Time dependent and temperature dependent properties of the forward voltage characteristic of InGaN high power LEDs

Abstract

Estimating the junction temperature and its dynamic behavior in dependence of various operating conditions is an important issue, since these properties influence the optical characteristics as well as the aging processes of a light-emitting diode (LED). Particularly for high-power LEDs and pulsed operation, the dynamic behavior and the resulting thermal cycles are of interest. The forward voltage method relies on the existence of a time-independent unique triple of forward-voltage, forward-current, and junction temperature. These three figures should as well uniquely define the optical output power and spectrum, as well as the loss power of the LED, which is responsible for an increase of the junction temperature. From transient FEM-simulations one may expect an increase of the temperature of the active semiconductor layer of some 1/10 K within the first 10 μs. Most of the well-established techniques for junction temperature measurement via forward voltage method evaluate the measurement data several dozens of microseconds after switching on or switching off and estimate the junction temperature by extrapolation towards the time of switching. In contrast, the authors developed a measurement procedure with the focus on the first microseconds after switching. Besides a fast data acquisition system, a precise control of the switching process is required, i.e. a precisely defined current pulse amplitude with fast rise-time and negligible transient by-effects. We start with a short description of the measurement setup and the newly developed control algorithm for the generation of short current pulses. The thermal characterization of the LED chip during the measurement procedures is accomplished by an IR thermography system and transient finite element simulations. The same experimental setup is used to investigate the optical properties of the LED in an Ulbricht-sphere. Our experiments are performed on InGaN LED chips mounted on an Al based insulated metal substrate (IMS), giving a comprehensive picture of the transient behavior of the forward voltage of this type of high power LED.

D5.3 - An Advanced Parametric Thermal Model for High Power LED Modules

“…The respective temperature dependent heat dissipation in the system is determined from characteristic maps and their analytical approximations. These properties of the LED sample are experimentally determined by the pulsed forward voltage method in an integrating Ulbricht sphere [5][6] [9]. Within certain limits, we may assume a constant temperature for the whole LED module equal to the junction temperature.…”

“…For practical reasons, the effective junction temperature together with the forward LED-current is used to describe the performance of the LED. The concept of a modified Shockley equation allows using the forward voltage of the LED to determine this junction temperature with a resolution better than 1 K [9]. For our simulations, we assume a (time independent) complete and unique description of the LED operation by the triplet of forward voltage, forward current, and junction temperature [10].…”