Vishay Telefunken 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 /l (nm). In order to permit effective recombination, the materials should have direct band-to-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 H Li Na K II III IV V VI VII Be Mg Ca B Al Ga C Si Ge N P As O S Se F Cl Br VIII He Ne Ar Kr In Sn Sb Te J Xe Zn Rb Sr Cd V III Al N AlN P AlP As AlAs Sb AlSb Ga 6.0 GaN 2.45 GaP 2.15 GaAs 1.65 GaSb In 3.4 * InN 2.26 InP 1.42 * InAs 0.73 * InSb 1.95 * 1.34 * 0.36 * 0.18 * Figure 1. Periodic system of elements and III-V compounds 10.00 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 will supercede silicon carbide as semiconducting material for blue light emitting diodes within the next few years. 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 1 Vishay Telefunken 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. 95 11428 orange 625nm yellow 590nm red 660nm green 565nm 0.570 AlAs Lattice Constant ( nm ) GaAs 0.565 Ga0.56Al0.44As 0.560 GaAs0.55P0.45 0.555 GaAs1–xPx:N 0.550 GaP 0.545 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 Bandgap ( eV ) Figure 2. Bandgaps and lattice constants 95 11463 3.0 450 Indirect Region Bandgap ( eV ) 500 AlAs 550 GaP Direct Region 600 650 700 2.0 AlInGaP Compositions Lattice Matched to GaAs 800 1.5 Wavelength ( nm ) AlP 2.5 GaAs InP 1.0 5.4 5.5 5.6 5.7 5.8 Lattice Constant ( A ) 5.9 Figure 3. Bandgaps and lattice constanst for InGaAlP 2 10.00 Vishay Telefunken 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. 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 reflector between the substrate and DH-structure and window layer on the DH-structure effect an extremely high luminescent efficiency in this type of LED. p-Electrode (Al) Si3N4 p-Diffusion Al p-Diffusion Si3N4 n–Ga(AsP):N x=0.4 x=0.6/0.85 n-GaAs1–xPx n-GaAs1–xPx n-GaAs Substrate n-GaP Substrate 96 11509 n-Electrode (Au) 96 11511 AuGe Figure 4. Standard red Figure 6. Orange and yellow The DH-red element consists of three epitaxial layers on p-GaAs substrate. The active layer with 35% Alcontent is manufactured between two layers of Gal-xAlxAs of 60% Al content. AuZn u p-GaP:N n-GaP:N n-Electrode (Au) u n-GaP n-Ga1–xAlxAs (x 0.6) p-Ga0.65Al0.35As p-Ga1–xAlxAs (x 0.6) u p-GaAs Substrate n-GaP Substrate 96 11513 AuGe 96 11510 Figure 7. Green p-Electrode (Au) p-Electrode (Al) 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. 10.00 p-Type Window Layer AlInGaP-DH-Structure Reflector n-GaAs Substrate 96 11512 n-Electrode (Au) Figure 8. AlInGaP technology 3 Vishay Telefunken Anode White LED GaN, InGaN SiC Substrate 16 026 Backside Metallization Cathode Figure 9. Blue GaN / InGaN 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 YAG–Phosphor converter and emitted at a longer wavelength. The complementary colors blue and yellow mix to form white. LED Characteristics 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 receiver, for example, the emitted radiated power fe is important. If, however, the receiver is the human eye, the light flux fv is decisive. Figure 10 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. 4 (lm/W) 1000 100 K m * Vl 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-to-band recombination, while other mechanisms and material inhomogeneities lead to wider emission. Here, it is evident that the efficiency drops as the wavelength is reduced. 10 acc. DIN 5031 part 3 565nm 668 lm/W green 650nm 73 lm/W red 1 400 440 480 520 560 600 640 680 720 760 800 96 11579 l – Wavelength ( nm ) Figure 10. 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. 