Thursday 2 August 2007

Light Emitting Diodes

Business India, December 19, 2005-January 1, 2006

Chips of light

LEDs convert electricity much more efficiently into light than say incandescent bulbs or fluorescent lamps

Shivanand Kanavi

Can semiconductor chips, which have revolutionized the way we live, give us light? Yes they can. Such chips for lighting are not made of silicon, which is used in electronics but more complex semiconductors, made of alloys of gallium, indium, arsenic, nitrogen, aluminum, phosphorous etc. They are fast becoming the coolest new technology in lighting.

It has been known since the turn of the century that some semiconductors emit light when a current is passed through them. However it has taken almost a hundred years for the technology to do it efficiently and inexpensively. Most of these semiconductors are what are called direct band gap semiconductors and they have led to the development of semiconductor lasers as well. Inexpensive semiconductor lasers drive your CD player, DVD player or even a laser pointer used during a presentation or even your TV’s remote control.

Semiconductor lasers are also extensively used in high speed data communication from the run-of-the-mill office computer networks called LANs(Local Area Networks) to mighty submarine fibre optic cable networks, like the ones acquired recently by VSNL (Tyco) and Reliance (Flag).

The discovery and perfection of direct conversion of electricity into light has also led to the reverse that is the development of more efficient solar panels to convert light into electricity.

The diodes, which emit light when they are conducting an electrical current, are called Light Emitting Diodes or LEDs. They are already becoming quite popular as Diwali or Christmas lights and in traffic signals. Those green and red light dots that indicate whether the device is active or in sleep mode in your digital camera, camcorder, DVD player and TV are also LEDs.

Compound semiconductors are considered the country cousins of the more flamboyant silicon chips that power our computers, cell phones and all electronics. However, without much ado their optical applications are increasing manifold in every day life.

The first bright LEDs to be invented were emitting red light and later orange and yellow. However attempts at producing green and blue LEDs were not very successful till a Japanese scientist Shuji Nakamura invented a bright blue LED and later white LED in the mid 90s. Nakamura’s work brightened up the whole field and intense activity ensued leading to fast growth. He worked hard with very little funding and repeated disillusionment for several years to come up with the blue LEDs. The company he worked for at that time, Nichia is today one of the world leaders in blue and white LEDs and lasers. A few years back he moved out of Nichia and is currently a faculty member in the University of California at Santa Barbara. While Nakamura works in optical properties of Gallium Nitride and other compound semiconductors his colleague Umesh Mishra researches into the electronic properties of Gallium Nitride to produce high powered transistors for cell phone companies and the US Defence Department. If successful Mishra’s Gallium Nitride transistors will replace the vacuum tubes from their last refuge—high power microwave systems in Radar and communication networks. Together Nakamura and Mishra have built up a formidable team of cutting edge researchers in Gallium Nitride at Santa Barbara.

Yes, all you Baywatch junkies, they also do serious science off the sands of Santa Barbara.

On a more serious note, the technology is evolving rapidly and in the next five years might revolutionise lighting. LEDs for lighting purposes have many advantages. They convert electricity much more efficiently into light than say incandescent bulbs or fluorescent lamps. In fact 90% of the energy is wasted in incandescent bulbs as heat. They also last much longer—upto 100,000 hours. That is more than 12 years of continuous operation! Where as in the case of incandescent lamps it is of the order of 1000 hours and in the case of fluorescent lamps it is of the order of 10,000 hours. They also consume much less electricity hence your batteries in a LED flashlight for example, seem to go on forever. That is ideal if you are in a remote area on your own as in camping, trekking or even a natural disaster. For example Pervaiz Lodhie a Pakistani entrepreneur in Southern California dispatched over 2000 solar powered LED flashlights to Kashmir soon after the earthquake hit the inaccessible Himalayan region. Last year his firm had also sent such items to South East Asia after the killer Tsunami hit the area.

What are the weaknesses of this promising lighting technology in an increasingly energy starved world? Primarily three. One the brightness that is measured in Lumens per Watt of electrical power is still nowhere near the standard required for high brightness lighting. Two, the products are still expensive. For example a decent flashlight costs around $15-40. Thirdly the light is extremely bright in one direction hence a LED light directed towards your work bench or a flashlight works well but if you try to light up your room with it then you end up using too many LEDs.

The US Department of Defence and the Department of Energy are heavily funding research into semiconductors to come up with high power lighting and electronics. As a result the developments are feverish in this field.

Recently, the venerable General Electric, a company that was founded by Thomas Edison to sell the light bulbs he invented, has announced Organic Light Emitting Diodes. In layman’s terms, soon there will be inexpensive plastic sheets, which will light up panels and curved surfaces. Cree Research Inc. a Nasdaq listed leader of LED chips, has produced very bright LEDs (more than 90 Lumens per Watt) two years ahead of industry’s expectations.

“A much less fashionable but critical area to work in, is encapsulation of LEDs” says Rajan Pillai of Nanocrystal Lighting Corporation, a research based start-up from New York. He is referring to the fact that the semiconductor chip is surrounded by a transparent lens capsule which act as a protective cover as well as an out let for light. All LEDs emit light of only one colour. In order to generate white light one introduces substances called phosphors into this casing. These phosphors then absorb the original light from the LED and emit light of different colours. An appropriate phosphor would thus create green light from a blue LED or white light from blue LED etc. Thus, if the phosphor can be improved, then the brightness of the led can be improved. Pillai claims that the new phosphors invented in Nanocrystals Lighting Corporation are smaller than the wavelength of light and hence invisible and that they can increase the brightness by about 20%.

You know when a technology has moved out of the lab and VC firms and into the market place, when you find the products on the Christmas shopping lists of visitors at Walmart and other retail chains. That is what LED flashlights have just achieved this holiday season, just as digital cameras and iPods did earlier.

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