The degradation of high power GaN/InGaN blue light-emitting diodes (LEDs) was investigated by considering the electrical, optical and thermal ageing characteristics. The LED samples were stressed at the elevated temperature of 85 °C with an injection current of 350 mA. Changes in the tunnelling current and series resistance for the electrical characteristics and an initial increase followed by a gradual decrease for the optical power were observed. Variations of the thermal resistance in the chip and package were found to be 2 °C W−1 and 0.3 °C W−1, respectively. The responsible factors were proposed to be: (a) the dopant activation and changes of defects in the chip level; (b) the yellowing of the optical lens and structural degradations such as generating voids or delaminations in the package level. The changes in the electrical, optical and thermal characteristics were found to depend on and affect each other. The internal relationship for the characteristics of the three aspects was explained.

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. 70.+f, 85.60.Bt, 85.60.Jb In this paper we report on the thermal analysis of high-power light-emitting diodes (LEDs) as a function of input power. The transient thermal measurement method using structure function was employed to investigate the thermal behavior of LEDs at different working conditions. It was shown that a 5 W LED package installed with four chips exhibited an increase of thermal resistance from 7.06 K/W to 8.83 K/W with the input current ranging from 50 mA to 700 mA at the ambient temperature of 25 ˚C. However, the thermal resistance for a 1 W LED package with one chip was decreased from 20.33 K/W to 19.54 K/W with the input current ranging from 50 mA to 350 mA. Based on our improved LED model, the different trends for the two packages are attributed to the difference in the power dissipated by internal series electrical resistances.

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