Humidity Effect on the Degradation of Packaged Ultra-bright White LEDs
ABSTRACT Many ultra-bright light-emitting diodes (LEDs), especially the white LEDs, are being actively developed for solid-state lighting and many other commercial applications. Hence, it is important to evaluate and understand the failure mechanisms that affect the performance characteristics and lifetimes of these new LEDs. This study concerns the humidity effect on the degradation of GaN-based packaged white LEDs. Under the accelerated humidity test, the LEDs showed a degradation of optical output. With the mixture statistical distribution analysis method, it is noted that the luminous flux degradation of the packaged white LEDs is dependent on more than two failure mechanisms. Two of the failure mechanisms are observed to follow the lognormal distribution. With detailed spectrum analysis and by employing the parameters extraction method, one of the two failure mechanisms that follow the lognormal distribution is observed to be caused by chip related failure due to the accumulated moisture in the encapsulation. For the other failure mechanism, phosphor degradation is noted to be the primary cause.
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ABSTRACT: Long life, on the order of 50,000–100,000 h, is one of the key features of light-emitting diodes (LEDs) that has attracted the lighting community to this technology. White LEDs have yet to demonstrate this capability. The goal of the study described in this manuscript was to understand what affects the long-term performance of white LEDs. Different types of LEDs have different degradation mechanisms. As a starting point, this study considered a commonly available commercial package, the 5 mm epoxy-encapsulated phosphor-converted (YAG:Ce) white LED. Based on past studies, it was hypothesized that junction heat and the amount of short-wavelength emission would influence the degradation rate of 5 mm type white LEDs, mainly due to yellowing of the epoxy encapsulant. Two groups of white LEDs were life-tested. The LEDs in one group had similar junction temperatures but different amplitudes for the short-wavelength radiation, and the LEDs in the second group had similar amplitudes for the short-wavelength radiation but different junction temperatures. Experimental results showed that the degradation rate depends on both the junction temperature and the amplitude of short-wavelength radiation. However, the temperature effect was much greater than the short-wavelength amplitude effect. Furthermore, the phosphor medium surrounding the die behaves like a lambertian scatterer. As a result, some portion of the light circulates between the phosphor layer and the reflector cup, potentially increasing the epoxy-yellowing issue. To validate this theory, a second experiment was conducted with LEDs that had the phosphor layer both close to the die and further away. The results showed that the LEDs with the phosphor layer away from the die degraded at a slower rate.Journal of Crystal Growth. 01/2004;
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ABSTRACT: We review the degradation mechanisms that limit the reliability of GaN-based light-emitting diodes (LEDs). We propose a set of specific experiments, which is aimed at separately analyzing the degradation of the properties of the active layer, of the ohmic contacts and of the package/phosphor system. In particular, we show the following: 1) low-current density stress can determine the degradation of the active layer of the devices, implying modifications of the charge/deep level distribution with subsequent increase of the nonradiative recombination components; 2) high-temperature storage can significantly affect the properties of the ohmic contacts and semiconductor layer at the p-side of the devices, thus determining emission crowding and subsequent optical power decrease; and 3) high-temperature stress can significantly limit the optical properties of the package of high-power LEDs for lighting applications.IEEE Transactions on Device and Materials Reliability 07/2008; · 1.52 Impact Factor
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ABSTRACT: We fabricated three types of white light emitting diodes (LEDs). The first is the white LED, which has a high general color rendering index (Ra) of 97 and CRI-No. 9 of 96. The CRI-No. 9 denotes the color reproduction in the red region. These values are higher than those of a tri-phosphor fluorescent lamp (Ra = 85 and CRI-No. 9 = 8). The second is the high efficiency white LED fabricated from the small-size high efficiency blue LED chip. The output power (Po), the external quantum efficiency (ηex) and wall-plug efficiency (WPE) of the small-size blue LED were 35.0 mW, 63.3% and 56.3%, respectively, at a forward-bias current of 20 mA. The luminous flux (Φ), luminous efficiency (ηL) and WPE of the second white LED are 8.6 lm, 138 lm/W and 41.7%, respectively. The luminous efficiency is 1.5 times greater than that of a tri-phosphor fluorescent lamp (90 lm/W). The third is the high power white LED fabricated from the larger-size blue LED chip. Po, ηex and W.P.E. are 458 mW, 47.2% and 39.7%, respectively, at 350 mA. Φ, ηL and WPE of the third white LED are 106 lm, 91.7 lm/W and 27.7% at 350 mA, respectively. Moreover, Φ of 247 lm and 402 lm at 1 A and 2 A are obtained, respectively. Φ at 2 A is equivalent to the total flux of a 30 W incandescent lamp. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Physica Status Solidi (A) Applications and Materials 05/2007; 204(6):2087 - 2093. · 1.46 Impact Factor