Life tests and failure mechanisms of GaN/AlGaN/InGaN light emitting diodes

Our studies of device lifetime and the main degradation mechanisms in Nichia blue LEDs date back to Spring 1994. Following the initial studies of rapid failures under high current electrical pulses, where metal migration was identified as the cause of degradation, we have placed a number of Nichia NLPB-500 LEDs on a series of life tests. The first test ran for 1000 hours under normal operating conditions (20 mA at 23/spl deg/C). As no noticeable degradation was observed, the second room temperature test was performed with the same devices but with a range of currents between 20 and 70 mA. After 1600 hours, some degradation in output intensity was observed in devices driven at 60 and 70 mA, but it was still less than 20%. The subsequent tests included stepping up the temperature by 10/spl deg/C in 500 h intervals up to a final temperature of 85/spl deg/C using the same currents applied in the second test. This work reviews the failure analysis that was performed on the degraded devices and the degradation mechanisms that were identified.