Vishay Semiconductors Physics of Optoelectronic Devices Light-Emitting Diodes This section deals with the principles and characteristics of the technically most important types of visible emitters which are formed, without special lateral structures, as whole-area emitters. Materials for LEDs The materials for light-emitting diodes in the visible spectrum (400 - 700 nm) are semiconductors with bandgaps between 1.8 and 3.1 eV, with Eg (eV) = hν = 1240 /λ (nm). In order to permit effective recombination, the materials should have direct bandto-band transitions or permit other recombination paths with high efficiency. Neither of these requirements are fulfilled by the well-known semiconductors silicon and germanium, as their bandgaps are too small and they are indirect by nature. As a further requirement, the materials have to enable manufacturing in the form of monocrystals (volume crystals or at least epitaxial layers) and a sufficiently developed technology must be available for processing. Element combination from groups III and V of the periodic element system results in semiconducting compounds with bandgaps between 0.18 eV (InSb) and approximately 6 eV (AlN); see figure 1. I II III IV V VI VII Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Zn Ga Ge As Se Br Kr Rb Sr Cd In Sn Sb Te J Xe H VIII He III V N P As Sb Al AlN 6.0 AlP 2.45 AlAs 2.15 AlSb 1.65 Ga GaN 3.4 * GaP 2.26 GaAs 1.42 * GaSb 0.73 * In InN 1.95 * InP 1.34 * InAs 0.36 * InSb 0.18 * Figure 1. Periodic system of elements and III-V compounds For every composition the bandgap is given in eV. Direct semiconductors are denoted by an asterisk. The largest direct bandgaps are found in the group III – nitrides GaN and InN. These nitrides are supercede silicon carbide as semiconducting material for blue light emitting diodes. Document Number: 80097 Rev. 1.2, 05-Jul-04 Gallium arsenide has a direct bandgap of 1.42 eV and is not only important for light emitting diodes, but is also a significant substrate material. III–V compounds can form mixed crystals with properties between those of the binary compounds. The most important mixed crystal systems pertaining to LEDs are GaAs–AlAs, GaAs – GaP, InP – GaP – AlP. The bandgap and the lattice constant of Gal-xAlxAs and GaAsl-xPx are shown in figure 2. The direct region is shown as a solid line and the indirect region as a dashed line. Up to x = 0.44 (band gap 1.96 eV), Gal-xAlxAs is direct, and the lattice constant changes only slightly over the whole mixture range. The highest direct bandgap for GaAsl-xPx is 1.99 eV (x = 0.45). In the indirect region of GaAs1-xPx, the efficiency of radiating recombination can be increased considerably by doping with nitrogen (isoelectronic center). This reduces the effective bandgap by approximately 0.06 eV (second dashed line). The bandgaps required for common LED colors are marked at the top of figure 2. Whereas red emission can be achieved with Ga0.6Al0.4As or GaAs0.6P0.4 (both direct), the indirect GaAsl-xPx:N is used for the other colors. In figure 3, the bandgaps and lattice constants of InGaAlP are presented. In this diagram, binary compounds are shown as points, ternary compounds as lines, and the quaternary mixed crystal InGaAlP as a shaded area. The binary crystals InP, GaP and AlP form the apecies. Their characteristic form is a result of the complex nature of the band structure and the transition from the direct to the indirect region and vice versa. The diagram shows which InGaAlP compositions are direct and which mixed crystals are lattice matched to a given substrate material. Both are necessary prerequisites for the manufacture of particularly bright light emitting diodes. The vertical dashed line drawn through the direct