The "Curious Phenomenon" and History of Light-Emitting Diodes (LEDs)

Our most popular electrical light-emitting device, the incandescent light bulb, was invented by Thomas Edison in 1879. LEDs are much newer, with their first appearance in the scientific world in 1907. Just like many other scientific discoveries, light-emitting diodes were discovered by accident. Silicon carbide (SiC) had just started production in some parts of the world, as the result of a process developed in the 1890s. SiC, known then as "carborundum" to some, was cheap to produce and was popular within the abrasives industry.

In 1907, Henry Joseph Round wanted to see if carborundum crystals could be used as alternatives to vacuum-tube diodes. When he ran a current through the crystalline, he observed a yellowish light. That was the world's first light-emitting diode. Round reported his discovery to Electrical World, describing the "curious phenomenon" of emitted yellowish light from samples of carborundum. He wired 10 to 110 volts through samples, noting their emission of light. Despite his initial discovery of the new technology, Round was unable to make much forward progress with it. Years later, people began using SiC for LED production. Blue LEDs used this material from the mid-1960s until the early 90s, despite its inefficiencies. The best SiC LEDs had an efficiency of only 0.03%.

Oleg Losev of Russia continued work on LEDs in the 1920s, testing whether the emission of light from LEDs was caused by heat. Losev put droplets of benzene on LEDs, finding that they evaporated very slowly. Thus, he proved that LEDs were not incandescent devices like the popular light bulb. Losev also showed that is was possible to turn an LED on and off very quickly, for suitable use in what he termed a "light relay".

The real progress and commercialization of LEDs came in the 1960s. In 1966, silicon-doped GaAs LEDs were developed. These emitted wavelengths from 900nm to 980nm (the infrared range), with efficiencies as high as 6%. Later, Texas Instruments began producing LEDs - the first commercially-available LEDs in the world. These LEDs were also infrared, emitting near 870nm. The cost of these LEDs was $130 per unit. As such, TI did not sell many of these devices.

Visible-light LEDs were also further developed and commercialized in the 1960s. In 1962, visible-light LEDs were created using GaAsP. This combination of materials is still in use today, for low-cost, low-brightness applications. It is popular for red status lights ("power" lights, status lights, etc.). General Electric (GE) offered the first commercially-available visible-light LED, which emitted red light. It cost even more than the Texas Instruments infrared LED, at a staggering $260 per unit. LEDs continued to be extremely expensive to buy until Monsanto began mass-producing LEDs in 1968. In the first two years of production, demand for LEDs was so high that sales doubled every few months. The efficiency of these first mass-produced LEDs was around 0.2%.

Meanwhile, AT&T Bell Laboratories was working on green LEDs. They doped GaP with nitrogen, creating LEDs with an efficiency of around 0.6%. These LEDs are still in use today, in low-brightness applications such as indicator lights and telephones. AT&T Bell wanted to use these LEDs in their telephones - there were huge gains from switching conventional bulbs out for LEDs in these devices. The company could eliminate the phone's 110-volt power cord, and power the entire device (phone and lighting system) using only the phone jack. Since the LEDs had a life expectancy of greater than 50 years in phones, the need for maintenance and repair of the devices was greatly reduced, also. The introduction of LEDs to the telephone market was the first major commercial success of the LED.

The last major LED development of the '60s was the GaN LED, which was designed to produce blue light. This was designed by RCA, because the company wanted to produce a TV that could be hung on the wall like a picture - very similar to today's LED TVs. Green and red LEDs were already available at the time, but nobody had developed blue ones that were commercially viable (SiC was not useful for that application). RCA needed three colors for its design, so they had to design their own blue LED.

The last major development of LEDs was in the '80s, with the Indium LED - more specifically, the AlGaInP LED. This is one of the most common types of LEDs, and it solved one of the biggest issues that the previous generations of LEDs faced - brightness. The other types of LEDs were not bright enough to be used in any applications that would require outdoor use (or use anywhere that was particularly bright). These LEDs were developed in Japan for use with visible-spectrum lasers. They appear now as modern, high-brightness LEDs in a variety of forms, with a variety of sub-compounds. The red R8030 LEDs used in the initial stages of this project were AlGaInP LEDs, as opposed to the low-brightness GaAsP red LEDs.

White LEDs

Keen readers will notice that I have not mentioned white LEDs yet. In the (simplified) history of LEDs above, I have mentioned red, yellow, infrared, green, and blue LEDs, but not white LEDs. That's because white LEDs are a more recent development.

There are a variety of ways to make white light using LEDs. The first is to use red, green, and blue LEDs together to create white light. That could have been done in the 1970s. But that takes a lot of different LEDs, reducing some of the benefits of using LEDs for a given white-light application in the first place. It is also possible to use semiconductors and dyes to achieve "white" light outputs. The most common technique, however, is to use a phosphor. In the early 1990s, researchers explored the use of phosphors in GaInN LEDs that produced blue light. The phosphor acts as a wavelength converter for the LED, expanding its effective emission range from just blue to a whole variety of spectra. Phosphors can be made from a variety of materials, but the most common seems to be a cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor. Basically a phosphor is hit by incoming photons emitted by the diode, and these photons are then re-emitted as longer wavelength light. Thus the phosphor LED outputs a variety of light, with a particular "peak" wavelength that is the diode's natural output.

This is important to know for this application, especially when we take a look at the spectral distribution of white-light LEDs. Below is the spectral response graph from the cool white LEDs (W6030) I tried at the beginning of the project:

The spectral distribution of the W6030 LEDs.
The spectral distribution of the W6030 LEDs.

These cool white LEDs appear to use a phosphor to achieve their visibly white output. We see a "cool" white because of the output spike at around 470nm. However, we see other parts of the visible spectrum emitted, so it's not just a blue LED - it is also emitting green, yellow, and red. But it looks white enough to us!

... now compare that to the spectral output of a "warm white" LED (another LED I used in this project, the WW7030):
The warm white LED has a higher peak (about 650 nm) and a far greater amount of light in other parts of the visible spectrum, which give the LED a "warm" color to our eyes.

Specific-color LEDs have very simple outputs - usually a very narrow range of the visible spectrum. Many do not even cover 65nm total. The graph below is for yet another tested LED, a red R8030:
A very simple graph indeed. This LED peaks at around 630nm, which isn't too hard to tell from the graph. As stated before, single-color LEDs have a very limited spectral output range. This one goes from about 600nm to 650nm, for a useful range of only 50nm.