Department of physics

Seminar I b

White LED

Author: Tomaz Suhovrsnik, dipl. fiz. (UN)

Mentor: prof. dr. Gorazd Planinsic

Ljubljana, July 1, 2014

Abstract

Today LEDs characteristics are better than any other type of light source. LEDs havehigh efficiency, longest lifetime, very little UV and IR emissions, no mercury and are veryrobust. After invention of blue LED, white LED was developed in late 90s and start toreplacing all kind of light sources all around us.

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Contents

1 Introduction 2

2 Semiconductors 22.1 Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 P-N junction 4

4 LED 4

5 White LED 6

6 Comparison of different light sources 9

7 Conclusions 11

1 Introduction

First LED that emit visible light (red) was made in 1962, by Nick Holonyak Jr.[1]. Few yearslater, after they become bright enough, they were used as indicators in many electrical devices.In 1972, a student of Holonyak made a first yellow LED. In 1976 first LEDs were build, powerfulenough to be used in telecommunications. First high-brightness blue LED was invented by ShujiNakamura in 1994 [2], based on InGaN, which is still most common today. After LEDs becomehigh efficient, white LEDs were quickly developed. Since 2000 LEDs have been replacing previousgenerations of light sources on almost all fields.

2 Semiconductors

Depending how well materials conduct electrical current, they are divided in conductors, isolatorsand semiconductors. Difference between all three types of material can be explained with bandgaps, which are ilustrated in Fig. 1. With red color are labeled valence bands and with greencolor conduction bands. Distance between them is called band gap. On vertical axis is energy.If conduction band is next to valence band, electrons from latter can pass freely to conductionband, like in conductor. In isolator they are wide separated apart, so very little electrons passthis long distance. Gap distance is for isolators is more than 4 eV. Gap in semiconductors issmaller, usually a few eV.

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Figure 1: Difference between conductor, isolator and semiconductor. Conduction band is ingreen color and valence band is in red color. The energy gap between them is called band gap.

2.1 Doping

Typical semiconductor elements are silicon, germanium and gallium. By adding impurities (dop-ing) to the semiconductor, one can get either excess of electrons or holes. First is called n-typeand second p-type semiconductor. In average only one out of 107 atoms of semiconductor isreplaced with dopant [3].

In case of band gaps, by applying excess of electrons, the conduction band is lowered as seenin Fig. 2 (c), so gap is smaller and by applying excess of holes, the valence band is elevated andagain the distance between valence and conduction band is smaller (in case (b)).

Figure 2: Three variants of semiconductor. (a) not doped, (b) is p-type and (c) is n-type.

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3 P-N junction

Then p-type and n-type are joined together. Because of diffusion electrons begin to travel top-type of semiconductor and holes to n-type. At the junction they recombine and what is leftare charged ions. These ions crate electrical field which is resisting movement of electrons andholes.

Because semiconductor is neutral, the charge in p-type has to be the same as in n-type [3]:

Naxp = Ndxn , (1)

where Na and Nd are concentrations of acceptor and donor atoms. xp and xn are depths of de-pletion region in p-type and n-type semiconductor. The depletion region is where recombinationis going on. From Eq. 1 one can see, that the more the material is doped, the thiner depletionregion is. Of course this also depends on what kind of impurities are used and on the density ofthem.

Charge density in part of p-type semiconductor is equal to +e0Na and in n-type –e0Nd. Sincecharge densities are known, one can calculate with Gauss’s law the magnitude of electric fieldand then the potential in depletion layer. If one define zero of potential in p-type part far awayfrom junction, one gets potential U0 in n-type on edge of the depletion region [3]:

U0 = –eNaNd(xn + xp)2

2ε0ε(Na + Nd), (2)

where ε is electric permittivity of semiconductor. xn + xp is total width of depletion region.Potential U0 is called built-in potential and presents the voltage between p- and n-type part,when there is no circuit.

4 LED

If one connects positive pole of battery to p-type and negative on n-type (forward bias), thevoltage between p-type and n-type of conductor is lowered (U0–U) and the magnitude of electricalfield, which is slowing down the diffusion, is smaller (see Fig. 3 (b)). So more recombinationbetween electrons and holes occurs. The current is described by Shockleys equation [3]:

I = I0

(exp

[ eU

ηkbT

]– 1)

, (3)

where I0 is saturation current and is affected by semiconductor characteristics like diffusionconstant and doping. If one electron and one hole recombine together in depletion layer, only onephoton emits. In this case η is 2. If they travel individually on the other side of depletion regionand each of them recombine individually with one hole and one electron, two recombinationsoccure, so two photons emit and in this case η is 1. In reality both cases are present, so η issomewhere between 1 and 2.

