Original URL: http://www.theregister.co.uk/2012/12/29/history_of_the_led/

The amazing magical LED: Has it really been fifty years already?

Simply a better way than making wires white-hot

By Dave Wilby

Posted in Vintage, 29th December 2012 10:02 GMT

Next time I hear Coldplay festively crooning "May all your troubles soon be gone, Oh Christmas Lights keep shinin' on," I'd like to think that far from lamenting some lost love, they're paying solemn tribute to the humble but illuminating LED.

The Light Emitting Diode celebrated its 50th anniversary this year. It's easy to forget LEDs have been around for so long, especially as they've become ubiquitous as cheap, energy-efficient sources of light in comparatively recent times.

Take those Christmas lights, for example. LEDs have almost completely replaced the tungsten bulbs we gazed at as children, and for good reason - they're up to 90 per cent more efficient, don't get dangerously hot on our tinder-flammable Christmas trees, don't need to be troublesomely filled with inert gas, and can last for up to 100,000 hours each.

The LED has become one of the most important electronic components in modern technology. The market for general lighting by LEDs is expected to be worth $19.5bn this year and reach $31.4bn by 2017.

General lighting accounts for just 35 per cent of the total LED market, though, with mobile devices - such as smart phones and tabs - accounting for 30 per cent.

Why has the LED proved quite so enduring and how much further can the minature glimmer go?

Any child with an electronics kit knows an LED is a semiconductor that lights up when you put it in a circuit the right way around. Ask why an LED works, and most people will probably mumble something about electroluminescence. But for those who haven't thought about the physics behind these things for some time, here's a very basic explanation of why they an LED makes light.

In each diode, two types of semiconductor material (such as silicon carbide or gallium arsenide) are bonded together. One half (n) is doctored to have extra electrons in it, and the other half (p) to have extra holes where electrons could be. The one-way boundary between the two is called a p-n junction.

When the diode is off, the extra electrons from the 'n' half neatly fill the holes in the 'p' half, forming what is called a 'depletion zone'. There are no free electrons, and charge doesn't flow.

If the diode is placed in a live circuit by adding a battery with its positive end connected to the free electron 'n' side of the diode, and the negative end connected to the extra holes 'p' side, things change. The 'n' side electrons are attracted to the positive electrode, and the 'p' side holes are attracted to the negative electrode.

No current flows because both the electrons and the holes are moving away from each other, and the depletion zone increases. This is the diode in reverse bias mode: it acts as an electronic valve by preventing current from flowing in a particular direction.

When you reverse the connections - by plugging the battery's positive end into the 'p' side and the battery negative terminal into the 'n' side of the LED - the electrons in the 'n' material are drawn towards the depletion zone and meet holes coming the other way from the 'p' half. The diode is now in forward bias mode, current is allowed to flow in this direction, and light is emitted as a result of the electrons and holes combining in the middle of the device.

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Cat's whisker electroluminescence was discovered as a phenomenon as far back as 1907 by Marconi's Henry Round, the first studies of embryonic LEDs were recorded by Oleg Vladimirovich Losev in 1928, and Rubin Braunstein discovered infrared from gallium arsenide at RCA in 1955.

But the "invention" of the LED is generally attributed to GEC researchers Robert Hall and Nick Holonyak in late 1961. At around the same time, Bob Biard and Gary Pittman were making similar breakthroughs at Texas Instruments, as was Marshall Nathan at IBM, and Robert Rediker from MIT.

It was Texas Instruments that secured the first patent the following year though, and the first commercial LED - known as the SNX-100 - was sold in 1962. IBM used them to replace the hot tungsten bulbs in their punched card readers.

The first visible light LEDs were red. Brighter, yellow versions followed a decade later in 1972. Basic blue versions were created in Japan in 1979, but didn't prove commercially viable until the 1990s. Thomas P. Pearsall began developing high-brightness LEDs for use with fibre optics in 1976.

By the 1980s computer companies such as Hewlett-Packard were developing LEDs bright enough to replace the traditional incandescent bulb in applications from digital clocks to car brake lights.

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And that's pretty much where LEDs stayed for about a decade: the preserve of very specific applications and used in electronics kits, but not really entering mainstream or widespread use as we see today.

Arguably the next big inflection point came courtesy of the Nichia Corporation in September 1991, when Naruhito Iwasa identified a production-friendly way of creating bright white LEDs through more advanced semiconductor doping techniques.

By revising his methods Iwasa redefined a 100x brighter blue LED in 1993, followed by a pure green version in 1995. Nichia continued to blaze the LED trail, forging the way for all the super-bright white and colour spectrum LEDs we see around us today.

Following the race to produce LEDs in multiple colours, the key focus of research quickly turned to brightness. The modern success of the LED in consumer and industrial applications can be attributed to the continuing evolution of super-bright LEDs, which some experts have noted appear to follow a kind of Moore's Law - becoming twice as bright every 18 months. Today LEDs are being developed that can produce 250 lumens per watt, and progress is extremely rapid.

As a result, our homes and workplaces can now be lit very effectively with LED bulbs. Every traffic signal in the US is now lit by LEDs. Every light in every new production vehicle is soon expected to be an LED rather than an incandescent bulb. New offices and retail buildings are being designed to be 100 per cent LED lit. And the extreme efficiency of LED lighting is allowing solar charged units to illuminate the developing world without the need for mains electricity.

Other than lighting, the most notable use of LEDs in our lives today is undoubtedly the introduction of brighter, more energy efficient screens. Multiple innovations in organic LEDs (OLEDs), which integrate a film of organic compound as the emissive electroluminescent layer, have helped transformed screen technology.

OLEDs do not require a backlight, so manufacturers have been able to produce screens that are thinner, lighter and of far higher contrast to earlier Liquid Crystal Display (LCD) or plasma designs. Our monitors, televisions, laptops, smartphones and tablets are now capable of delivering a picture quality that makes us grin without forcing us to recharge often enough to make us groan.

Fifty years on, lighting purists criticise LED light for not being "warm" enough, especially white light in a domestic setting, but the truth is that the relentless and rapid evolution of the LED makes it too efficient to ignore.

It's likely that the end price of an LED bulb will go down by half between now and 2020, which along with its impressive energy efficiency gains, boosts the LED's economic and green credentials so much higher than incandescent light that for most markets the LED is now a no-brainer. On average an LED bulb uses upwards of 80 per cent less energy than a traditional 60W incandescent bulb*, costs a tenth of the price to run each year, and lasts 50 times as long.

And, in the world of technology, the applications are seemingly endless. Traditional LEDs are used in pretty much every device indicator panel and remote control we use daily, as well as the disc drives of our computers, media players and games consoles.

What's next? With innovation as rapid as that experienced over the last 50 years, you should keep a (shaded) eye out for: huge billboards, multi-screen video walls, medical implants, cryptic watches, modern art, interactive medical centres, tattoos, the open source satellite initiative, and gestural music ware. Also, keep an eye on groups such as the Li-Fi Consortium, who are working on next-generation optical wireless communications. ®

Bootnote

*Although it may be worth noting that in the winter, when buildings are warmed by thermostat-controlled central heating, the energy which the old incandescent bulbs used to "waste" in the form of heat will now be automatically replaced by the heating running a little harder and more often. However gas for the boiler is cheaper than electricity for the lights, and when the heating is off this won't happen (and the air-conditioning, if present, won't have to fight the lights so hard in summertime). - Ed