Rapid Optical Degradation of GaN-Based Light-Emitting Diodes by a Current-Crowding-Induced Self-Accelerating Thermal Process

GaN-based light-emitting diodes (LEDs) rapidly degraded by a current-crowding-induced self-accelerating thermal process were investigated. The LEDs stressed at a dc current of 200 mA and a temperature of 100 °C exhibited a rapid degradation rate of 2.7×10^-3 h^-1, while those stressed at 100 mA showed a relatively slow degradation rate of 3.0×10^-4 h^-1, in which the degradation rate corresponds to a slope of optical output power versus time plots. The corresponding degradation kinetics were also different depending on the stress current, i.e., the generation of nonradiative recombination defects was mainly responsible for the degradation of the 100-mA stressed LEDs, while multiple degradation kinetics including a drastic increase of defects, contact failure, and chip detachment from the lead frame were responsible for the rapid optical degradation of the 200-mA stressed LEDs. The origin of the multiple degradation kinetics could be the current-crowding effect, which causes local heating of the device and hence a self-accelerating thermal process. The rapidly degraded LEDs consistently showed a very large junction temperature from 36 °C (pristine) to 74 °C (failed). These results show that the LEDs should be avoided when current crowding occurs under harsh conditions in reliable lifetime testing and/or for practical use.