Further information More extensive semiconductor device treatments of the led are contained in Semiconductor devices and Integrated Circuits by A G Milnes( Van Nostrand Reinhold, New York)and in Introduction to Optical Electronics by K.A. Jones(Harper and Row, New York). E. Uiga provides more interfacing and design detail for the LED a circuit element in optoelectronics(Prentice-Hall, Englewood Cliffs, NJ, 1995). Wang [1989]considers second-order effects extensively. In Semiconductor Optoelectronics by J Singh(McGraw-Hill, New York, 1996), the emphasis is on communications applications, but the temperature dependence and frequency response issues covered there are also relevant to displays Chapter 2 of Gage et al. [1981]contains detailed information on the optical and thermal design constraints on the LEd package and on LEd back-lit display systems. Chapter 6 considers filtering and other techniques for the contrast enhancement required for direct sunlight viewing Professional society magazines are good sources of up-to-date information at the non-specialist level, espe- cially the occasional special issues devoted to topic reviews. IEEE Spectrum is a good example, as is the IEEE Circuits d Devices 83.2 Liquid-Crystal displays James E. Morris In a low-power CMOS digital system, the dissipation of a light-emitting diode(LED) or other comparable splay technology can dominate the total systems power requirements. In such circumstances the low-power dissipation advantage of CMOS technology can be completely lost. This is the situation in which liquid-crystal display(LCD)technology must be used. The LED(or other active system, such as a plasma or vacuum fluorescent display) emits optical power supplied(comparatively inefficiently) by the system battery or other source. The passive LCD is fundamentally different in that the optical power is supplied externally(by sunlight or room lighting typically) and the system source need supply only the relatively minute amount of power (microwatts per square centimeter)required to change the device's reflective optical properties Materials classed as liquid crystals are typically liquid at high temperatures and solid at low temperatures, but the intermediate temperature range they display characteristics of both. Although there are many different crystals used, we will concent the use of ne the most common by far The essential feature of a liquid crystal is the long rod-like molecule. In a nematic crystal, the molecules lign as shown in Fig. 83.12. If the container surface is microscopically grooved, the interface molecules will be aligned by the grooves and intermolecular forces will maintain that orientation across the liquid crystal ig. 83. 12(a)]. The molecules will align in an electric field, and beyond a critical value, the field may be sufficient to overcome the alignment with the grooves Fig 83. 12(b)].(In practice, the transition is not so abrupt, and groove alignment persists at the interface itself Fig 83. 13].) The process of alignment in the electric field is the result of the anisotropic dielectric constant characteristic of liquid crystals. For the electric field parallel to the molecular alignment, e,=Ep and for a perpendicular field, E,=Er. In a"positive"liquid crystal, E, > Ep and the molecules align parallel to the field as described above in order to minimize the systems potential energy. The principle of the twisted nematic cell is illustrated in Fig. 83. 14. The confining plates, typically 10 um apart, are grooved orthogonally, forcing the molecular orientation to spiral through 90 degrees [Fig 83. 14(a)] In the LCD, two polarizers and a mirror are added as shown in Fig. 83. 14(b) Incident ambient light is polarized and enters the liquid-crystal cell with the plane of polarization parallel to the molecular orientation. As the light traverses the cell, the plane of polarization is rotated by the twist in the liquid crystal, so that it reaches the opposite face with a polarization 90 degrees to the original direction, but parallel now to the direction of the second polarizer, through which it may therefore pass. The light is then reflected from the mirror and passes back through the cell, reversing the prior sequence. e 2000 by CRC Press LLC© 2000 by CRC Press LLC Further Information More extensive semiconductor device treatments of the LED are contained in Semiconductor Devices and Integrated Circuits by A. G. Milnes (Van Nostrand Reinhold, New York) and in Introduction to Optical Electronics by K. A. Jones (Harper and Row, New York). E. Uiga provides more interfacing and design detail for the LED as a circuit element in optoelectronics (Prentice-Hall, Englewood Cliffs, NJ, 1995). Wang [1989] considers second-order effects extensively. In Semiconductor Optoelectronics by J. Singh (McGraw-Hill, New York, 1996), the emphasis is on communications applications, but the temperature dependence and frequency response issues covered there are also relevant to displays. Chapter 2 of Gage et al. [1981] contains detailed information on the optical and thermal design constraints on the LED package and on LED back-lit display systems. Chapter 6 considers filtering and other techniques for the contrast enhancement required for direct sunlight viewing. Professional society magazines are good sources of up-to-date information at the non-specialist level, espe￾cially the occasional special issues devoted to topic reviews. IEEE Spectrum is a good example, as is the IEEE Circuits & Devices magazine. 83.2 Liquid-Crystal Displays James E. Morris In a low-power CMOS digital system, the dissipation of a light-emitting diode (LED) or other comparable display technology can dominate the total system’s power requirements. In such circumstances the low-power dissipation advantage of CMOS technology can be completely lost. This is the situation in which liquid-crystal display (LCD) technology must be used. The LED (or other active system, such as a plasma or vacuum fluorescent display) emits optical power supplied (comparatively inefficiently) by the system battery or other source. The passive LCD is fundamentally different in that the optical power is supplied externally (by sunlight or room lighting typically) and the system source need supply only the relatively minute amount of power (microwatts per square centimeter) required to change the device’s reflective optical properties. Principle of Operation Materials classed as liquid crystals are typically liquid at high temperatures and solid at low temperatures, but in the intermediate temperature range they display characteristics of both. Although there are many different types of liquid crystals used, we will concentrate here on the use of nematic crystals in twisted nematic devices, the most common by far. The essential feature of a liquid crystal is the long rod-like molecule. In a nematic crystal, the molecules align as shown in Fig. 83.12. If the container surface is microscopically grooved, the interface molecules will be aligned by the grooves and intermolecular forces will maintain that orientation across the liquid crystal [Fig. 83.12(a)]. The molecules will align in an electric field, and beyond a critical value, the field may be sufficient to overcome the alignment with the grooves [Fig. 83.12(b)]. (In practice, the transition is not so abrupt, and groove alignment persists at the interface itself [Fig. 83.13].) The process of alignment in the electric field is the result of the anisotropic dielectric constant characteristic of liquid crystals. For the electric field parallel to the molecular alignment, er = e||, and for a perpendicular field, er = e^. In a “positive” liquid crystal, e|| > e^, and the molecules align parallel to the field as described above in order to minimize the system’s potential energy. The principle of the twisted nematic cell is illustrated in Fig. 83.14. The confining plates, typically 10 mm apart, are grooved orthogonally, forcing the molecular orientation to spiral through 90 degrees [Fig. 83.14(a)]. In the LCD, two polarizers and a mirror are added as shown in Fig. 83.14(b). Incident ambient light is polarized and enters the liquid-crystal cell with the plane of polarization parallel to the molecular orientation. As the light traverses the cell, the plane of polarization is rotated by the twist in the liquid crystal, so that it reaches the opposite face with a polarization 90 degrees to the original direction, but parallel now to the direction of the second polarizer, through which it may therefore pass. The light is then reflected from the mirror and passes back through the cell, reversing the prior sequence