Absorbed Light Output GaAs Transparent Encapsulation GaAs FIGURE 83.4 Effect of(a)opaque substrate, (b)transparent substrate, and(c)encapsulation on photons emitted at the pn Junction and where t, and t, are the minority carrier lifetimes associated with the radiative and nonradiative recombi nation processes, and t is the effective lifetime. The radiative efficiency is defined as n=R/(R1+Rm)=t/τ (83.10) and the internal quantum efficiency is ni= ny 83.11) (c) It is clear from Fig. 83. 4 that many of the photons generated on either side of the junction will pass through sufficient bulk semiconductor to be reabsorbed. In fact the photon energy may be ideally suited to reabsorption if it exceeds the semiconductor direct bandgap. It is obvious, then, why GaAs is opaque and Gap transparent to photons from Ga(As: P) junctions. Clearly, a greater efficiency might be expected from the transparent substrate with reflecting contact [Fig. 83. 4(b) The photon must strike the LED surface at an angle less than the critical angle for total internal reflection, 1/n (83.12) and next, nLED are the external and internal refractive indices, respectively. For air, next=1, but critical angle loss be reduced by encapsulating the device in an epoxy lens cap[ Fig 83. 4(c)] to increase both nat >1 and the angle of incidence at the air interface. Even within angles less than 0, there is Fresnel loss, with transmission ratio T=4n/(1+n)2 (83.13) The total external quantum efficiency is then the fraction of photons emitted [Neamen, 1992], given by n。=1/(1+v/AT e 2000 by CRC Press LLC
© 2000 by CRC Press LLC and where tr and tnr are the minority carrier lifetimes associated with the radiative and nonradiative recombination processes, and t is the effective lifetime. The radiative efficiency is defined as h = Rr/(Rr + Rnr) = t/tr (83.10) and the internal quantum efficiency is hi = hg (83.11) (c) It is clear from Fig. 83.4 that many of the photons generated on either side of the junction will pass through sufficient bulk semiconductor to be reabsorbed. In fact the photon energy may be ideally suited to reabsorption if it exceeds the semiconductor direct bandgap. It is obvious, then, why GaAs is opaque and GaP transparent to photons from Ga(As:P) junctions. Clearly, a greater efficiency might be expected from the transparent substrate with reflecting contact [Fig. 83.4(b)]. The photon must strike the LED surface at an angle less than the critical angle for total internal reflection, qc, where sin qc = next /nLED = 1/n (83.12) and next, nLED are the external and internal refractive indices, respectively. For air, next = 1, but critical angle loss can be reduced by encapsulating the device in an epoxy lens cap [Fig. 83.4(c)] to increase both next > 1 and the angle of incidence at the air interface. Even within angles less than qc, there is Fresnel loss, with transmission ratio T = 4n/(1 + n)2 (83.13) The total external quantum efficiency is then the fraction of photons emitted [Neamen, 1992], given by [Yang, 1988] he = 1/(1 + avo/AT) (83.14) FIGURE 83.4 Effect of (a) opaque substrate, (b) transparent substrate, and (c) encapsulation on photons emitted at the pn junction. A Absorbed Photons Graded Alloy GaAs1-y Py (y=0 0.4) Graded Alloy GaAs1-y Py Reflective Contact Emitted Photons Emitted Photons Predominantly Electron Injection Transparent Plastic Encapsulation Al Top Contact Light Output Insulating Layer Al Back Contact GaAs GaAs P GaAs1-yPy Ga P B n p n n p (a) (b) (c) qc p
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 Applications 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