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Figure 3: PN junction: (a) not connected to battery, electrical field (E) in depletion region isgenerated by ions. (b) positive pole of battery is connected to p-type and negative pole to n-type(forward bias). Magnitude of electrical field in depletion region is smaller, so electrons and holesdiffuse easier and more recombinations happen. (c) If poles of battery are switched (reversebias), electrical field in depletion region is bigger and diffusion of electrons and holes is smaller[4].

Figure 4: Fermi levels in different cases. In case a) p and n-type of semiconductor are stillseparated. Fermi level in p-type is lower than in n-type. In case b) there are joined togetherin pn-junction. In equilibrium Fermi level is equal through whole material. In case c) barrier islowered with external voltage (U). Electric field in depletion region is smaller (see Fig. 3, caseb), which leads to more recombinations of electron-hole pairs. In case d) the poles of battery areswitched (see Fig. 3 case c) and the electrical field in depletion region is even higher [4].

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If battery is turned around, so that now negative pole is connected to p-type and positivepole to n-type (reverse bias), magnitude of electrical field in depletion region is bigger (see Fig.3). Diffusion along depletion region of electrons and holes is now even smaller than in case of pnjunction not connected to battery. Electrons are attracted to positive pole so they move towardsn-type, away from junction. The same happens to holes which move to negative pole.

What is going on when pn junction is connected to external voltage can be explained alsowith energy bands. In undoped semiconductors the Fermi level (chemical potential of electronsin material), is in the middle between conduction and valence band. If one adds donors, Fermilevel moves to conduction band and if one adds acceptors it moves to valence band (see Fig. 4,case a). So in n-type Fermi level is higher than in p-type. When p and n-type are put together(see Fig. 4, case b), in equilibrium and without external voltage, Fermi level equals throughwhole material. This can not happen any other way, than that energy bands curve. Electric fieldin depletion region is energy barrier for electrons and holes. The height of barrier is eU0, whereU0 is built-in potential. Because the energy is product of charge and voltage (E = eU), one canchange the barrier with external voltage: e(U0 – Uexternal).

On Fig. 5 is presented electrical current dependency on voltage, described by Shockleysequation (Eq. 3). In case of forward biased pn junction (see Fig. 3 case (b)) electrical currentgrows exponentially with applied voltage. In second case, called reverse biased pn junction (seeFig. 3 case (c)), there is only small current I0 present. One would assume there will be nocurrent, because electrons and holes are attracted to move apart from each other, away fromjunction. Reason is in thermally generated electron-hole pairs in depletion region. If voltagein reverse bias is increased over few 10 V, electron avalanche occur. Accelerated electrons willdamage LED.

Figure 5: Electrical current dependency on voltage. Electrical current (I) presents moving elec-trons and holes along depletion region. U presents voltage of external source, which keeps movingelectrons and holes [4].

5 White LED

LEDs today are available in all the colors of rainbow. The spectra of emitted photons from LEDare very narrow (see Fig. 9, left picture). Typical values of FWHM are from 10 to 40 nm [5]. Inorder to get different tones, one has to mix together different colors. In order to get white light

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with LEDs, there are usually two options.First option is to combine red, green and blue LED. With mixing red, green and blue color,

one gets white light. This option is usually used as backlighting for large displays. Second morecommon, especially due to lower manufacturing price, is white LED (WLED), which is made ofblue LED and phosphorus material, which emits green-yellow light. In addition to lower costsof WLED to RGB LED, studies show that the WLED have twice as better luminous efficacy ascombination of RGB LEDs [6]. This is due to the low quantum efficiency of green emitters, whichis around 10%. Meanwhile quantum efficiency of blue and red emitters are 30% and 50%, respec-tively. Most common combination for white LED is blue LED based on indium gallium nitride(InGaN) and yellow phosphorus material cerium-doped yttrium aluminium garnet (Ce3+:YAG).

First blue LED was invented by Nakamura and colleagues in 1994. Their LED produced 1.5mW of blue light at forward bias of 3.6 V and at 20 mA. Quantum efficiency at that current was2.7 % and luminoux flux of 2 lm (1.2 cd in cone viewing angle of 15◦). Luminous efficacy was 28lm/W. In Fig. 6 is presented spectra at different currents. Each of them has a wavelength peakat 450 nm and FWHM of 70 nm.