bandgap region shows the compositions that are exactly lattice matched to, and can be manufactured on a GaAs substrate. The corresponding notation In0.5(GaxAl1–x)0.5P gives the ratio of the amounts of atoms in the crystal lattice. The bandgap is direct if the aluminium concentration is below x = 0.7. The spectral region from red via orange and yellow to green can be covered by variation of x in InGaAlP. Extremely high luminescent effiencies for orange and yellow are obtained in double heterostructures of InGaAlP situated on a Bragg reflector on a GaAs substrate. www.vishay.com 49 Vishay Semiconductors p-Electrode(Al) p-Diffusion Si3N4 yellow 590 nm orange 625 nm green 565 nm red 660 nm 95 11428 x = 0.4 0.570 AlAs Lattice Constant ( nm ) GaAs 0.565 n-GaAs1–x Px Ga0.56 Al 0.44As 0.560 n-GaAs Substrate GaAs 0.55P0.45 0.555 96 11509 GaAs1–xPx:N 0.550 n-Electrode (Au) GaP 0.545 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 Figure 4. Standard red Bandgap ( eV ) The DH-red element consists of three epitaxial layers on p-GaAs substrate. The active layer with 35 % Al content is manufactured between two layers of Gal-xAlxAs of > 60 % Al content. Figure 2. Bandgaps and lattice constants 95 11463 3.0 450 Indirect Region AlP 500 AlAs 550 GaP Direct Region 600 650 700 2.0 AlInGaP Compositions Lattice Matched to GaAs 800 1.5 n-Electrode (Au) Wavelength ( nm ) Bandgap ( eV) 2.5 n-Ga1–x Al xAs (x > 0.6) p-Ga 0.65 Al 0.35 As p-Ga 1–x Al x As (x > 0.6) p-GaAs Substrate GaAs InP 96 11510 1.0 5.4 5.5 5.6 5.7 5.8 Lattice Constant ( °A ) 5.9 Figure 3. Bandgaps and lattice constants for InGaAlP Technologies The figures below show two structures on a GaAs substrate: figure 4, standard red and figure 5, DH-red. Standard red consists of GaAs0.6P0.4 which was deposited with a GaAs1–xPx buffer layer on a n-GaAs substrate by vaporphase epitaxy. The element is manufactured in a planar process with masked Zn diffusion. The substrate is opaque in this case. This means that the rear contact can be provided over the complete area. www.vishay.com 50 p-Electrode (Au) Figure 5. DH-red Other standard LEDs are shown in figures 6 and 7. The chips for orange and yellow are similar to the standard red type. Due to the large amount of P, 0.6 or 0.85, the material is deposited with suitable buffer layers on the transparent GaP substrate, and a reflective rear contact is provided. The green element consists entirely of GaP and can therefore be manufactured with liquid-phase epitaxy. The grown PN junction is divided by mesa etching in this example to enable measurement of individual chips on the wafer. A new type of LED for applications where a particularly high brightness is required is shown in figure 8. The structure is similar to that of a DH-red element. A double heterostructure is arranged on a light-absorbing GaAs substrate. A combination of a Bragg reflec- Document Number: 80097 Rev. 1.2, 05-Jul-04 Vishay Semiconductors tor between the substrate and DH-structure and window layer on the DH-structure effect an extremely high luminescent efficiency in this type of LED. Al p-Diffusion Anode GaN Si3N4 n–Ga(AsP):N SiC Substrate x = 0.6/0.85 16 026 n-GaAs1–x Px Backside Metallization Cathode n-GaP Substrate Figure 9. GaN / InGaN / straight wall Anode (+) 96 11511 InGaN AuGe Figure 6. Orange and yellow AuZn SiC Substrate p-GaP:N n-GaP:N Cathode (-) Backside Metallization InGaN n-GaP 19007 Figure 10. Mushroom shaped n-GaP Substrate n ITO 96 11513 AuGe p Epi Layer ITO Organic Binder Figure 7. Green Al p-Electrode(Al) Si Substrate p-Type Window Layer AlInGaP-DH-Structure Reflector n-GaAs Substrate 96 11512 n-Electrode (Au) Figure 8. AlInGaP technology Document Number: 80097 Rev. 1.2, 05-Jul-04 18990 Figure 11. OMA-Chip LED