LED 2N2907A ve- (a) For use when LSTTL drives an LED (b) For use when a logic high is needed to drive an LED FIGURE 83.7 LED interfacing for(a)low-power transistor-transistor logic,(b)logic high drive, and(c)CMOS ( Source: M. Forbes and BB Brey, Digital Electronics, Indianapolis: Bobbs-Merrill, 1985, P. 242. With permission. +Vcc NATIONAL L-- LED2 立=LED1 BAR GRAPH DISPLAY POSITION INDICATOR DISPLAY FIGURE 83.8 Operational amplifiers or voltage comparators used to decode an analog signal into a bar graph or position dicator display.( Source: S. Gage et al, Optoelectronics/Fiber-Optics Applications Manual, 2nd ed, New York: Hewlett Packard/McGraw-Hill, 1981, P. 23.3. With permission. used to indicate signal level on audio equipment, with a modification of the position indicator seen in Fig. 83.8 to guide fine tuning. Matrix LED arrays can be used for flexible, high-density panel displays [Fig 83.9(a)] and are conventionally controlled by row or column strobing[ Fig 83.9(b)] controlled by a microprocessor interface. Multiple LEDs are commonly packaged together in a single integrated device, organized in one of the standard display fonts Fig 83. 10(a)l, with decoding often included within the package [Fig. 83. 10(b)]. The 7-segment e 2000 by CRC Press LLC
© 2000 by CRC Press LLC used to indicate signal level on audio equipment, with a modification of the position indicator seen in Fig. 83.8 to guide fine tuning. Matrix LED arrays can be used for flexible, high-density panel displays [Fig. 83.9(a)] and are conventionally controlled by row or column strobing [Fig. 83.9(b)] controlled by a microprocessor interface. Multiple LEDs are commonly packaged together in a single integrated device, organized in one of the standard display fonts [Fig. 83.10(a)], with decoding often included within the package [Fig. 83.10(b)]. The 7-segment FIGURE 83.7 LED interfacing for (a) low-power transistor-transistor logic, (b) logic high drive, and (c) CMOS. (Source: M. Forbes and B.B. Brey, Digital Electronics, Indianapolis: Bobbs-Merrill, 1985, p. 242. With permission.) FIGURE 83.8 Operational amplifiers or voltage comparators used to decode an analog signal into a bar graph or position indicator display. (Source: S. Gage et al., Optoelectronics/Fiber-Optics Applications Manual, 2nd ed., New York: HewlettPackard/McGraw-Hill, 1981, p. 23.3. With permission.)
■口口 ■自日口 口口口 口口口 日口 日口 口口口 百口 口■ 口口口 百日口口口口口 日口■口口 口 X1×2X3 自口口口口 (a 口 口口 FIGURE 83.9 Matrix displays. (a)One LEd will be turned on by applying the proper signal to one x axis and one y axis. (b) Character generation using column strobe methods. Source: S. Gage et al., Optoelectronics/Fiber-Optics Applications Manual, 2nd ed, New York: Hewlett-Packard/McGraw-Hill, 1981, pp 2.25, 5.44. With permission 图 日日口 FONT A: 7-SEGMENT FONT C: 16-SEGMENT FONT E: 5x7 DOT MATRIX FONT GLASS WINDOW ATED CIR NTRAST 接 ETIC SEAL AT EXTERNAL L BACK CERAMIC SUBSTRATE URE 83.10(a)Display fonts used in LED displays.( b) Construction features of a hermetic LED display.( Source: S iage et al, Optoelectronics/Fiber-Optics Applications ManuaL, 2nd ed, New York: Hewlett-Packard/McGraw-Hill, 1981, Pp 5.3, 5.6. with permission. display is adequate for hexadecimal applications, but the 16-segment is required for alphanumerics. To limit pin-out requirements, the LEDs of a single package are connected in either the common anode or commor cathode configuration Fig 83. 11(a)), with multiple display digits multiplexed as illustrated in Fig. 83. 11(b) e 2000 by CRC Press LLC
© 2000 by CRC Press LLC display is adequate for hexadecimal applications, but the 16-segment is required for alphanumerics. To limit pin-out requirements, the LEDs of a single package are connected in either the common anode or common cathode configuration [Fig. 83.11(a)], with multiple display digits multiplexed as illustrated in Fig. 83.11(b). FIGURE 83.9 Matrix displays. (a) One LED will be turned on by applying the proper signal to one x axis and one y axis. (b) Character generation using column strobe methods. (Source: S. Gage et al., Optoelectronics/Fiber-Optics Applications Manual, 2nd ed., New York: Hewlett-Packard/McGraw-Hill, 1981, pp. 2.25, 5.44. With permission.) FIGURE 83.10 (a) Display fonts used in LED displays. (b) Construction features of a hermetic LED display. (Source: S. Gage et al., Optoelectronics/Fiber-Optics Applications Manual, 2nd ed., New York: Hewlett-Packard/McGraw-Hill, 1981, pp. 5.3, 5.6. With permission.)