10.00 Vishay Telefunken 50 Forward Current ( mA ) 40 LED 30 U = US – (R IF) US = 5V R = 140W 20 F– 10 I Due to the rapid current rise in the forward direction, LEDs are always connected in series with a current limiting element. Figure 11 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. 0 0 1 2 3 4 5 VF – Forward Voltage ( V ) 96 11580 Figure 11. LED operating point Table 1. Typical technology characteristics@ IF = 10 mA Color Technology gy Red GaAlAs on GaAs Red GaAsP on GaP Red AlInGaP on GaAs Red AlInGaP on GaAs Softorange AlInGaP on GaAs Softorange GaAlAs on GaP Yellow AlInGaP on GaAs Yellow GaAsP on GaP Green GaP on GaP Pure Green GaP on GaP True Green InGaN on SiC Blue Green InGaN on SiC Blue InGaN on SiC Blue GaN on SiC White InGaN/YAG on SiC lp ld nl nm nm nm 650 648 20 635 620 38 643 630 15 620 618 20 610 605 17 610 605 36 590 588 20 585 590 38 565 570 38 555 560 22 518 523 35 503 505 30 463 470 25 428 466 65 5500K not x =0.33 / y =0.33 defined fv fe mlm 60 30 150 300 300 25 200 30 35 12 250 200 75 25 220 mW 0.82 0.20 1.44 1.15 0.92 0.06 0.39 0.05 0.05 0.02 0.55 0.79 1.21 0.96 1.21 VF V 1.80 2.00 1.90 1.85 1.90 2.00 1.90 2.00 2.00 2.00 3.10 3.20 3.60 3.70 3.60 tr ns 100 300 45 45 45 300 45 300 450 450 30 30 30 30 30 tf ns 100 150 30 30 30 150 30 150 200 200 30 30 30 30 30 Efficiency* lm/W 3.3 1.5 7.9 16.2 15.8 1.3 10.5 1.5 1.8 0.6 8.1 6.3 2.1 0.7 6.1 *This table gives an overview comparision for all major Vishay technologies. Some LED datasheets might state minor differences. 10.00 5 Vishay Telefunken Conversion Tables Table 2. Corresponding radiometric and photometric definitions, symbols and units. Definition Power Radiometry Symbol Radiant flux (radiant power) Radiant emittance/ exitance Radiant intensity Output power per unit area Output power per unit solid angle Output power Radiance per unit solid angle and unit emitting area Input power Irradiance per unit area Energy Radiant energy Energy per unit area Radiant exposure (irradiation) Photometry Symbol Unit Luminous flux (luminous power) Luminous exitance Luminous intensity Fv Unit Lumen lm Mv lm/m2 Iv candela, cd Fe Watt, W Me W/m2 Ie W/sr Le W/m2*sr Luminance Lv cd/m2 Ee W/m2 Illuminance Ev Lux, lx lx = lm/m2 Qe Ws Luminous energy (quantity of light) Qv lm * s Hv (lm*s)/m2 He (W*s)/m2 Light exposure Table 3. Luminance Conversion Units (DIN 5031 part 3) Unit cd * m–2 asb sb -2 1 cd*m 1 π 10-4 1 asb (Apostilb) 1/π 1 1/π∗10-4 1 sb (stilb) 104 π∗104 1 4 4 1 L (Lambert) 1/π 10 10 1/π 1 cd*ft-2 10.764 33.82 1.076*10-3 1 fl (Footlambert) 3.426 10.764 3.426*10-4 1 cd*in -2 1550 4869 0.155 L π∗10-4 10-4 π 1 3.382*10-3 1.0764*10-3 0.4869 cd * ft–2 fL cd * in–2 Notes -2 -4 9.29*10 0.2919 6.45*10 2.957*10-2 0.0929 2.054*10-4 929 2919 6.452 2 2.957*10 929 2.054 1 π 6.94*10-3 ft = foot 1/π 1 2.211*10-3 144 452.4 1 in = inch Table 4. Illuminance Conversion Units Unit 1 lx 1 lm*cm–2 1 fc (footcandle) 6 lx 1 104 10.764 lm * cm–2 10–4 1 10.764*10–4 fc 0.0929 929 1 Notes instead of lm*cm–22, formerly y Phot ((ph)) 10.00 Vishay Telefunken Special Notes and Conversion Diagrams [ [ 164 lx a) At standard illuminant A: 1 klx 6.1 mW/cm2 or 1 mW/cm2 b) At 555 nm it is valid: 683 lm 1W 634.5 lm/ft2 1 mW/cm2 [ [ c) 1 lumen/ft2 = 1 footcandle 4 p candlepower = 1 lumen (lm) 250 1.0 200 0.8 150 0.6 540 560 580 ld y 500 100 0.4 600 W S(x,y) 490 v E /E e ( lx/mW/cm2 ) 520 50 0.2 700 400 0 2000 2200 2400 2600 2800 Tf – Color Temperature ( K ) 96 11581 0 0 3000 Figure 12. Ev/Ee (Tf) 96 11583 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 x Figure 14. Chromaticity diagram acc. to CIE 1931 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 0 350 400 450 500 550 600 650 700 750 800 96 11582 λ – Wavelength ( nm ) Figure 13. Color matching functions acc. to CIE 1931 10.00 7