Determination of junction temperature and thermal resistance in the GaN-based LEDs using direct temperature measurement

This paper reports on the experimental methods of the determination of junction temperature and thermal resistance in GaN-based LEDs. For the direct temperature measurement and investigation of thermal distribution on the operating LED chip, nematic liquid crystal thermographic technique was employed. Hot spot was observed and its size was increasing with the driving input power. The initial hot spot with an anisotropic-isotropic transition of 29°C appeared near the cathode region under the drive voltage of 2.95 V and the current of 8.1 mA. The size of the hot spot was increased with input power. Micro thermocouple was embedded into the epoxy package for the investigation of its size effects on thermal behavior. For the specific structure of LED package investigated the thermal resistances were calculated to be 265°C/W and 215°C/W for the low epoxy domed package and high epoxy domed package, respectively.