; and for specialists who make semiconductor LEDs and laser diodes, it will serve as a useful connection to the scientific literature. The book is also accessible to nonspecialists such as engineers and scientists who use LEDs or to those who simply wish to learn more about their operation and general characteristics.
