Bright sparks

LED lights are long-lasting, use very little electricity and generate almost no heat – everything their incandescent cousins are not. But how do they produce light?

At night I go out on deck and watch the LED Anchor light. It sits up there at the top of the mast doing its work, but asking very little of the batteries. It is a marvel of science. It is also a marvel that I shouldn’t have to climb up there to replace it for quite some time. Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicone carbide and a cat’s whisker detector.

Russian inventor Oleg Losev reported creation of the first LED in 1927. His research was distributed in Soviet, German and British scientific journals, but it was consigned to the category of “useless invention” until the 1960s when the first commercial light emitting diodes (LEDS) were developed. These were dimmer than dishwater and only emitted light in the red frequencies of the spectrum.

Shuji Nakamura invented the first bright blue LED in 1994. This was the first step toward the white LED, which employed a phosphor coating to partially convert the emitted blue light to red and green frequencies creating a light that appears white. This invention also awarded Nakamura and his team the 2014 Nobel Prize for physics.

By the early 2000s LEDS began to appear in the marine industry and they have slowly taken over as the light source of choice for the energy sensitive 12-volt system found on most boats.

HOW IT WORKS

Traditional incandescent light bulbs are really just wires attached to a source of energy. They emit light because the wire heats up and gives off some of its heat energy in the form of light.

An LED, on the other hand, emits light by electronic excitation rather than heat generation. Diodes are electrical

valves that allow electrical current to flow in only one direction, just as a one-way valve might in a water pipe.

When the valve is “on,” electrons move from a region of high electronic density to a region of low electronic density. This movement of electrons is accompanied by the emission of light. The more electrons that get passed across the boundary between layers, known as a junction, the brighter the light.

Manufacturing LED lights follows a basic process:

MAKING SEMICONDUCTOR WAFERS

The raw material of the LED is a crystalline semiconductor and it is “grown” in a high-temperature, high-pressure chamber where gallium, arsenic, and phosphor are purified and mixed together. The heat and pressure liquefy and press the components together so that they are forced into a solution.

After the elements are mixed in a uniform solution, a rod is dipped into the solution and pulled out slowly. The solution cools and crystallises on the end of the rod as it is lifted out of the chamber, forming a long, cylindrical crystal ingot. Each ingot is sliced into very thin wafers before being cleaned and polished smooth.

ADDING EPITAXIAL LAYERS

Additional layers are added to the semiconductor wafer through a process with the frightening name of Liquid Phase Epitaxy (LPE). Epitaxial layers – semiconductor layers that have the same crystalline orientation as the substrate below – are deposited on a

wafer while it is drawn under reservoirs of molten gallium, arsenic, and phosphor. The deposited layers will become a continuation of the wafer’s crystal structure and are several microns thick.

After depositing epitaxial layers, it may be necessary to add additional dopants to alter the characteristics of the diode for color or efficiency. To do this the wafer is again placed in a high temperature furnace tube, where it is immersed in a gaseous atmosphere containing the dopants such as nitrogen or zinc ammonium.

ADDING CONTACTS

Patterns are created on the wafer before contact metal is heated until it vapourises and condenses onto the pattern in vacuumedsealed, high-temperature chamber. The metal contact and the semiconductor bond together chemically. A single 50mm diameter wafer produced in this manner will have the same pattern repeated up to 6,000 times on its surface. The wafer is then cut into small segments called dies.

MOUNTING AND PACKAGING

Individual dies are mounted on a bulb-like structure and the LED is connected with wires. The wires and die are suspended inside a mold which is then filled with epoxy. The epoxy is cured, and the LED package is complete.

The average lifetime of a small incandescent light bulb is 5-10 years – modern LEDS should last 100 years before failure. This makes them suitable for applications where it is difficult or impossible to replace parts, such as undersea, outer space, or the top of my mast. B

“Diodes allow electrical current to flow in only one direction.”