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Figure 1: Simplified Cross-section of an LED illustrating the
important components associated with degradation.
E N
E C
E V
Photon
Defect
Lens
Encapsulant
Phosphor
Semiconductor Die
Conduction
Band
Valence
Band
- electron
- hole
Figure 2: Band Diagram Illustrating radiative emission and nonradiative
recombination via defects.
compounds. Moreover, they are single crystal structures
relying on epitaxial growth via chemical vapor
deposition for their formation. These layers are grown
on substrates such as sapphire or silicon carbide. Often
they are complex layered structures,-so called “quantum
wells” that carefully manipulate electronic processes to
maximize the conversion of electrical charges into light.
One consequence of these combinations of dissimilar
materials are defects that arise due to mismatches in
the atomic lattice dimensions and thermal expansion
coefficients among the layers. The consequence of
these imperfections are atomic defects in the lattice
structure, both in the bulk of the material as well as at the
interfaces between the different materials. To create light
electrons injected from the majority carrier, n-type doped
layer recombine with holes injected from the p-type
contact within the junction to form blue light. However,
not all electrons and holes recombine to generate light,
otherwise we would have vastly higher performance!
Non-radiative recombination of carriers may happen via
several mechanisms, but the most important from the
standpoint of reliability occurs at these defects within
the semiconductor. Because these defects lie at a lower
energy level than the conduction and valence band of the
semiconductor, they act as a means by which electrons
and holes recombine non-radiatively, giving off heat
instead of light. This energy can be quite large, on the
order of the energy of the chemical bonds and thereby
creates more defects through the displacement or
rupture of chemical bonds. This initiates a “snowballing
effect” that accelerates with time. Figure 2 illustrates a
simplified band diagram of the semiconductor showing
the various recombination processes.
Phosphors. Phosphors convert the 450 nm blue light from
the LED to the various colors of the visible spectrum to
create white light. They do so by absorbing the blue light
and losing a portion of the photon’s energy in a controlled
fashion, down-converting the blue to red, green and blue
over a broad band of wavelengths. These phosphors are
often complex rare-earth silicates or oxides, and may
be doped to ensure specific wavelengths of emission.
While these materials are polycrystalline and already
contain numerous atomic defects, recombination as
described in the previous section is active here too.
In addition chemical processes such as reaction with
water vapor or other chemical compounds can lead to
degradation. Because these effects are highly dependent
on the chemical composition of the phosphors, and the
phosphors used are part of proprietary designs, there
may be considerable variation among LEDs. Often even
within a manufacturer’s product line different phosphors
are used, or they are applied in a different fashion that
results in a particular behavior.
Lens & Encapsulant. The clear lens that acts to collimate
light emanated from the semiconductor die-phosphor
structure and the protective encapsulant material must
remain highly transmitting throughout the life of the
LED. Because LEDs operate at elevated temperature
and humidity, degradation may occur here as well. In
addition, the blue light exiting the LED phosphor also
may play a role in the darkening process. Once more, the
specific chemical composition and structure of the lens
will determine its behavior under normal and adverse
circumstances and is highly process and composition
dependent.
Conclusion. The complex electrical and chemical
processes that occur during the operation of an LED
and give rise to decrease in light output are difficult
to quantify via a simple analytical expression. While a
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