Contact N GaALoz Asos 2.1 ev P GaAs P GaAlos Asoa GaAs FIGURE 83.5 A GaAlAs heterojunction LED: (a)cross-sectional diagram;(b) energy-band diagr OPEN COLLECTOR GATES ACTIVE PULLUP. TOTEM POLE GATE MAY BE R LOGIC LOGIC SERIES SWITCHING SERIES SHUNT WITCHING CURRENT TO LED FIGURE 83.6 Digital logic can interface directly to LED lamps. Source: S Gage et al, Optoelectronics/Fiber-Optics appli- cations Manual, 2nd ed, New York: Hewlett-Packard/McGraw-Hill, 1981, P. 2. 20. With permission. where a is the average absorption coefficient, v, is the LED volume, and A is the emitting area In considering LED effectiveness for display purposes, one must also include radiation wavelength in relation to the spectral response of the human eye [Sze, 1985]. Although the Gap green LED is intrinsically less efficient than the GaAsP red LED, the eye compensates for the deficiency with a greater sensitivity to green. More recently developed heterojunction LEDs(Fig. 83.5)offer two mechanisms to improve LED efficiencies [Yang, 1988]. The electron injection efficiency can be enhanced, but, in addition, absorption losses through the wider 2. 1-ev bandgap n-type layer are essentially eliminated for photons emitted by recombination in the lower 2.0-ev bandgap p-type region. Interfacing In circuit design applications, the LEd may be treated much as a regular diode, but with a much greater forward voltage, V, Since one usually seeks maximum brightness from the device, it is usually conducting heavily and VE approaches the contact potential. As one moves from GaAs to GaP [Fig 83.3(a), VE varies from about 1.5 to around 2.0 V. The variation in VE with temperature(at constant current)follows similar rules as apply to conventional diodes, but radiant power and wavelengths also change [Gage et al, 1981 Single LEDs are commonly driven by logic gates, perhaps as status indicators, and some of the simplest interface circuits are shown in Fig. 83. 6. In many cases, the gate output will not be able to source or sink sufficient current for visibility, and an amplifier will be required, as in Fig. 83. 7. Bar graph displays are commonly© 2000 by CRC Press LLC where a is the average absorption coefficient, vo is the LED volume, and A is the emitting area. In considering LED effectiveness for display purposes, one must also include radiation wavelength in relation to the spectral response of the human eye [Sze, 1985]. Although the GaP green LED is intrinsically less efficient than the GaAsP red LED, the eye compensates for the deficiency with a greater sensitivity to green. More recently developed heterojunction LEDs (Fig. 83.5) offer two mechanisms to improve LED efficiencies [Yang, 1988]. The electron injection efficiency can be enhanced, but, in addition, absorption losses through the wider 2.1-eV bandgap n-type layer are essentially eliminated for photons emitted by recombination in the lower 2.0-eV bandgap p-type region. Interfacing In circuit design applications, the LED may be treated much as a regular diode, but with a much greater forward voltage, VF. Since one usually seeks maximum brightness from the device, it is usually conducting heavily and VF approaches the contact potential. As one moves from GaAs to GaP [Fig. 83.3(a)], VF varies from about 1.5 to around 2.0 V. The variation in VF with temperature (at constant current) follows similar rules as apply to conventional diodes, but radiant power and wavelengths also change [Gage et al., 1981]. Single LEDs are commonly driven by logic gates, perhaps as status indicators, and some of the simplest interface circuits are shown in Fig. 83.6. In many cases, the gate output will not be able to source or sink sufficient current for visibility, and an amplifier will be required, as in Fig. 83.7. Bar graph displays are commonly FIGURE 83.5 A GaAlAs heterojunction LED: (a) cross-sectional diagram; (b) energy-band diagram. FIGURE 83.6 Digital logic can interface directly to LED lamps. (Source: S. Gage et al., Optoelectronics/Fiber-Optics Appli￾cations Manual, 2nd ed., New York: Hewlett-Packard/McGraw-Hill, 1981, p. 2.20. With permission.) N GaAl0.7 As0.3 P GaAl0.6 As0.4 N GaAl0.7 As P GaAl0.6 As0.4 p GaAs P GaAs Contact Contact 2.1 eV 2.0 eV 1.42 eV Eƒ