Electrical stressing and self-heating effects on GaN-based LEDs' degradation under extremely low temperature

To investigate the electrical current and self-heating effects on GaN-based LED's degradation, highly accelerated electrical stressing tests at extremely low temperature (-60 degrees C) are conducted. The very large bias current (2A) can make sure the remarkable self-heating inside LEDs. Two groups of LED devices with/without phosphor coating, i.e. blue LEDs and white LEDs, are packaged on ceramic substrates employing the same batch of vertical-structure GaN-based LED chips. The aging duration is 1440hours in total. Contrary to the common sense, for the electromigration effects and p-type GaN barrier generated under high current density, the increase of optical flux and luminous efficiency is accompanied by the increase of reverse leakage currents level of LEDs. The dominant degradation mechanisms in blue and white LEDs are different. For white LEDs, the weakness is the phosphor layer between the GaN-based LED chip and encapsulated silicone lens, where cracks initiate and extend to the lens free surface. For blue LEDs, the weakness is the GaN-epilayer, where electromigration paths forms and leads to increased reverse leakage currents. According to this work, it seems that high power white LEDs have the advantages to blue LEDs working at extreme cold environment, the injection current can be larger than the rated current.