The LED light has gone from a little-considered technology to a true lighting revolution in just a single century. While some visionaries saw LED lighting as the eventual successor to incandescent fixtures, not even the most ambitious expected it would happen so soon. And even decades after its invention, its role as a light source was discounted by most engineers. However, this belief would be overturned quickly as the LED was improved upon again and again, soon leading to a light that was unmatched in efficiency and reliability.
Now, LEDs are considered a major element of green power initiatives, and according to the Department of Energy, current outdoor municipal lighting costs taxpayers $10 billion every year. The adoption of LED lighting could reduce this cost by as much as half, eliminating a huge portion of energy waste, much of which is generated by polluting coal and gas plants.
In fact, this is the intention behind the Presidential Challenge for Advanced Outdoor Lighting, a federal initiative that will work with cities to triple the amount of lighting poles using LED sources. The Department of Energy suggests that by adopting LED lighting and improved lighting controls, cities may cut the amount they spend on street lighting by up to 70 percent.
But that’s the future. Let’s start at the beginning, back in the early 20th century, when LEDs were little more than a novelty.
Light and Electricity: An Intimate Relationship
More than a century ago, in 1907 to be precise, one of the most important concepts in optics was discovered. That concept was electroluminescence, and it was detected by H. J. Round, an experimenter from Britain. Electroluminescence is the term used to describe the light given off by a material when it is exposed to an electrical current or field. It’s not, however, a phenomenon singular to LEDs, as it can be used to describe any material, such as phosphor powders, that lights in the presence of electricity.
Alone, electroluminescence can be used to create a variety of lights, but it is limited by poor efficiency and output, so it is only used in a handful of applications, including automobile dashboards and nightlights.
But when the principle is adapted for use in a fixture optimized for light output, the results can be impressive.
Semiconductors: A Foundation of Modern Electronics
Twenty years after the discovery of electroluminescence, Oleg Losev, a Soviet inventor, created the first LED. Losev produced his invention after observing how certain diodes installed in radio sets would illuminate during function. Most of his work was focused on the conductive properties of silicon carbide, which was freely available at the time in the form of an abrasive. Today, it is still found in LEDs that produce light in the blue range.
While Losev’s work was freely distributed among Soviet and western scientists, no practical use was found for the LEDs right away, which pushed his work to the side for decades. It wasn’t until the 1950s when engineers were ready to make further use of the technology.
Further advancements in the field were largely a byproduct of researching semiconductors. In the late 50s and early 60s, several companies, including General Electric, Bell, Texas Instruments and RCA, were on the hunt for better semiconductors. Semiconductors are a critical element of modern electronics, and represent a categorical upgrade over previous circuitry components, such as vacuum tubes.
The LED: An Incidental Invention
In 1961, building off of Losev’s work, James Baird and Gary Pittman with Texas Instruments produced the first infrared LED. As an infrared diode, this early LED did not produce light visible to the human eye, but it produced light all the same. Baird and Pittman’s work with gallium arsenide semiconductors was the lynchpin in the infrared LED, and gallium arsenide semiconductors are still found in many products, including solar cells, optical windows and circuits operating at microwave frequencies.
The first LEDs, though not capable of providing enough light for illuminating a space, were extremely useful in optical communications. For example, the infrared LED replaced tungsten bulbs installed in punch card readers as a way to deliver optical information. The LEDs were patented and sold by Texas Instruments at $130 each, which was a lot of money at the time. Their superior lifespan and efficiency, though, made the investment worth it.
Just one year after Baird and Pittman’s invention, Nick Holonyack created the first red LED at General Electric. This breakthrough also represented the first visible LED in existence, though Holonyack wasn’t trying to invent a visible LED. Instead, he was attempting to produce a semiconductor laser, which many GE researchers were working on at the time. And though he was just a few weeks behind a team that built the first infrared laser, he foresaw a future where his visible LEDs would become a mainstay in the lighting industry.
Holonyack’s suspicions about the future of LED lighting start to take shape a decade later, when George Craford of Monsanto invents an LED that emits yellow light. Craford’s LED was built off of a gallium arsenide phosphide semiconductor, which is the same semiconductor Holonyack used with his red LEDs.
In 1978, LEDs intended for display purposes were finally ramped up into full commercial production. At first, they were only used in machine displays and as indicator lights, but eventually found their way into a variety of consumer products as well.
