Editing Light-emitting diode (section)

== White LEDs ==

There are two primary ways of producing [[white]] light-emitting diodes. One is to use individual LEDs that emit three [[primary color]]s—red, green and blue—and then mix all the colors to form white light. The other, more often used method is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, similar to a [[fluorescent lamp]]. The yellow phosphor is [[cerium]]-doped [[Yttrium aluminium garnet|YAG]] crystals suspended in the package or coated on the LED. This YAG phosphor causes white LEDs to appear yellow when off, and the space between the crystals allow some blue light to pass through in LEDs with partial phosphor conversion. Alternatively, white LEDs may use other phosphors like manganese(IV)-doped [[potassium fluorosilicate]] (PFS) or other engineered phosphors. PFS assists in red light generation, and is used in conjunction with conventional Ce:YAG phosphor.

In LEDs with PFS phosphor, some blue light passes through the phosphors, the Ce:YAG phosphor converts blue light to green and red (yellow) light, and the PFS phosphor converts blue light to red light. The color, emission spectrum or color temperature of white phosphor converted and other phosphor converted LEDs can be controlled by changing the concentration of several phosphors that form a phosphor blend used in an LED package.<ref>{{Cite web | url=https://www.ledinside.com/news/2014/11/seeing_red_with_pfs_phosphor |title = Seeing Red with PFS Phosphor}}</ref><ref>{{Cite web | url=https://www.ledsmagazine.com/architectural-lighting/retail-hospitality/article/16696629/ge-lighting-manufactures-pfs-red-phosphor-for-led-display-backlight-applications | title=GE Lighting manufactures PFS red phosphor for LED display backlight applications| date=March 31, 2015}}</ref><ref>{{cite journal | url=https://sid.onlinelibrary.wiley.com/doi/abs/10.1002/sdtp.10406 | doi=10.1002/sdtp.10406 | title=62.4: PFS, K<sub>2</sub>SiF<sub>6</sub>:Mn<sup>4+</sup>: The Red-line Emitting LED Phosphor behind GE's TriGain Technology™ Platform | date=2015 | last1=Murphy | first1=James E. | last2=Garcia-Santamaria | first2=Florencio | last3=Setlur | first3=Anant A. | last4=Sista | first4=Srinivas | journal=Sid Symposium Digest of Technical Papers | volume=46 | pages=927–930 | url-access=subscription }}</ref><ref>{{Cite journal|doi = 10.1149/2.0251801jss|title = Full Spectrum White LEDs of Any Color Temperature with Color Rendering Index Higher Than 90 Using a Single Broad-Band Phosphor|year = 2018|last1 = Dutta|first1 = Partha S.|last2 = Liotta|first2 = Kathryn M.|journal = ECS Journal of Solid State Science and Technology|volume = 7|pages = R3194–R3198| s2cid=103600941 |doi-access = free}}</ref>

The 'whiteness' of the light produced is engineered to suit the human eye. Because of [[Metamerism (color)|metamerism]], it is possible to have quite different spectra that appear white. The appearance of objects illuminated by that light may vary as the spectrum varies. This is the issue of color rendition, quite separate from color temperature. An orange or cyan object could appear with the wrong color and much darker as the LED or phosphor does not emit the wavelength it reflects. The best color rendition LEDs use a mix of phosphors, resulting in less efficiency and better color rendering.{{citation needed|date=October 2020}}