After 20 years of development characteristics have improved. Today through average blueLED runs a lot higher current (350 mA) and is connected to voltage of 2.9 V. They emit around40 lm, which gives a luminous efficacy around 40 lm/W. Within viewing angle of 160◦, 90% ofluminous flux is captured. Peak wavelength is at 470 nm and FWHM is 20 nm [7].

Figure 6: Spectra of first high-brightness blue LED, invented by Nakamura, at currents of 10,20, 30 and 40 mA [2].

Blue light is then used to excite phosphorus material. In Fig. 7 is schematically shownprocess of color conversion. 90% of blue light is absorbed in phosphorus material and emittedas green-yellow color. In Fig. 8 are shown absorption and emission spectra of Ce3+:YAG. Inabsorption spectra one can see two peaks, at 340 nm and 460 nm. Blue LEDs emit light withpeak wavelength around second absorption peak of Ce3+:YAG. This is the reason blue LEDand Ce3+:YAG are most common combination. The better this two peaks match, the better isefficiency of color conversion. Spectra of emitted light from Ce3+:YAG is broad. It ranges from

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500 to 700 nm. Fluorescence intensity is exponential decay with decay time of 62 ns [8]. Spectraof white LED can be seen in Fig. 9, 12.

Figure 7: Schematic view of conversion from blue to white light [8].

Figure 8: Left picture presents absorption spectra of phosphorus material Ce3+:YAG, with twopeaks at 340 nm and 460 nm. The right picture presents fluorescence spectra of Ce3+:YAGunder excitation wavelength at 460 nm [8].

In Fig. 10 is shown difference in construction between blue, white and magenta LED. In caseof blue LED (a) there is nothing special. When turned on one can see light coming from pnjunction (shinning blue color). There is no phosphorus material filled in front of semiconductor

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like in case of white and magenta LED. When white LED (b) turned off, one can clearly seephosphorus material which is green-yellow color.

Figure 9: On the left spectra of blue, green, yellow and red LEDs. On the right, spectra of whiteand magenta LEDs [9].

Figure 10: Closer look reveals difference between blue (a) and white (b) or magenta LED (c). Inabove row LEDs are turned off and below are turned on [9].

6 Comparison of different light sources

Light source can beside visible light produce also other electromagnetic radiation, like ultravioletand infrared light, which human eye is blind for. First generation lamps, called incandescent,which are still being used today, emit most of photons in infrared region (see Fig. 11), which isinvisible to naked eye and though useless for illumination.

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Figure 11: Red dashed line presents spectra of incandescent bulbs. Only small area of spectra isin visible range [10].

How well a light source produces visible light is usually measured with luminous efficacy (η).It is defined as a as a ratio of how much visible light is produced and how much total poweris needed to produce this amount of visible light. Amount of light is measured in lumens andpower in watts:

η =luminous flux

power

[ lm

W

], (4)

where lumen is defined as:

1 W of produced light with wavelength of 555 nm is equal to 680lm ,

where is defined that human eye is the most sensitive for 555 nm (green color). Luminous fluxtakes into account sensitivity of human eye to different wavelengths of visible light (see Fig. 12).

Figure 12: Black curve presents spectra of white LED. The left narrow part is spectra of blueLED and the wider and lower part is spectra of phosphorus materials fluorescence. Dashed linepresents sensitivity of human eye to different wavelengths [11].

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In Table 1 are presented characteristics of light sources. Output amount of lumens andrequired power supply are different between light sources. So one usually compares luminousefficacy, which is the ratio of how much visible light one gets per watt of power. The secondimportant parameter is color rendering index (CRI), which tells something about the quality ofproduced light. It goes from 0 to 100 and tells how many of all the colors in the emission spectraare present. Incandescent and halogen lamp have usually CRI of 100, because they have verywide spectra, that extends also outside the region of visible light. The third part is about money.Here important parameters are lifetime and price of light source. Note that all prices except forwhite LED are from single retailer [12], and they may vary from shop to shop.

