Degradation mechanisms of high-power white LEDs activated by current and temperature

This paper presents a study of the degradation mechanisms that limit the reliability of commerciallyavailable white LEDs. Purely thermal stress and biased iso-thermal stress were carried out for several thousands hours on 1W-power LEDs, produced by a leading manufacturer. Results reveal that temperature and operating current have different roles in determining the optical degradation of these devices: (i) pure thermal stress induces a short-term optical power decay, strictly correlated to the decrease in the reflectivity of the package/reflector system and with no effects on the electrical characteristics of the blue chip; the activation energy of thermally-induced degradation is equal to 1.8 eV; (ii) constant current stress induces a long-term degradation process, with a degradation rate which is strongly dependent on the stress current level. In this latter case, optical degradation is ascribed to the degradation of the blue semiconductor chip: details are provided through the analysis of forward voltage and wavelength shift during stress time.