Thermal characterization of light-emitting diodes in the frequency domain

We report on a method for the measurement of thermal relaxation time constants within light-emitting diodes (LEDs) in the frequency domain. The method is based on the phase shift of the forward voltage waveform with respect to that of the harmonically modulated forward current due to the sensitivity of the forward voltage to junction temperature. The phase shift was shown to exhibit dips at angular frequencies equal to inverse thermal time constants. Extraction of thermal time constants was demonstrated for common low-power and high-power LEDs. The measured thermal time constants (similar to 0.1-100 ms) were linked to heat flows between the LED components. (c) 2008 American Institute of Physics.