Characteristics The most important characteristics of the LEDs dealt with here are summarized in table 1. The emission of LEDs is almost monochromatic and can be characterized by a peak wavelength (column 4) and a spectral half bandwidth (column 6). The lowest spectral half bandwidth is generated by LEDs with direct band-toband recombination, while other mechanisms and material inhomogeneities lead to wider emission. Here, it is evident that the efficiency drops as the wavelength is reduced. Liquid-phase elements generally have a higher efficiency than comparable vapor-phase elements. For applications in which a Si detector is to be used as a www.vishay.com 51 Vishay Semiconductors receiver, for example, the emitted radiated power φe is important. If, however, the receiver is the human eye, the light flux φv is decisive. Figure 12 shows the sensitivity of the human eye in accordance with DIN 5031. The maximum value is 683 lm/W at 555 nm. Green LEDs are close to this maximum value, while the curve for red emission drops rapidly. An orange LED (630 nm), for example, appears approximately 5 times brighter than a red LED (660 nm) at the same efficiency. White LED In general white light is a mixture of the 3 basic colors red, green and blue. A cost effective solution is white light out of only one chip by using the physical principle of luminescence conversion. A high brightness InGaN-based LED chip, mounted in the reflector cup is covered by a luminescence converter consisting of an inorganic phosphor material dissolved in Silicon, while the whole system is embedded in transparent epoxy resin. Part of the blue light is absorbed by a TAG–Phosphor converter and emitted at a longer wavelength. The complementary colors blue and yellow mix to form white. Due to the rapid current rise in the forward direction, LEDs are always connected in series with a current limiting element. Figure 13 shows how the working point is set with a series resistor. If the supply voltage varies greatly, a constant current source is used. Digital displays and similar devices are often operated in multiplex mode. The seven data lines carry the information for the digits in a time-staggered manner. Each LED is operated for one-quarter of the time with four times the current in order to achieve the same intensity as in continuous operation. In displays with larger numbers of LEDs, it is advisable to reduce the number of driver stages and connecting leads. Due to the diode characteristics of LEDs, a large number of LEDs can be operated with only a few leads if the circuit is designed accordingly. 1000 100 K m * Vλ (lm/W) acc.DIN 5031 part 3 96 11579 10 565nm 668 lm/W green 650nm 73 lm/W red 1 400 440 480 520 560 600 640 680 720 760 800 λ– Wavelength ( nm ) Figure 12. Human eye sensitivity diagram Electrically, LEDs are ordinary PN diodes (except for GaN). The most important parameter for normal operation is the forward voltage VF. (Table 1, third column from the right) where the numerical value in V corresponds approximately to the bandgap of the semiconductor used. The speed of LEDs is characterized by their switching times (last columns); red LEDs made of direct materials are fastest, while indirect types are considerably slower due to the special recombination mechanism. Generally, LEDs are robust and their lifetimes (more than 105 hours) are more than sufficient for practically all applications. www.vishay.com 52 Document Number: 80097 Rev. 1.2, 05-Jul-04 Vishay Semiconductors 40 LED 30 U = US – (R x IF) US = 5 V R = 140 Ω 20 10 I F– Forward Current ( mA ) 50 0 0 96 11580 1 2 3 4 5 VF – Forward Voltage ( V ) Figure 13. LED operating point Document Number: 80097 Rev. 1.2, 05-Jul-04 www.vishay.com 53 Vishay