The first white light-emitting diodes (LEDs) were offered for sale in the autumn of 1996.<ref>{{Cite journal | doi = 10.1002/lpor.201600147 | last1 = Cho | first1 = Jaehee | last2 = Park | first2 = Jun Hyuk | last3 = Kim | first3 = Jong Kyu | last4 = Schubert | first4 = E. Fred | title = White light-emitting diodes: History, progress, and future | journal = Laser & Photonics Reviews | volume = 11 | issue = 2 | pages = 1600147 | year = 2017| bibcode = 2017LPRv...1100147C | s2cid = 53645208 | issn = 1863-8880 | url=https://onlinelibrary.wiley.com/doi/10.1002/lpor.201600147 | url-access = subscription }}</ref> Nichia made some of the first white LEDs which were based on blue LEDs with Ce:YAG phosphor.<ref>{{cite book | url=https://books.google.com/books?id=GEFKDwAAQBAJ&dq=ce+yag+led&pg=PA36 | isbn=978-0-9863826-6-6 | title=Light-Emitting Diodes (3rd Edition, 2018) | date=February 3, 2018 | publisher=E. Fred Schubert }}</ref> Ce:YAG is often grown using the [[Czochralski method]].<ref>{{cite book | url=https://books.google.com/books?id=aJyCDAAAQBAJ&dq=growing+ce+yag&pg=PA113 | isbn=978-1-119-23600-9 | title=Additive Manufacturing and Strategic Technologies in Advanced Ceramics | date=August 16, 2016 | publisher=John Wiley & Sons }}</ref>

=== RGB systems ===
[[File:RGB_LED_Spectrum.svg|thumb|Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. [[Full width at half maximum|FWHM]] spectral bandwidth is approximately 24–27 nm for all three colors.]]

[[File:RGB-Led-projection.jpg|thumb|An RGB LED projecting red, green, and blue onto a surface]]

Mixing red, green, and blue sources to produce white light needs electronic circuits to control the blending of the colors. Since LEDs have slightly different emission patterns, the color balance may change depending on the angle of view, even if the RGB sources are in a single package, so RGB diodes are seldom used to produce white lighting. Nonetheless, this method has many applications because of the flexibility of mixing different colors,<ref>{{Cite journal | doi = 10.1364/OE.15.003607 | last1 = Moreno | first1 = I. | last2 = Contreras | first2 = U. | title = Color distribution from multicolor LED arrays | journal = Optics Express | volume = 15 | issue = 6 | pages = 3607–3618 | year = 2007 | pmid = 19532605| bibcode = 2007OExpr..15.3607M | s2cid = 35468615 | doi-access = free }}</ref> and in principle, this mechanism also has higher [[quantum efficiency]] in producing white light.<ref>{{Cite web|url=http://spie.org/newsroom/1069-making-white-light-emitting-diodes-without-phosphors?SSO=1|title=Making white-light-emitting diodes without phosphors {{!}} SPIE Homepage: SPIE|last1=Yeh|first1=Dong-Ming|last2=Huang|first2=Chi-Feng|website=spie.org|access-date=2019-04-07|last3=Lu|first3=Chih-Feng|last4=Yang|first4=Chih-Chung}}</ref>

There are several types of multicolor white LEDs: [[:wiktionary:dichromatic|di-]], [[trichromatic|tri-]], and [[tetrachromatic]] white LEDs. Several key factors that play among these different methods include color stability, [[color rendering index|color rendering]] capability, and luminous efficacy. Often, higher efficiency means lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. Although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.<ref>{{cite book |last1=Cabrera |first1=Rowan |title=Electronic Devices and Circuits |date=2019 |publisher=EDTECH |isbn=978-1839473838}}</ref>

One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but as of 2010 few green LEDs exceed even 100 lumens per watt. The blue and red LEDs approach their theoretical limits.{{citation needed|date=October 2020}}

Multicolor LEDs offer a means to form light of different colors. Most [[color#Perception|perceivable colors]] can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. Their emission power [[exponential decay|decays exponentially]] with rising temperature,<ref>{{Cite journal |last1=Schubert |first1=E. Fred |last2=Kim |first2=Jong Kyu |journal=Science |volume=308 |issue=5726 |doi=10.1126/science.1108712 |pmid=15919985 |pages=1274–1278 |year=2005 |title=Solid-State Light Sources Getting Smart |bibcode=2005Sci...308.1274S |s2cid=6354382 |url=https://www.ecse.rpi.edu/~schubert/Reprints/2005%20Schubert%20and%20Kim%20(Science)%20Solid-state%20light%20sources%20getting%20smart.pdf|archive-url=https://web.archive.org/web/20160205165109/https://www.ecse.rpi.edu/~schubert/Reprints/2005%20Schubert%20and%20Kim%20(Science)%20Solid-state%20light%20sources%20getting%20smart.pdf |archive-date=February 5, 2016 }}</ref>
resulting in a substantial change in color stability. Such problems inhibit industrial use. Multicolor LEDs without phosphors cannot provide good color rendering because each LED is a narrowband source. LEDs without phosphor, while a poorer solution for general lighting, are the best solution for displays, either backlight of LCD, or direct LED based pixels.