回回回回 MULTI PLEXINO FIGURE 83.11 (a)generalized drive circuits for strobed operation. ( b)Block diagram of a strobed (multiplexed) six-digit LED display. (Source: S. Gage et al, Optoelectronics/Fiber-Optics Applications Manual, 2nd ed, New York: Hewlett-Pack ard/McGraw-Hill, 1981, Pp 5.25, 5. 23. with permis Defining Terms External quantum efficiency: The proportion of the photons emitted from the pn junction that escape the device structure(but sometimes alternatively defined as nine) Injection electroluminescence: Electroluminescence is the general term for optical emission resulting from the passage of electric current; injection electroluminescence refers to the case where the mechanism involves the injection of carriers across a pn junction Internal quantum efficiency: The product of injection efficiency and radiative efficiency corresponds to the ratio of power radiated from the junction to electrical power supplied. Related Topic 22 1 Physical Properties References J. Allison, Electronic Engineering Semiconductors and Devices, 2nd ed, London: McGraw-Hill, 1990 M. Forbes and B. B Brey, Digital Electronics, Indianapolis: Bobbs-Merrill, 1990. S. Gage, D. Evans, M. Hodapp, H. Sorensen,R Jamison, and R. Krause, Optoelectronics/Fiber-Optics Applications Manual, 2nd ed. New York: Hewlett-Packard/McGraw-Hill, 1981 D Jiles, Introduction to the Electronic Properties of Materials, London: Chapman Hall,1994 S. Nakamura, " A bright future for blue/green LEDs, " IEEE Circuits Devices, 11(3),19-23, 1995 D. A. Neamen, Semiconductor Physics and Devices: Basic Principles, Boston: Irwin, 1992 R. E Pierret, Semiconductor Device Fundamentals, New York: Addison-Wesley, 1996. S. M. Sze, Semiconductor Devices: Physics and Technology, New York: Wiley, 1985 S. Wang, Fundamentals of Semiconductor Theory and Device Physics, Englewood Cliffs, N J. Prentice-Hall, 1989 E S. Yang, Microelectronic Devices, New York: McGraw-Hill, 1988 e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Defining Terms External quantum efficiency: The proportion of the photons emitted from the pn junction that escape the device structure (but sometimes alternatively defined as hi he). Injection electroluminescence: Electroluminescence is the general term for optical emission resulting from the passage of electric current; injection electroluminescence refers to the case where the mechanism involves the injection of carriers across a pn junction. Internal quantum efficiency: The product of injection efficiency and radiative efficiency corresponds to the ratio of power radiated from the junction to electrical power supplied. Related Topic 22.1 Physical Properties References J. Allison, Electronic Engineering Semiconductors and Devices, 2nd ed., London: McGraw-Hill, 1990. M. Forbes and B. B. Brey, Digital Electronics, Indianapolis: Bobbs-Merrill, 1990. S. Gage, D. Evans, M. Hodapp, H. Sorensen, R. Jamison, and R. Krause, Optoelectronics/Fiber-Optics Applications Manual, 2nd ed., New York: Hewlett-Packard/McGraw-Hill, 1981. D. Jiles, Introduction to the Electronic Properties of Materials, London: Chapman & Hall, 1994. S. Nakamura, “A bright future for blue/green LEDs,” IEEE Circuits & Devices, 11(3), 19–23, 1995. D. A. Neamen, Semiconductor Physics and Devices: Basic Principles, Boston: Irwin, 1992. R. F. Pierret, Semiconductor Device Fundamentals, New York: Addison-Wesley, 1996. S. M. Sze, Semiconductor Devices: Physics and Technology, New York: Wiley, 1985. S. Wang, Fundamentals of Semiconductor Theory and Device Physics, Englewood Cliffs, N.J.: Prentice-Hall, 1989. E. S. Yang, Microelectronic Devices, New York: McGraw-Hill, 1988. FIGURE 83.11 (a) Generalized drive circuits for strobed operation. (b) Block diagram of a strobed (multiplexed) six-digit LED display. (Source: S. Gage et al., Optoelectronics/Fiber-Optics Applications Manual, 2nd ed., New York: Hewlett-Packard/McGraw-Hill, 1981, pp. 5.25, 5.23. With permission.)