Editing Light-emitting diode (section)

=== 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>