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Accelerated Life Test of High Brightness Light Emitting Diodes


Abstract:

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 manufactur...Show More

Abstract:

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.
Published in: IEEE Transactions on Device and Materials Reliability ( Volume: 8, Issue: 2, June 2008)
Page(s): 304 - 311
Date of Publication: 10 June 2008

ISSN Information:


I. Introduction

The improving performances of light-emitting-diode (LED)-based application and the worldwide policy on environment care are the main factors involved in the impressive growth of gallium-nitride-based LEDs' market. In the last few years, great efforts have been spent in the improvements of materials and structure in terms of process and reliability. Novel technologies for low defect densities in active region allow a better efficiency, and an efficacy of 110 lm/W at 350 mA has been demonstrated [1]. In addition, the recent development of technologies for phosphor deposition, together with new alloy configurations, allows the design of LEDs with color render index of up to 90% [2]. Finally, the development of processes like flip-chip technology has permitted a better heat dissipation and an increasing external efficiency for chips grown on sapphire.

References

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