Luminous efficacy of incandescent and halogen lamps is at least four times lower from otherthree types of light sources. Fluorescent lamps currently maybe still have the best characteristics,but are very large, fragile and contain mercury. The same is with CFL lamps, only that theyare smaller. While white LEDs are small and safe, due to absence of mercury, thought at themoment still quite expensive. But one has to keep in mind that development of fluorescent lampsis at the end, while LEDs are constantly under fast and massive development. This also meansthat characteristics in Table 1 for commercial white LED will be quickly outdated. For example,in 2010, the Japanese group has developed white LED with luminous flux of 203 lm and luminousefficacy of 183 lm/W [15].

Table 1: Characteristics [13] and prices [12] for various light sources (white LED [7], price [14]).

Light source Incandescent Halogen Fluorescent CFL1 White LEDOutput lumens[lm]

830 900 2900 850 470

Power supply[W]

60 50 28 13 8

Luminous effi-cacy [ lmW ]

14 18 104 65 59

CRI2 [1-100] 100 100 84 86 80Lifetime [hours] 1000 3000 30000 10000 25000Price [AC] 1.8 3 4.3 3 24

7 Conclusions

LEDs have already been with us since 60s as indication lamps. Recently mass developmentof LED is pushing their efficacies to the limit every day. With mass production their price isgetting lower. Not to mention all other LEDs characteristics, like stiffness, lifetime, size andnarrow spectra. It seemed they are perfect in every way. They are also healthier. Not onlyto the fact, that they don’t contain mercury, but because they use less power, they are moreefficient, so indirectly less carbon dioxide is produced in process of generating electricity. Theyhave also the smallest time interval, from turning on, till they start to shine.

1CFL=Compact Fluorescent Lights2Color Rendering Index. 100 = full color range: incandescent. CRI is a measure of a light source’s ability to

show object colors ”realistically” or ”naturally” compared to a familiar reference source, either incandescent lightor daylight.

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Because LEDs work on very low voltages, they are ideal for pedagogical purpose. LED itselfis very interesting electric device. Students can discover things like asymmetric I-V curve, narrowspectra, how semiconductors work and that LED can transform electric energy to light and alsolight to electric energy [9] [16].

References

[1] Nick Holonyak and S. F. Bevacqua, Coherent (Visible) Light Emission from Ga(As1–xPx)Junctions, Appl. Phys. Lett. 1, 82 (1962).

[2] Shuji Nakamura, Takashi Mukai, and Masayuki Senoh, Candela-class high-brightness In-GaN/AlGaN double-heterostructure blue-light-emitting diodes, Appl. Phys. Lett. 64, 1687(1994).

[3] Safa Kasap, PN Junction Devices and Light Emitting Diodes, an e-Booklet (2001).

[4] Jure Ausec, Svetlece diode pri pouku fizike, bachelor’s degree (2013).

[5] Michael Bass et al., Handbook of Optics, Volume IV: Optical Properties of Materials, Non-linear Optics, Quantum Optics (Vol. 4. McGraw Hill Professional 2009).

[6] A. Keppens, P. C. Acuna, H. T. Chen, G. Deconinck and P. Hanselaer, Efficiency evaluationand comparison of phosphor-white and RGB packages for high-power light-emitting diodeapplications, Journal of Light & Visual Environment 35.3 (2011).

[7] http://www.lighting.philips.com/main/led/masterled.wpd(May 31, 2014)

[8] Ran Huang, Sai Li, Shaolin Xue, Zhixing Jiang, Shuxian Wu, Preparation and Characteri-zation of YAG:Ce3+ Phosphors by Sol-solvothermal Process, 2012 International Conferenceon Future Environment and Energy IPCBEE vol.28 (2012).

[9] Gorazd Planinsic and Eugenia Etkina, Light Emitting Diodes: Learning new physics, sendfor publication in The Physics Teacher (May 2014).

[10] http://laserboyfriend.blogspot.com/2012/09/lightbulbs-and-lumen.html (May 31, 2014)

[11] http://www.osram.com (May 31, 2014)

[12] http://www.merkur.si (May 25, 2014)

[13] http://www.ledaladdin.com (July 1, 2014)

[14] http://www.ledrise.com (May 25, 2014)

[15] Yukio Narukawa, Masatsugu Ichikawa, Daisuke Sanga, Masahiko Sano and Takashi Mukai,White light emitting diodes with super-high luminous efficacy. J. Phys. D: Appl. Phys. 43(2010).

[16] G. Planinsic and E. Etkina, Light-Emitting Diodes - A Hidden Treasure, The PhysicsTeacher, Volume 52, issue 2 (2014).

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