Page 108 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

102 Photonic Devices

means that where there was once one laser, now there are about fifty.
This has produced a windfall for component manufacturers. But once
these systems are installed, what next? A business needs to grow
every year in the world of semiconductor devices, because prices of in-
dividual devices are continually declining.
When we were working in the laboratory developing new LED
structures, we would often ask each other what we would have to do
so that everyone in the world would own at least one LED, hopefully
under circumstances that would require periodic replacement. At the
beginning of the 1990s, the best that had been achieved along these
lines was that almost everyone owned an LED in the form of a red
lamp that shows that the television is on. The more fortunate had
LED displays in microwave ovens or CD players. However, the pres-
ence of LEDs in lighting means that everyone will own thousands of
these devices in perhaps the not too distant future.
In the beginning, a light-emitting diode was a p-n junction made
from a semiconductor with a direct band gap. Most of these devices
emitted light at wavelengths invisible to the human eye. These LEDs
have found a home in the remote control of televisions and other elec-
tronic devices. Red, green, and blue (RGB) emitters are needed in or-
der to produce a light source capable of displaying all the visible col-
ors and, of course, white light. In the 1970s it was widely accepted by
intelligent scientists that a blue LED was probably not possible to
make because of fundamental physical arguments about the difficulty
of obtaining useful levels of p-type doping in wide band gap semicon-
ductors. Fortunately, many of these scientists retired after becoming
managers and deciding to stop research on blue LEDs. An unintended
benefit of stopping research on blue LEDs was that people also
stopped remarking that such a device was impossible. The quiescence
in this discussion has permitted a few innovative device engineers to
look at the challenge with fresh ideas and energy. The first commer-
cial blue LEDs made from SiC were demonstrated by Cree Research
in the latter part of the 1980s. At the beginning of the 1990s, new ex-
periments from the group of Isamu Akasaki, then at Nagoya Univer-
sity, showed that efficient blue LEDs and eventually lasers could be
made from GaN. This work is recognized as the critical step that al-
lowed Shuji Nakamura of the Nichia Corporation to move GaN opto-
electronic devices from the list of unobtainable effects to commercial
reality. Now there are blue and ultraviolet semiconductor lasers and
LEDs made from GaN and related compounds.
The new age of LEDs is made possible by more than p-n junctions of
semiconductors like GaAs and InP. Efficient, bright emission is also
achieved using organic crystals and polymers. It is now evident that
polymer LEDs will be formidable competitors of semiconductor LEDs.

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