4.9. LIGHT EMITTING DIODE (LED) 193
have no voltage drop in the forward direction and no leakage current in the reverse direction.
In the presence of a reverse leakage currentIrev, additional loss ofIrevVinoccurs in the free-
wheeling diode when the transistor is ON; and if the forward diode drop is non- negligible,
IoVdiodewill be lost in the diode when the transistor is OFF. Both these losses are important
since in the first caseVinis large and in the secondIois large. It is imperative that in these
applications the device behave as a nearly- perfect diode with Vdiodeand Irevboth being as
small as possible. Furthermore, the diode should switch off faster than the transistor, to reduce
transient dissipation in it. Schottky diodes which are unipolar and have short switching times are
emerging as preferred diodes in free-wheeling applications.
One of the most important applications is in the area of optoelectronic devices. Essentially
all the semiconductor devices catering to optoelectronics use the diode concept. These include
detectors, avalanche photodetectors, optical modulators, as well as light-emitting diodes and
semiconductor lasers. In this section we discuss the operation of the light emitting diode
4.9 Light emitting diode (LED) ...........................
The simplicity of the light-emitting diode (LED) makes it a very attractive device for display
and communication applications. The basic LED is ap-njunction that is forward biased to
inject electrons and holes into thep-andn-sides respectively. The injected minority charge
recombines with the majority charge in the depletion region or the neutral region. Indirect
bandsemiconductors,thisrecombinationleadstolightemissionsinceradiativerecombination
dominatesinhigh-qualitymaterials.Inindirectgapmaterials,thelightemissionefficiencyis
quitepoorandmostoftherecombinationpathsarenonradiative,whichgeneratesheatrather
thanlight. In the following section we will examine the important issues that govern the LED
operation.
We will briefly outline some of the important considerations in choosing a semiconductor for
LEDs or laser diodes.
4.9.1 EmissionEnergy ..............................
The light emitted from the device is very close to the semiconductor bandgap, since the in-
jected electrons and holes are described by quasi-Fermi distribution functions. The desire for a
particular emission energy may arise from a number of motivations. In figure 4.29 we show the
response of the human eye to radiation of different wavelengths. Also shown are the bandgaps
of some semiconductors. If a color display is to be produced that is to be seen by people, one has
to choose an appropriate semiconductor. Very often one has to choose an alloy, since there is a
greater flexibility in the bandgap range available. In figure 4.30 we show the loss characteristics
of an optical fiber. As can be seen, the loss is least at 1.55μm and 1.3μm. If optical communi-
cation sources are desired, one must choose materials that can emit at these wavelengths. This
is especially true if the communication is long haul, i.e., over hundreds or even thousands of
kilometers. InP-based materials are used for these applications. Materials like GaAs that emit at
0.8μm can still be used for local area networks (LANs), which involve communicating within
a building or local areas. The area of displays and lighting is filled dominantly by GaN-based