posed of different layers. The material used in each layer determines the color of the light, and growing the layers carefully affects the amount of light extracted. To make red, orange, and amber LEDs, LumiLeds uses aluminum gallium indium phosphide, or phosphide for short. For blue and green, LumiLeds uses a newer style of chip made of gallium indium nitride, which is grown on a sapphire substrate.
Two central issues, he said, are light generation within the chip and light extraction from the chip. If the first stage is done correctly, light can be generated inside the chip with nearly 100-percent efficiency (i.e., every electron generates a photon). Light can also be extracted with an efficiency of 100 percent, in theory. In practice today the highest light extraction efficiency is about 50 percent. There are many practical barriers to higher extraction efficiencies, including internal reflection and absorption at the metallic contacts, within the semiconductor layers, or in the encapsulation/package.
General Electric sold the first LEDs in 1962, based on the work of Nick Holonyak Jr., then with General Electric Research laboratories. They were comparatively dim and very expensive, costing several hundred dollars each. Since the late 1960s, when Monsanto and Hewlett-Packard started marketing LEDs in high volume, their light output has increased by about 10 times per decade. The brightest LED today, he said, produces about 100 lumens per watt (lm/W). He demonstrated one of the small, dim, red diodes made 30 years ago and compared it with a much brighter 100 lm/W LED flashlight. “We’ve come a long way in red,” he said.
The value of LEDs can be seen easily in some of their red applications, such as brake lights and taillights. The early Edison bulbs emitted about 2 lm/W; modern incandescent lamps emit around 15 lm/W. When a red brake light is created by putting a red filter over an incandescent bulb, most of the light value is lost, and the output drops to 3 to 4 lm/W. With a red LED, virtually all the light output is functional.
As a result, most of the car manufacturers in the world now use LEDs for their high, center-mounted brake lights. In the Cadillac Deville all the tail lights and brake lights are LEDs, which are not only brighter but can also be mounted in smaller spaces in the car. As a result no space is lost to house bulbs and fixtures. They cannot be seen at all in some applications unless they are turned on. LEDs have also begun to replace neon for signage because of their energy efficiency, reliability, and lack of toxic mercury.
The other successful application discussed at the symposium was traffic signals. The early solid-state models held about 700 LEDs. As brightness improved,