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Investigating The Curious Case Of The Blue LED

May 5, 2011
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Welcome to our second annual One Bright Issue. We’ve gathered a group of articles that cover LEDs and solid-state lighting (SSL) as well as the semiconductors that support these applications. Yet I’d like to talk about a special LED here, one that has all the makings of a Hollywood movie.

The intrigue began when Nick Holonyak Jr. invented the first practical LED while he was at General Electric in 1962. The first LEDs became commercially available in the late 1960s—red ones. Green LEDs showed up in the 1970s.

Blue LEDs didn’t appear until the early 1990s and didn’t become commercially available until the mid-1990s. Everyone was happy to see them finally arrive, though, since red, green, and blue together can create any color of light, but most importantly white.

Wearing A Disguise

Although a white LED can be created by combining red, green, and blue LEDs, there’s another way to do it. Just put a mask over the blue LED—disguise it. Since blue plus yellow makes white, you can make white LEDs by adding yellow-emitting phosphors on top of the materials that make up the blue LED. The phosphors absorb the blue light from the chip and then re-emit it as white light.

I thought it was interesting when I illuminated my white ceiling with a with a device I purchased from Radio Shack for nighttime reading in bed. It had a single white LED. But when I pointed the light at the ceiling and thus enlarged it, I could see a ring of blue on the outer edge of the circular illumination pattern—there you are, blue LED.

Apparently, this technique comes at a price. When I spoke with Jason Hartlove, president and CEO of Nanosys, he explained a new technology the company had created for backlighting LCDs—currently an important job for white LEDs. With the Nanosys technology, the light from blue LEDs passes through a quantum rail.

One benefit of this technique is lower cost, since blue LEDs cost less than white LEDs. Why? White LEDs have a licensing fee associated with them due to patents that combine a blue LED with yellow phosphor.

Not only does the blue LED come late to the party and then disguise itself as a white LED to increase its popularity, but eventually these white LEDs are taken to task for not being all that everyone expected them to be.

White LEDs are a key ingredient of LCD backlighting and low-power applications like flashlights and reading lights. Since they worked so well in these applications, people started thinking that maybe they should be used to replace incandescent and fluorescent lighting. Yikes.

A Tragic Flaw?

While the world waited, scientists found out that the blue LED had a tragic flaw—droop. Richard Stevenson famously pointed this out in an article he wrote in the August 2009 issue of IEEE Spectrum called “The LED’s Dark Secret.”

Stevenson said that if you crank up the current of a blue LED, efficiencies will plummet. Of course, this news was not good, since the average person expects a blue LED to be flawless, provide awesome light output under any circumstances, and last forever to boot.

The article drew a truckload of comments, mostly positive. But Duilio Passariello alerted Stevenson to a comment on the SSL Pioneers LinkedIn group noting that Stevenson used facts that were quite old in his article. Half of what Stevenson said was true five years ago, but not today, the comments added. So the plot thickens.

I headed over to SSL Pioneers and joined the group, but it was mostly marketing guys poo-pooing Stevenson’s claims. Passariello himself wanted some science to back up their comments. I took a look on the Web myself and found that science is indeed making progress on the droop problem.

Stevenson all but admitted this himself in a January 2011 article he wrote in IEEE Spectrum called “LED Lighting: Blue + Yellow = White” hailing the advances made by Osram Opto Semiconductors, Cree, Philips Lumileds, and Nichia.

Meanwhile, another paper published by Compound Semiconductor called “Triple Layer Barriers Combat LED Droop” is based on research by a partnership led by Samsung LED Co., which involves contributions from researchers at Pohang University of Science and Technology, Korea, and Rensselaer Polytechnic Institute (RPI), New York.

So, it’s probably a safe bet that blue LEDs will come through in the end for a perfect Hollywood ending.

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