Semiconductors Color Technology Red Red Red Red Red (OMA) Softorange Softorange Yellow Yellow Yellow Green Green Pure Green Pure Green True Green Blue Green Blue Blue White GaAlAs on GaAs GaAsP on GaP AlInGaP on GaAs AlInGaP on GaAs AlInGaP on Si AlInGaP on GaAs GaAsP on GaP AlInGaP on GaAs GaAsP on GaP AlInGaP on GaAs GaP on GaP GaP on GaP AlInGaP on GaAs InGaN on SiC InGaN on SiC InGaN on SiC GaN on SiC InGaN/TAG on SiC lp ld Dl fv fe VF tr tf Efficiency* nm 650 635 643 620 622 610 610 590 585 574 565 555 561 518 503 463 428 nm 648 620 630 618 615 605 605 588 590 572 570 560 562 523 505 470 466 nm 20 38 15 20 18 17 36 20 38 20 38 22 20 35 30 25 65 not defined mlm 60 30 200 400 600 400 25 200 30 80 35 12 30 250 200 75 25 220 mW 0.82 0.20 1.22 1.5 2.5 1.2 0.06 0.78 0.05 1.12 0.05 0.02 0.05 0.55 0.79 1.21 0.96 1.21 V 1.80 2.00 1.85 1.85 2.8 1.90 2.00 1.90 2.00 2.0 2.00 2.00 2.00 3.10 3.20 3.60 3.70 3.60 ns 100 300 45 45 45 45 300 45 300 45 450 450 45 30 30 30 30 30 ns 100 150 30 30 30 30 150 30 150 30 200 200 30 30 30 30 30 30 lm/W 3.3 1.5 10.8 21.6 28 21.1 1.3 21.1 1.5 4 1.8 0.6 1.5 8.1 6.3 2.1 0.7 6.1 5500K x =0.33 / y =0.33 Table 1: Typical technology characteristics @ IF = 10 mA *This table gives an overview comparision for all major Vishay technologies. Some LED datasheets might state minor differences. Conversion Tables Radiometry Definition Symbol Φe Unit Watt, W W/m2 W/sr Power Radiant flux (radiant power) Output power per unit area Output power per unit solid angle Output power per unit solid angle and unit emitting area Radiant emittance/exitance Radiant intensity Radiance Me e W/m2*sr Input power per unit area Energy Irradiance Radiant energy Ee W/m2 Qe Ws Energy per unit area Radiant exposure (irradiation) He (W*s)/m2 Unit Ie L Photometry Symbol Luminous flux Φv (luminous power) Luminous exitance Luminous intensity Luminance Mv v cd/m2 Illuminance Luminous energy (quantity of light) Ev lx = lm/m2 Lux, lx Qv lm *s Light exposure Hv (lm*s)/m2 Table 2: Corresponding radiometric and photometric, symbols and units asb sb L fL cd * m cd * ft–2 -4 -2 1 p π*10-4 0.2919 10 9.29*10 -4 -4 -2 1/p 1 0.0929 1/π*10 10 2.957*10 4 4 1 p 929 2919 10 π*10 4 4 2 1/π 1 929 1/π 10 10 2.957*10 10.764 33.82 1 p 1.076*10-3 3.382*10-3 3.426 10.764 1/π 1 3.426*10-4 1.0764*10-3 1550 4869 0.155 0.4869 144 452.4 –2 -2 1 cd*m 1 asb (Apostilb) 1 sb (stilb) 1 L (Lambert) 1 cd*ft-2 1 fl (Footlambert) 1 cd*in -2 Unit 1 lx 1 lm*cm–2 1 fc (footcandle) lx 1 104 10.764 Table 3: Luminance Conversion Units (DIN 5031 part 3) fc lm * cm–2 10–4 1 –4 10.764*10 0.0929 929 1 Unit Lumen,lm lm/m2 candela, cd Iv L Notes cd * in–2 -4 6.45*10 2.054*10-4 6.452 2.054 6.94*10-3 2.211*10-3 1 ft = foot in = inch Notes instead of lm*cm–2, formerly Phot (ph) Table 4: Illuminance Conversion Units www.vishay.com 54 Document Number: 80097 Rev. 1.2, 05-Jul-04 Vishay Semiconductors Special Notes and Conversion Diagrams a) At standard illuminant A: 1 klx ≈ 6.1 mW/cm2 or 1 mW/cm2 ≈ 164 lx At 555 nm it is valid: 683 lm ª 1 W 634.5 lm/ft2 ≈ 1 mW/cm2 1 lumen/ft2 = 1 footcandle 4 π candlepower = 1 lumen (lm) b) c) 1.0 520 540 0.8 560 0.6 580 500 d y 250 0.4 S(x,y) 490 E v/Ee ( lx/mW/cm2 ) 200 600 W 0.2 700 400 150 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 x 96 11583 100 Figure 16. Chromaticity diagram acc. to CIE 1931 50 0 2000 2200 2400 2600 2800 3000 Tf – Color Temperature ( K ) 96 11581 Figure 14. EV/Ee (Tf) 1.8 Z 1.6 1.4 X, Y, Z 1.2 Y=V(λ) 1.0 X 0.8 0.6 0.4 X 0.2 0.0 350 400 450 500 550 600 650 700 750 800 λ– Wavelength ( nm ) 96 11582 Figure 15. Color matching functions acc. to CIE 1931 Document Number: 80097 Rev. 1.2, 05-Jul-04 www.vishay.com 55 Vishay Semiconductors www.vishay.com 56 Document Number: 80097 Rev. 1.2, 05-Jul-04