Still, LEDs were a ways off from full commercial and residential lighting. The primary challenge was still the color of the LEDs, along with their output. Violet LEDs made from gallium nitride and magnesium were invented in 1972, and in 1979, Shuji Nakamura produced the first blue LED also using gallium nitride.
Nakamura is considered by many to be the mastermind behind modern LED lighting, as his work to create a bright blue LED, which finally paid off in 1994, paved the way for the LEDs used in today’s lighting applications. Nakamura’s bright blue LED was made possible with the use of sapphire as the substrate with a gallium nitride semiconductor. In the early 2000s, the switch to silicone substrates and indium gallium nitride semiconductors made commercial production relatively inexpensive and significantly ramped up output.
White LEDs owe their existence directly to the invention of bright blue LEDs. By applying a phosphor coating to bright blue LEDs, some of the blue light is absorbed and emitted as yellow light through fluorescence. This precise combination of blue and yellow light is perceived by the human eye as white, though by altering the amount of phosphor and light output levels, it’s possible to create green and red light as well. White light can also be created by combining this green and red light with the LED’s blue light, and this approach ensures superior color rendering as well.
Now, modern LEDs are undergoing a constant cycle of improvements, usually spurred on by better combinations of substrates, semiconductor materials and doping techniques. This march of progress has doubled the output of LEDs many times over, about once every 36 months, according to findings published in Nature Photonics, a scientific journal chronicling the laser science and optoelectronics disciplines.
This doubling motif is similar to Moore’s Law, which states that the number of transistors installed on integrated circuitry doubles every couple years. Moore’s Law predicted the extremely fast rise of computing power in today’s computers, and so it’s possible to imagine a near future where LEDs form the bedrock of the lighting industry.
The Future of LEDs
For many lighting experts, the future is already here, with LED lights that outmatch other fixtures in nearly every conceivable way. The only significant hurdle left to overcome is manufacturing costs, which still exceed the costs involved in most other lighting technologies. This price, though, is falling all the time, and some manufacturers claim to have produced LED fixtures that can produce 1,000 lumens per dollar spent.
The falling costs, superior efficiency, wide spectrum of colors, small size and extended life are gaining traction in the commercial and residential sectors. For instance, several MLB stadiums, including Safeco Field and Minute Maid Park, have implemented LED stadium lighting. This is considered a watershed moment for many members of the lighting industry, and a sign of what’s to come.
The Science of Semiconductors
As semiconductors are at the heart of what makes an LED work, they deserve further examination. The exact science behind semiconductors is complex, reaching down to the quantum level, but the high level principles are easier to visualize.
First, what exactly is a semiconductor? In short, it’s a material that expresses a few uncommon electrical properties, and these materials are present in the natural world or can be fabricated at the molecular level.
These unique electrical properties include:
Greater electrical resistance, existing between true conductors and insulators.
Reduced resistance as the material is heated, which is the opposite of most other materials.
Chemistry that can be easily altered to manipulate the semiconductor’s electrical properties further.
This last point is of particular importance because it is how engineers create semiconductor devices. To do so, the semiconductor is “doped,” or altered at the atomic level with impurities that create electrical imbalances at “junctions.” In short, the doped impurities may be atoms (such as arsenic or phosphorus) that contain an extra electron in their outer valence than the atoms in the semiconductor. This means there are suddenly a lot of free electrons floating around, ready to be snatched up and bonded to by something else. This is known as N-type doping. The other type of doping is P-type doping, which is when a material with one fewer electron in its outer valence (such as boron), is placed in the semiconductor. This extra “hole” puts pressure on other electrons to bond with it.
Put P-type and N-type materials close enough together, and the resulting junction is a hotbed of electrical activity, as the electrons jump through the semiconductor to fill the holes on the other side of the junction. This movement of electrons is harnessed into creating light through various means, as atoms give off photons once they are stabilized with the addition of an electron. The precise color that’s emitted depends on the exact materials involved in the process, as electrons release higher frequency light (such as greens and blues) if they drop from a high energy state to a low energy state quickly. This requires precise calibration of the electrons in the material, and is why red and yellow LEDs were possible before blue LEDs.
Again, it’s complex science, but it’s been fine-tuned to the extent that semiconductors are incredibly precise, down to the atomic level. That’s what makes them so much more efficient than other lighting technologies, which rely on producing light through heating up material (such as metal filament lights) or by exciting gasses (such as fluorescent lights).
In all, that’s pretty impressive for a humble, dim light that was considered an accident even by its creators. One might say that the future is rather bright for LEDs.