Bench Talk

LED lights have been a long time coming but are now rapidly replacing less efficient conventional illumination

There is little debate that LED lighting has hit the mainstream with the technology projected to achieve a market share of 84% in the general illumination market by 2030 (saving the equivalent of the total energy consumed by nearly 24 million U.S. homes compared to today’s inefficient lighting, according to the U.S. Department of Energy). But like many emerging technologies, LED lighting has taken a long time to actually emerge – much longer than you might think.

Many people interested in technology might remember Nick Holonyak’s key role in the development of the modern LED. In 1962, Holonyak was a researcher with Bell Labs. He was working with diodes in an attempt to construct semiconductor lasers when he observed that adding more phosphorus to the semiconductor increased its band gap, shortening the wavelength of the emitted light from the infrared to the visible red. Since the light wasn’t coherent (i.e. the photons’ waveforms were not in phase), Holonyak serendipitously succeeded in producing the first red LED rather than a laser.

Holonyak wasn’t alone in his endeavors. In that same year, four research groups in the U.S. (two groups from GEC, one from IBM and another from MIT) reported a functioning LED semiconductor laser based on gallium arsenide crystals. But Holonyak was the first to produce visible light from a semiconductor diode … or so it was thought. However, the history of the LED actually began over five decades previously, with the discovery of electroluminescence.

Electroluminescence occurs when a material emits light in response to an electric current. LEDs comprise a sandwich of n-type and p-type semiconductors. In the presence of a bias voltage the electron carriers in the n-type combine with the holes in the p-type and (with the appropriate choice of band gap) emit photons of a visible wavelength. Altering the make-up of the semiconductor changes the wavelength (and hence color) of the emitted photons.   

Briton Henry Joseph Round is sometimes credited with discovering the “light emitting diode” after his work in 1907. But while it’s true that Round––an assistant of Guglielmo Marconi––sent a letter to the editors of Electrical World reporting a glow from a crystal of silicon carbide, there was no follow-up publication. Round’s work was more about the discovery of electroluminescence rather than an LED as we understand such a device today. Much less is known about the true inventor of the LED, talented Soviet scientist Oleg Vladimirovich Losev. Losev spent his working life as a technician in radio laboratories, before meeting a tragic death at the age of 39 during the Nazi siege of Leningrad.

In the mid-1920s, Losev observed light emission from zinc oxide and silicon carbide crystal rectifier diodes used in radio receivers when a current was passed through them. His paper describing the phenomenon was published in 1927. From then, in a total of 16 papers published up to 1930, Losev provided a comprehensive study of the LED and outlined its applications. You can find out more about Oleg Losev in an article published in Nature Photonics by Nickolay Zhelodev of the University of Southampton, U.K.

Round, Losev and Holonyak’s work paved the way for LED development but there was still lots to do before mainstream LED lights were to become reality. The problem was that LEDs produced a single wavelength of light, and as such worked well as indicators but were of little use for ‘white’ light applications such as illumination. (White light is made up of multiple wavelengths.)

The Japanese took up the challenge and came up with three inventions that made LED lighting practical. First, in 1986, Isamu Akasaki and Hiroshi Amano of Nagoya University in Japan found a way of growing semiconductor crystals on a sapphire substrate and producing a p-type version of the material – both vital steps in the development of high-brightness LEDs. Meanwhile, Shuji Nakamura, an employee of Nichia, found a less expensive way than Akasaki and Amano to produce p-type material. The process enabled the commercial development of the blue LED, a critical component of LED lighting because it produces short-wavelength high energy photons that generate bright luminosity with high efficiency. (You can find out more Asaki, Amano and Nakamura’s work in this paper which celebrates their award of the 2014 Nobel prize in physics for the invention of the blue LED.) Later, Nakamura’s fellow researcher Yasunobu Noguchi invented a phosphor material which Kensho Sakano combined with the blue LED to produce ‘white’ light.

Nichia’s first commercial white LED from 1996 produced a relatively puny output, but by 2003 the company improved the LED’s output by an order of magnitude. Since then, LED manufacturers such as Cree, OSRAM, Lumileds and Seoul Semiconductor have spent millions on improving luminosity and efficiency. Today’s commercial devices boast efficiencies around thirty times that of early LEDs and form the basis of most consumer LED lighting today. (Alternative technology, which combines the light from red, green and blue LEDs to form white light is now making inroads.)

The latest development for LED lighting has taken a different track; lighting maker Philips is now offering Hue ‘smart’ LED bulbs that pulsate to the beat of the home owner’s music via a third-party app. Such antics are uncomfortably reminiscent of the disco booth at a 1990s office Christmas party, but perhaps Millennials, who were spared that experience, will find rhythmic LED lights the hot ticket for 2016. That’s probably not a development that Losev, Round, Holonyak, Nakamura and their collaborators would have predicted for their inventions all those decades ago.