Reliability issues in GaN-based light-emitting diodes: Effect of dc and PWM stress

This paper describes an extensive analysis of the degradation of high-power white LEDs, submitted to dc and pulsed stress. By means of combined electrical and optical characterization we provide experimental evidence for the following: (i) dc stress can induce a significant decrease in the luminous flux of the devices; (ii) degradation rate has a linear dependence on the stress current level, thus indicating that current is a major driving force for degradation; (iii) optical degradation is significantly correlated to the increase in the defect-related current components, and to the red-shift of the emission wavelength of the main blue peak. On the basis of these results, degradation is ascribed to the increase in the defectiveness of the active region, with subsequent generation of defective or shunt paths; (iv) PWM stress determines a stronger degradation with respect to dc stress, for devices aged under similar conditions. The different mechanisms that may contribute to increasing the degradation rate during PWM stress with respect to dc case are discussed in the paper.