Page 109 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

Light-Emitting Diodes 103

The range of colors and the level of efficiency are similar, but the pro-
cessing of polymers is much less complicated and much less expen-
sive. Direct screen printing is a leading technology for making poly-
mer circuits. The substrate can be a flexible sheet of plastic, instead of
a more expensive single crystal substrate. There is still a large but
slowly disappearing body of thought that claims that polymers will
never be used for commercial electronic applications because they are
unstable or unreliable, or perhaps another other excuse. Be careful of
such pessimism!
The history of progress in optoelectronics is clear on a couple of
points: this is a field that is marked with dramatic advances by engi-
neers and scientists who do not accept preconceptions of what is not
possible, a field with exciting possibilities for both commercial and
fundamental scientific developments that will define the kind of
world we live in.
LEDs built from semiconductor diodes operate on the principle of
minority carrier injection that occurs in forward bias. Operating con-
ditions for a polymer-based LED are somewhat different: hot carriers
are injected over a barrier where they can recombine with carriers of
the opposite type in order to produce luminescence. All LEDs have the
feature that the emitted light comes from the recombination of elec-
trons and holes. The excess carrier concentrations are created in dif-
ferent ways, but the recombination of excess carriers leads directly to
light emission. Although some of the operating conditions vary from
semiconductor to polymers, the methods of experimental analysis re-
main the same. In this chapter, we will investigate the performance
characteristics of LEDs from an analytical and experimental view-
point. In order to maintain continuity with the presentation on detec-
tors in Chapter 3, we present LEDs in the framework of p-n junctions.
The operating characteristics of primary concern are output optical
power, optical wavelength, efficiency, and modulation bandwidth. The
LED output intensity is proportional to the drive current. It is a direct
quantum conversion of electrons to photons. The optical wavelength
of emission is located near the band gap energy. A primary concern in
high bit rate communications applications is the modulation band-
width of the emitter and the detector. The bandwidth of all electronic
devices depends on both circuit factors [i.e., the resistance–capaci-
tance (RC) product] and intrinsic factors such as carrier transit time
and carrier lifetime. The structure of detectors—low-doping, low-ca-
pacitance diodes—has the result that detector diodes have a natural
advantage over LEDs in terms of the RC time constant. LEDs are by
design highly doped diodes, and they are operated in forward bias,
further increasing capacitance, which will limit the modulation band-
width.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.