We review the failure modes and mechanisms of
gallium nitride (GaN)-based light-emitting diodes (LEDs). A number
of reliability tests are presented, and specific degradation
mechanisms of state-of-the-art LED structures are analyzed. In
particular, we report recent results concerning the following
issues: 1) the degradation of the active layer induced by direct
current stress due to the increase in nonradiative recombination;
2) the degradation of LEDs submitted to reverse-bias stress tests;
3) the catastrophic failure of advanced LED structures related
to electrostatic discharge events; 4) the degradation of the ohmic
contacts of GaN-based LEDs; and 5) the degradation of the optical
properties of the package/phosphors system of white LEDs. The
presented results provide important information on the weaknesses
of LED technology and on the design of procedures for
reliability evaluation. Results are compared with literature data
throughout the text
Short-term accelerated life test activity on high brightness light emitting diodes is reported. Two families of 1-W light-emitting diodes (LEDs) from different manufacturers were submitted to distinct stress conditions: high temperature storage without bias and high dc current test. During aging, degradation mechanisms like light output decay and electrical property worsening were detected. In particular, the degradation in light efficiency induced by thermal storage was found to follow an exponential law, and the activation energy of the process was extrapolated. Aged devices exhibited a modification of the package epoxy color from white to brown. The instability of the package contributes to the overall degradation in terms of optical and spectral properties. In addition, an increase in thermal resistance was detected on one family of LEDs. This increase induces higher junction temperature levels during operative conditions. In order to correlate the degradation mechanisms and kinetics found during thermal stress, a high dc current stress was performed. Results from this comparative analysis showed similar behavior, implying that the degradation process of dc current aged devices is thermal activated due to high temperatures reached by the junction during stress. Finally, the different effects of the stress on two families of LEDs were taken into account in order to identify the impact of aging on device structure
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