Dimming a multicolor LED source to match the characteristics of incandescent lamps is difficult because manufacturing variations, age, and temperature change the actual color value output. To emulate the appearance of dimming incandescent lamps may require a feedback system with color sensor to actively monitor and control the color.<ref>{{cite journal | title = Sensors and Feedback Control of Multicolor LED Systems | format = PDF | first1 = Thomas | last1 = Nimz | first2 = Fredrik | last2 = Hailer | first3 = Kevin | last3 = Jensen | journal = Led Professional Review: Trends & Technologie for Future Lighting Solutions | publisher = LED Professional | date = November 2012 | issue = 34 | issn = 1993-890X | pages = 2–5 | url = http://www.mazet.de/en/english-documents/english/featured-articles/sensors-and-feedback-control-of-multi-color-led-systems-1/download#.UX7VXYIcUZI | archive-url = https://web.archive.org/web/20140429162806/http://www.mazet.de/en/english-documents/english/featured-articles/sensors-and-feedback-control-of-multi-color-led-systems-1/download#.UX7VXYIcUZI | url-status = dead | archive-date = 2014-04-29 }}</ref>

=== Phosphor-based LEDs ===
[[File:White LED.png|class=skin-invert-image|thumb|upright=1.6|Spectrum of a white LED showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband [[Stokes shift|Stokes-shifted]] light emitted by the Ce<sup>3+</sup>:YAG phosphor, which emits at roughly 500–700 nm]]

This method involves [[coating]] LEDs of one color (mostly blue LEDs made of [[InGaN]]) with [[phosphor]]s of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs).<ref>{{cite book|title=Fifth International Conference on Solid State Lighting|author1=Tanabe, S. |author2=Fujita, S. |author3=Yoshihara, S. |author4=Sakamoto, A. |author5=Yamamoto, S.|chapter=YAG glass-ceramic phosphor for white LED (II): Luminescence characteristics |journal=Proceedings of SPIE|chapter-url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5941/594112.pdf|archive-url=https://web.archive.org/web/20110511182527/http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5941/594112.pdf|archive-date=2011-05-11|volume=5941|doi=10.1117/12.614681|page=594112|year=2005|bibcode=2005SPIE.5941..193T |s2cid=38290951 |editor1-last=Ferguson |editor1-first=Ian T |editor2-last=Carrano |editor2-first=John C |editor3-last=Taguchi |editor3-first=Tsunemasa |editor4-last=Ashdown |editor4-first=Ian E }}</ref> A fraction of the blue light undergoes the Stokes shift, which transforms it from shorter wavelengths to longer. Depending on the original LED's color, various color phosphors are used. Using several phosphor layers of distinct colors broadens the emitted spectrum, effectively raising the [[Color Rendering Index|color rendering index]] (CRI).<ref>{{Cite journal|title=Color rendering and luminous efficacy of white LED spectra|author=Ohno, Y.|journal=Proc. SPIE|doi=10.1117/12.565757|url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5530/5530-88.pdf|archive-url=https://web.archive.org/web/20110511182632/http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5530/5530-88.pdf|archive-date=2011-05-11|volume=5530|page=89|year=2004|series=Fourth International Conference on Solid State Lighting|bibcode=2004SPIE.5530...88O|s2cid=122777225|editor1-last=Ferguson|editor1-first=Ian T|editor2-last=Narendran|editor2-first=Nadarajah|editor3-last=Denbaars|editor3-first=Steven P|editor4-last=Carrano|editor4-first=John C}}</ref>

Phosphor-based LEDs have efficiency losses due to heat loss from the [[Stokes shift]] and also other phosphor-related issues. Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the [[luminosity function]]).

