INDUSTRY LEDs
Generating white light The LEDs that are used in lighting today are an evolution of the first white-emitting LEDs, which emerged from Japan in the 1990s and employed a blue-emitting chip to pump a yellow phosphor (see Figure 2a). In this form of white-emitting device, not all of the blue emission is absorbed by this phosphor, and white light results from colour mixing of blue and yellow. This approach, still widely used today, has the merit of a very high efficiency. However, the colour rendering index is low, due to the lack of emission in the red and orange regions of the visible spectrum.
To improve the CRI of solid-state sources, manufacturers are increasingly turning to a two- or even three-phosphor strategy. A broader range of CCTs and higher CRIs are possible by supplementing the yellow phosphor with one that emits in the red, while even higher values can be obtained with the combination of a red and green phosphor (see Figures 2b and 2c).
The pioneers of the white LED based on a blue-emitting chip – Shuji Nakamura of Nichia and the academic group led by Isamu Akasaki and Hiroshi Amano from Nagoya University – were first to discover that the combination of YAG and a blue emitting semiconductor chip delivered white light with high luminance. To protect their discovery, Nichia filed patents on this combination and in 1996 was granted US Patent 5,998,925. This patent, which describes the use of a phosphor that ‘absorbs part of the blue light and emits a yellowish light’, places restrictions on the use of a blue chip in combination with any garnet composition containing at least one element from: the group of yttrium, lutetium, selenium, lanthanum and samarium; and one element from the group aluminium, gallium and indium and being activated by cerium.
It is worth noting that the Nichia patent does not restrict the manufacture and sale of YAG phosphor, but rather, its application to create a white LED. Consequently, companies that have a
Figure 1: Intematix produces a range of phosphor materials, including those based on YAG, silicates, nitrides and aluminates.
license under this patent may purchase YAG from any manufacturer for use in their LED devices. Nevertheless, the license imposes an extra cost that most LED manufacturers would like to avoid by using phosphors other than YAG.
To address this need, in 2006 we began commercial production of phosphors based on silicate materials. Being one of the early producers with unique compositions, our silicates quickly became widely used, thanks to their high brightness, wide range of available colours, and relatively narrow emission bandwidths – characteristics highly valued by manufacturers of displays, the dominant market for white LEDs. Three years later, we expanded our offering with our own brand of garnet phosphor, which we called NYAG.
In the last few years, we have continued to broaden our product line with a focus on meeting the requirements of the emerging lighting market by launching green and yellow aluminate (GAL) and red nitride products. These innovations have increased the number of high-
June 2014
www.compoundsemiconductor.net 45
performance phosphor families available to LED manufacturers and we continue to fine-tune and optimise each family for brightness and particle size, shape, finish and consistency. We now offer one of the industry’s widest ranges of particle sizes, allowing customers to choose the best size and distribution for a particular application. For example, smaller particles are easier to disperse in silicone and generally give the best angular colour uniformity, while larger particles tend to deliver higher brightness.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108