Thermal characterization of light‐emitting diodes in the frequency domain

Abstract

AbstractWe report on a method for measurement of thermal relaxation time constants within light‐emitting diodes (LEDs) using harmonic modulation of the driving current. The method is based on the phase shift of the forward voltage waveform in respect to that of the modulated current. The phase shift is due to the sensitivity of the forward voltage to junction temperature, which responds to the modulation of the heat generation depending on the thermal relaxation rate. The frequency dependence of the phase shift was shown to exhibit characteristic dips at angular frequencies equal to inverse thermal time constants. Such an approach for thermal characterization was demonstrated for common GaP, AlGaAs, AlInGaP, and InGaN LEDs. In particular, low‐power p‐n and double‐heterostructure LEDs as well as high‐power truncated‐inverted‐pyramid and flip‐chip LEDs were investigated. The measured thermal time constants (∼ 0.1–100 ms) were tentatively assigned to heat flows within the multilayer structure of the LEDs and collated with the numerical estimates based on thermal resistance and heat capacitance of the LED components. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)