ABSTRACT
Light-emitting semiconductor diodes (LEDs) are light sources that were developed in the last few
decades. For most of this time, they have been used as small indicator lights in a wide range of consumer
applications. Some 10 years ago, two new material systems, AlGaInP and InGaN, entered the LED
arena and gave birth to a new generation of light-emitting diodes: the high-brightness LEDs. This was
an important breakthrough for the entire LED business, which enhanced the prospects of LED use in a
much wider range of applications. Now, with InGaN covering the emission range from blue to green and
AlGaInP from yellow to red, the entire visible spectrum has become accessible to LED light (figure
B1.1.1). Furthermore, the continuously improving material quality, together with better chip and
package designs, have led to much enhanced performances in terms of efficiency and total output power.
Today, at certain wavelengths, LEDs achieve more than 50% energy efficiency in the laboratory and
ones with more than 20% efficiency are commercially available. The internal conversion of electrical
power into light is sometimes close to 100% and the only task left is to extract as much light as possible
out of the semiconductor material without it being lost internally. It can be projected that the good
performance at certain colours will eventually be extended to the entire spectrum and the efficiency of
commercial LEDs is expected to exceed 50%. Another attractive feature of LEDs is their very long
lifetime, of at least some ten thousand hours or several years of continuous operation. Finally, the
availability of highly efficient LEDs covering the range from violet to red has now allowed the
generation of white light and enabled the LED to enter the wide field of illumination and lighting.
