The blue light-emitting diode, arguably the greatest optoelectronic advance of the past 25 years, harbors a dark secret: Crank up the current and its efficiencies will plummet. The problem is known as droop, and it’s not only puzzling the brightest minds in the field, it’s also threatening the future of the electric lighting industry.

Tech visionaries have promised us a bright new world where cool and efficient white LEDs, based on blue ones, will replace the wasteful little heaters known as incandescent lightbulbs. More than a dozen countries have already enacted legislation that bans, or will soon ban, incandescent bulbs. But it’s hard to imagine LEDs dislodging incandescents and coming to dominate the world electric lighting industry, unless we can defeat droop.

In flashlights, in backlights for screens in cellphones and now televisions, and in a bunch of other applications, white LEDs already constitute a multibillion-dollar market. But that’s just a US $5 billion niche compared to the overall lighting industry, whose sales next year should reach $100 billion, according to the market research firm Global Industry Analysts. The trick will be to make LEDs turn electricity into light efficiently enough to offset their relatively high cost—roughly 16 cents per lumen, at lightbulb-type brightness, as opposed to about 0.1 cents or less for incandescents.

Look at the competition and you’d think the job was easy. Today’s garden-variety incandescent bulbs aren’t much different from the ones Thomas Edison sold more than a century ago. They still waste 90 percent of their power, delivering roughly 16 lumens per watt. Fluorescent tubes do a lot better, at more than 100 lm/W, but even they pale next to the best LEDs. The current state-of-the-art white LED pumps out around 250 lm/W, and there’s no reason why that figure won’t reach 300 lm/W.

Unfortunately, these LEDs perform at their best only at low power—the few milliamps it takes to backlight the little screen on your mobile phone, for instance. At the current levels needed for general lighting, droop kicks in, and down you go, below 100 lm/W.

Illustration: Bryan Christie Design

LED Architecture

At the heart of every white LED is a semiconductor chip made from nitride-based materials. The chip is traditionally positioned on top of the cathode lead. Applying several volts across this device makes the chip emit blue light. Passing the light through a yellow phosphor yields white light. Modern, high-power LEDs are variants of this architecture, featuring more complex packages for superior thermal management. Click on image for a larger view.

 

The first-ever report of light emission from a semiconductor was by the British radio engineer Henry Joseph Round, who noted a yellowish glow emanating from silicon carbide in 1907. However, the first devices at all similar to today’s LEDs arrived only in the 1950s, at Signal Corps Engineering Laboratories, at Fort Monmouth, in New Jersey. Researchers there fabricated orange-emitting devices; green, red, and yellow equivalents followed in the ’60s and ’70s, all of them quite inefficient.

The great leap toward general lighting came in the mid-1990s, when Shuji Nakamura, then at Nichia Corp., in Tokushima, Japan, developed the first practical bright-blue LED using nitride-based compound semiconductors. (Nakamura’s achievement won him the 2006 Millennium Technology Prize, the approximate equivalent in engineering of a Nobel Prize.) Once you’ve got blue light, you can get white by passing the blue rays through a yellow phosphor. The phosphor absorbs some of the blue and reradiates it as yellow; the combination of blue and yellow makes white.

All LEDs are fabricated as aggregated sections, or regions, of different semiconductor materials. Each of these regions plays a specific role. One region serves as a source of electrons; it consists of a crystal of a compound semiconductor into which tiny amounts of an impurity, such as silicon, have been introduced. Each such atom of impurity, or dopant, has four electrons in its outer shell, compared with the three in an atom of gallium, aluminum, or indium. When a dopant takes a place that one of these other atoms would normally occupy, it adds an electron to the crystalline lattice. The extra electron moves easily though the crystal, acting as a carrier of negative charge. With this surfeit of negative charges, such a material is called n -type.