Compared to incandescent lightbulbs, LEDs produce a lot more lumens per watt of input power — they’re more efficient at producing light. Of course, that means that incandescent light bulbs are more efficient at producing heat, and as the days get shorter, and the nights get colder, somewhere, someone who took the leap to LED lighting has a furnace that’s working overtime. And that someone might also wonder how we got here: a world lit by esoteric inorganic semiconductors illuminating phosphors.

The fact that diodes emit light under certain conditions has been known for over 100 years; the first light-emitting diode was discovered at Marconi Labs in 1907 in a cat’s whisker detector, the first kind of diode. This discovery was simply a scientific curiosity until another discovery at Texas Instruments revealed infrared light emissions from a tunnel diode constructed from a gallium arsenide substrate. This infrared LED was then patented by TI, and a project began to manufacture these infrared light emitting diodes.

But infrared light is invisible to the human eye, and not useful for any sort of indication or lighting. The first visible-spectrum LED was built at General Electric in 1962, with the first commercially available (red) LEDs produced by the Monsanto Company in 1968. HP began production of LEDs that year, using the same gallium arsenide phosphate used by Monsanto. These HP LEDs found their way into very tiny seven-segment LED displays used in HP calculators of the 1970s.

From the infrared LEDs of the early 1960s to the red LEDs of the late 1960s, the 1970s saw orange-red, orange, yellow, and finally green LEDs. There’s a trend to these developments, and it has to do with electron gaps. For a diode to generate light, you must first put energy into an electron. This energy makes the electron jump from its natural state in a valence band to a conduction band. This energy isn’t enough to keep the electron in the conduction band, so it will eventually fall back into the hole it left in the valence band. In doing so, it releases energy back again in the form of a photon.

The more energy it took to move the electron into a conduction band, the more energy that is released as a photon, in the form of higher frequency light. The reason that infrared LEDs came before red LEDs, and green LEDs came after that is that it’s simply harder to climb these bandgaps and find an LED substrate that will emit higher frequencies of light.

Infrared, red, and even green LEDs were “easy”, but blue LEDs require a much larger bandgap, and therefore required more exotic materials. The puzzle behind making a high-brightness blue LED was first cracked in 1994 at the Nichia Corporation using indium gallium nitride. At the same time, Isamu Akasaki and Hiroshi Amano at Nagoya University developed a gallium nitride substrate for LEDs, for which they won the 2014 Nobel Prize in Physics. With red, green, and blue LEDs, the only thing …read more

Source:: Hackaday