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.
Highlights
A precise estimation of the junction temperature is an important issue for all types of semiconductor devices, in general
The light-emitting diode (LED) chip with an active InGaN MQW-layer-structure metallically bonded to a silicon substrate has a square shape area of 880 x 880 μm, a thickness of 170 μm, and is mounted on an Al-based insulated metal substrate (IMS)
From the results of the power spectral density measurements shown in Fig. 4 it can be concluded that the center frequency shifts with the cooler temperature, but does not depend on the applied current pulse width, since all the curves with different tPulse and the same θCool are almost exactly superimposed
Summary
The geometric structure of the chip and the thermal properties of all the implemented materials have to be known with sufficient accuracy for a reliable calculation of the junction temperature and the temperature distribution within the chip. For most of the practical applications the required data (e.g. the thermal conductivity of some materials) is not fully available. In this context, it has to be considered that the properties of a thin layer with interfaces to the neighboring layers may strongly differ from the bulk material’s properties. When the junction temperature and the temperatures at the surfaces of the chip are known, a (reverse) thermal simulation can be applied to estimate the missing geometric or material parameters and calculate the corresponding temperature profile. Whereas a couple of established junction temperature measurement techniques already exist with a dynamic of several dozens of microseconds, a reliable measurement in the range of microseconds and the interpretation of the electrical, thermal and optical effects is still a challenge
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