Lighting the 21st century

LED basics

Organic compounds called conjugated organics or conjugated polymers have many of the qualities of semiconductors, including light-emitting properties. The structure of the organic layers and the choice of anode and cathode are designed to maximize the recombination process in the emissive layer, and generate the maximum amount of light.

Unlike conventional incandescent and gaseous discharge sources, LEDs are semiconductor devices that convert electrical energy into a discrete color of light. The original gallium arsenide red LED was invented in the 1960s. Next came different colors such as amber and green. Then in the 90s came high-brightness LEDs, which yielded performance increases and let LEDs cross the threshold from indicator lights to sources of illumination.

LEDs are solid-state diodes made up of p-type and n-type semiconductors. The former have an excess of holes, or positive charges, while the latter have an excess of negative charges. Bringing the two materials together and applying a voltage of the right polarity causes electrons to flow into the p-type material and holes into the n-type material. But this combination has too much energy and is unstable, so some of the energy gets released as photons of light. The specific wavelength, and hence color, of light depends on the difference in energy levels as well as the type of semiconductor material used.

The standard LED chip mounts in a reflector cup of a lead frame and is encased in a solid epoxy lens. Chip packaging determines whether the light beam is narrow or wide.

Over the years, improved manufacturing techniques, packaging innovations, and better semiconducting materials have led to the development of brighter and multicolored LEDs. Once limited to simple status indicators, LEDs now play prominent roles in back lighting, panel indication, decorative illumination, emergency lighting, and animated signage.

Unlike incandescent bulbs that give off the full spectrum of light in a spherical pattern, LEDs emit a focused beam of a single wavelength. New techniques that dope semiconductors with more charge carriers increase LED light output as much as 20 times over earlier generations. Such techniques make possible daylight-visible LEDs in virtually any color of the spectrum. Still, LED production is a tricky process; devices from the same wafer can vary widely in color and light output.

Die manufacturers and packagers are often different companies. Many innovative LED designs depend heavily on die packaging, requiring a lot of cooperation to get an integrated design. Some newer designs use different reflector cups, die geometries, resin materials, bonding techniques, and thermal mechanisms.

The traditional T-1-3/4, or 5-mm, bullet shape is still what comes to mind when most people think about LEDs. But this package has poor thermal performance when used with high-output devices. LEDs can couple more intimately to the housing through use of gap pad materials, thermal epoxies, and convection currents. More recently, copper-based metal core boards have been used to further improve thermal transfer.

In addition to red, yellow, and amber, LEDs now sport colors ranging from green to ultrablue. LEDs even generate white light, long difficult to produce. The approach to generate it employs either a combination of red, blue, and green light, or shines light from a blue LED onto a phosphor layer, which then gives off white light. Indeed, the generation of blue light was a big hurdle for LEDs. The biggest difficulty was identifying a material with suitable bandgap energy. Use of gallium nitride (GaN) helped perfect not just blue but also green LEDs.