is defined as
the emission of optical radiation (ultraviolet, visible, or infrared light
rays) as a result of a material being subjected to electronic excitation.
emitting diodes (LED's)
are examples of semiconductor applications that employ luminescence to
perform their intended functions, and are therefore grouped together in
Note that luminescence does not encompass the phenomenon of optical
radiation as a result of the temperature of the emitting material, which
is termed as
Since luminescent devices involve photons in their operation, they are
also known as
devices. There are, however, other classes of photonic devices that
do not involve luminescence. These photonic devices, which can
considered as luminescent devices, include: 1)
which are devices that detect optical signals; and 2)
devices or solar cells, which convert optical radiation into electrical
The variety of colors and broad energy range characterizing the radiation
emitted by electroluminescent devices today stem from the discovery of
many semiconductor materials that
are useful for luminescent applications as well as the development of
different techniques for exciting them into luminescence. These
resulted in different devices that emit a vast range of
Luminescence may be classified into four (4) categories, depending on its
wherein the excitation is provided by optical radiation; 2)
wherein the excitation comes from an electron beam or a cathode ray; 3)
wherein the excitation involves fast particles or high-energy radiation;
wherein an electric field or current provides the excitation.
Electroluminescence may be triggered in various ways, e.g., intrinsic,
avalanche, tunneling and injection processes.
interest to this discussion is
which results from injection of minority carriers into the region of a p-n
junction where radiative transitions occur. A
defined as a transition between two states of a molecular entity, with the
energy difference being emitted or absorbed as
absorption and emission can occur in three ways: 1) when an electron
from a filled state in the valence band to an empty state in the
conduction band, in which case a photon is
2) when a photon
of another photon from an electron that transitions from a filled state in
the conduction band to an empty state in the valence band; and 3) when an
electron in the conduction band
to an empty state in the valence band, wherein a photon is also
in the process.
of an electron from the conduction band to the valence band that results
in the emission of a photon is known as
Certain materials, known as
(DBS), have their radiative recombinations converted primarily into photon
emissions, making them efficient for light emission purposes.
materials, the momentum of the electrons at the bottom of the conduction
band and that of the holes at the top of the valence band are equal.
in DBS materials that result in photon emission are highly probable, with the energy of the emitted
the bandgap energy of the DBS.
transitions are unlikely in
(IBS), since their radiative recombination processes involve phonons and
other scattering agents. Thus, IBS materials need to be 'altered' in order
to enhance their radiative processes. This is usually accomplished by
(such as nitrogen) into the IBS to form efficient radiative recombination
centers within them.
or LED's, are basically p-n junctions that emit external spontaneous
optical radiations (infrared, visible, or ultraviolet) under
conditions. Visible Light Emitting Diodes, or visible LED's, involve
semiconductors with energy bandgaps greater than
since this is the minimum energy of light to which the human eye is
sensitive (λ = 0.7 µm). The color of the light emitted by an LED depends
on the semiconductor material used, which is characterized by its own
Relative eye sensitivity,
the measure of the effectiveness of light in stimulating the human eye, is
a function of wavelength, with maximum stimulation attained for λ = 0.555 µm,
for which V(0.555) = 1.0.
The value of V(λ)
decreases as λ
moves away from 0.555 µm to either
side, reaching almost zero at λ = 0.39 µm
and λ = 0.77 µm.
of radiant energy, on the other hand, describes the effectiveness of the
available radiant energy on human vision.
There are three
that decrease photoemission: 1)
photons within the LED material itself; 2)
which occurs when the photons pass from one medium type to another (e.g.,
from LED material to air), causing some photons to be reflected back to
the media interface; and
critical angle loss,
caused by the over-all internal reflection of photons incident to the
surface at angles greater than a critical value defined by Snell's Law.
This critical angle is about 16 degrees for GaAs and 17 degrees for GaP.
are fabricated on substrates of
which is a direct bandgap semiconductor. LED's of other
colors (orange, yellow, and green) are usually fabricated on substrates of
which is an indirect bandgap semiconductor.
can be manufactured using ZnS or SiC.
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