Due to the simplicity of manufacturing, the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex [[#RGB systems|RGB]] system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.{{citation needed|date=October 2020}}
[[File:1 watt 9 volt SMD LED.jpg|thumb|1 watt 9 volt three chips SMD phosphor based white LED]]
Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. As of 2010, the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stokes shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.{{citation needed|date=October 2020}}

Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Alternatively, the LED might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Remote phosphors provide more diffuse light, which is desirable for many applications. Remote phosphor designs are also more tolerant of variations in the LED emissions spectrum. A common yellow phosphor material is [[cerium]]-[[Doping (Semiconductors)|doped]] [[yttrium aluminium garnet]] (Ce<sup>3+</sup>:YAG).{{citation needed|date=October 2020}}

White LEDs can also be made by [[coating]] near-ultraviolet (NUV) LEDs with a mixture of high-efficiency [[europium]]-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way [[fluorescent lamp]]s work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.<ref>{{cite journal|author=Xu, Yulin, Bohua Zhang, Zhiqiang Xu, Weihao Ye, Baoyan Guo, Jianle Zhuang, Chaofan Hu, Bingfu Lei, Guangqi Hu, and Yingliang Liu|date=June 2024|title=Preparation of carbon dots using aminoquinoline as nitrogen source as full ultraviolet bands absorber and application of LED UV leakage protection|journal=Journal of Dyes and Pigments|volume=225|doi=10.1016/j.dyepig.2024.112060}}</ref> 

A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids the typical costly [[sapphire]] [[Substrate (materials science)|substrate]] in relatively small 100- or 150-mm wafer sizes.<ref name="electronicdesign.com">[http://electronicdesign.com/europe-news/next-generation-gan-si-white-leds-suppress-costs Next-Generation GaN-on-Si White LEDs Suppress Costs], Electronic Design, 19 November 2013</ref> The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It was predicted that since 2020, 40% of all GaN LEDs are made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment.<ref>[http://www.isuppli.com/Semiconductor-Value-Chain/News/Pages/GaN-on-Silicon-LEDs-Forecast-to-Increase-Market-Share-to-40Percent-by-2020.aspx GaN-on-Silicon LEDs Forecast to Increase Market Share to 40 Percent by 2020], iSuppli, 4 December 2013</ref>

=== Mixed white LEDs ===
[[File:Led Lights Panel.jpg|thumb|Tunable white LED array in a floodlight]]
There are RGBW LEDs that combine RGB units with a phosphor white LED on the market. Doing so retains the extremely tunable color of RGB LED, but allows color rendering and efficiency to be optimized when a color close to white is selected.<ref>{{cite web |title=All You Want to Know about RGBW LED Light |url=https://www.agcled.com/blog/all-you-want-to-know-about-rgbw-led-light.html |website=AGC Lighting}}</ref>

Some phosphor white LED units are "tunable white", blending two extremes of color temperatures (commonly 2700K and 6500K) to produce intermediate values. This feature allows users to change the lighting to suit the current use of a multifunction room.<ref>{{cite web |title=Tunable White Application Note |url=https://support.enlightedinc.com/hc/en-us/articles/360031886233-Tunable-White-Application-Note |website=enlightedinc.com}}</ref> As illustrated by a straight line on the chromaticity diagram, simple two-white blends will have a pink bias, becoming most severe in the middle. A small amount of green light, provided by another LED, could correct the problem.<ref>{{Cite web|url=https://leducation.org/green-light-can-maximize/|title=2021 How Green Light Can Maximize the Quality of Tunable White – LEDucation|website=leducation.org}}</ref> Some products are RGBWW, i.e. RGBW with tunable white.<ref name=EG.COL.T>{{cite web |title=Understanding LED Color-Tunable Products |url=https://www.energy.gov/eere/ssl/understanding-led-color-tunable-products |website=Energy.gov |language=en}}</ref>

A final class of white LED with mixed light is dim-to-warm. These are ordinary 2700K white LED bulbs with a small red LED that turns on when the bulb is dimmed. Doing so makes the color warmer, emulating an incandescent light bulb.<ref name=EG.COL.T/>

=== Other white LEDs ===
Experimental white light-emitting diodes (LEDs) have been developed using homoepitaxially grown zinc selenide (ZnSe) on ZnSe substrates. This approach eliminates the need for phosphors, distinguishing it from conventional white LEDs that typically combine blue or ultraviolet LEDs with phosphors to produce white light.<ref>{{Cite web |title=Joint venture to make ZnSe white LEDs |url=https://optics.org/article/16534 |access-date=2025-05-29 |website=optics.org}}</ref>

