Degradation Mechanisms of InGaN-based High Power LEDs
Date Issued
2009
Date
2009
Author(s)
Chen, Shao-Yu
Abstract
Reliability issues of light-emitting diodes (LEDs) have gathered great importance in recent years because the LED-based technologies are popular and have been widely used in daily life. For researches on reliability of LEDs, burn-in test is typically adopted to estimate the lifetime and performance of LEDs. Through a burn-in test, the devices are impacted by high junction temperature, high current density and high intensity of near-UV radiation simultaneously. As a result, the degradation mechanisms of LEDs are superimposed and hence are difficult to separately analyzed.By applying LEDs under well-designed aging conditions, the heat, current and near-UV radiation induced degradation are separately analyzed. For heat induced degradation, the analyzed LEDs are sent in temperature controlled ovens without current driving. For current and near-UV induced degradation, the LEDs are driven under 350 mA with various junction temperature. The mechanisms of chip level and package level degradation are further examined by considering the variation of optical and electrical properties for LEDs with various packaging styles. In addition, the degradation of silicone, silver paste, Ohmic contacts and current spreading characteristics are separately analyzed.Results of typical and accelerated burn-in tests show the optical output power is decreased over time. Also, the forward voltage, reverse leakage current and forward leakage current are increased. Further analysis show following: (1) High temperature stress can greatly impact the optical and thermal properties of package materials, including plastic leadframe, silicone, silver paste, silver reflectors and YAG phosphor; (2) A great encapsulant (e.g. silicone) is needed to minimize the chemical reaction of package material with atmosphere; (3) high temperature stress can significantly affect the Ohmic contact between p-type GaN and ITO and hence cause the modifications of I–V characteristics in forward region; (4) high current density can induce the increase of reverse and forward leakage current and is related to the variation of energy efficiency; (5) the optical properties of LEDs, including peak wavelength, FWHM and CCT, varied after burn-in tests; (6) the junction temperature and temperature resistance from junction to ambient of LEDs is increased after burn-in tests.To conclude, a well thermal management can enhance the reliability of InGaN-based high power LEDs. We suggest that the operation junction temperature which is lower than 100 ℃ is a reasonable value to minimize the degradation of LED processing and chip. To overcome the near-UV induced packaging degradation, the researches of high durability materials are believed to obviously enhance the reliability of devices.
Subjects
light-emitting diodes
high power LEDs
reliability
InGaN
junction temperature
SDGs
Type
thesis
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