In these ZnSe-based LEDs, the active region emits blue light, while the conductive ZnSe substrate emits yellow light. The combination of these emissions results in white light output. This method offers advantages such as lower operating voltage (approximately 2.7 V), reduced packaging complexity, and the potential for a broader range of color temperatures (3500–8500 K) compared to GaN-based devices.<ref name=":5">{{Cite journal |last=Katayama |first=K. |last2=Matsubara |first2=H. |last3=Nakanishi |first3=F. |last4=Nakamura |first4=T. |last5=Doi |first5=H. |last6=Saegusa |first6=A. |last7=Mitsui |first7=T. |last8=Matsuoka |first8=T. |last9=Irikura |first9=M. |last10=Takebe |first10=T. |last11=Nishine |first11=S. |last12=Shirakawa |first12=T. |date=June 2000 |title=ZnSe-based white LEDs |url=https://ui.adsabs.harvard.edu/abs/2000JCrGr.214.1064K/abstract |journal=Journal of Crystal Growth |language=en |volume=214-215 |issue=1-2 |pages=1064–1070 |doi=10.1016/S0022-0248(00)00275-X |issn=0022-0248}}</ref>

A study published in the Journal of Crystal Growth in 2000 demonstrated that these ZnSe-based white LEDs exhibited a color temperature around 3400 K and a color rendering index (CRI) of 68. At a forward current of 20 mA, the optical output power was 2.0 mW, and the luminous efficiency was estimated at 10.4 lm/W, comparable to incandescent lamps and commercial InGaN-based white LEDs.<ref name=":5" />

However, challenges remain, particularly concerning device degradation. Reports indicate that the lifetime of these ZnSe-based white LEDs is limited, with some studies suggesting a half-life of approximately 800 hours at 20°C . Additionally, the commercialization of these devices has been hindered by issues such as electron overflow and the need for enhanced p-type carrier concentration.<ref name=":5" />

Despite these challenges, the development of phosphor-free ZnSe-based white LEDs represents a significant step toward more efficient and versatile lighting solutions. Ongoing research aims to address the limitations and improve the performance and longevity of these devices.<ref>{{Cite web |title=Radware Bot Manager Captcha |url=https://validate.perfdrive.com/fb803c746e9148689b3984a31fccd902/?ssa=c27adc9c-2c68-47d7-92c2-4217d3044d84&ssb=43834282869&ssc=https%3A%2F%2Fiopscience.iop.org%2Fjournal%2F1674-1056%3F&ssi=f987b99d-cnvj-4ec1-8498-78e6ae7bf8cb&ssk=botmanager_support@radware.com&ssm=55157618935069704101449635340940&ssn=b9aff80796e106d5e08b5e4a28555350a98f3aec2dae-3010-4ede-b34970&sso=62d18fd1-b82aae4c1869a96c9e110bf174224e0b84e5079c9dd2faf0&ssp=67594533161748545119174852467951695&ssq=70284985460792714936054607916373967502819&ssr=MjA4LjgwLjE1My4xMDk=&sst=Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/110.0.0.0 Safari/537.36 Citoid/WMF (mailto:noc@wikimedia.org)&ssu=&ssv=&ssw=&ssx=eyJfX3V6bWYiOiI3ZjYwMDBmMGZjY2Q4ZS0yMzI0LTRlMzctODY0NS1jMWU0MzRlMzc3NWYxNzQ4NTU0NjA3ODAzMC1hNGVjOGVjOTIzNmJlODgwMTAiLCJ1em14IjoiN2Y5MDAwMWRjODI2NjEtZWU2Ni00YzM1LWFlZWEtODMzNDNiM2I2MzkzMS0xNzQ4NTU0NjA3ODAzMC0wMDFjZWIzNGYyMTNjZDMzMTAiLCJyZCI6ImlvcC5vcmcifQ== |access-date=2025-05-29 |website=validate.perfdrive.com}}</ref>