Junction breakdown is a limiting factor in semiconductor devices and can occur through various mechanisms. One of these mechanisms is known as thermal instability/runaway. At sufficiently high junction temperatures, the leakage current and with it the electric power can increase exponentially. In the context of wafer-level reliability, this is a pressing issue because current generations of power devices operate at increasingly high power densities and temperatures that make them prone to this breakdown phenomenon. Described is an attempt to realize a wafer-level High-Temperature-Reverse-Bias (HTRB)-test at the nominal device specifications of an IGBT power device. First, the thermal instability and thermal resistance of the setup were experimentally characterized. Several approaches to the problem were pursued and are documented. One is the reduction of the thermal resistance of the measurement setup using various methods. Another one is the pulsed application of stress bias in order to provide the device with periodical phases of reduced power, allowing the junction temperature to decrease. The reduction of the thermal resistance did not yield a usable solution for the problem. Thus, the focus of the work switched to pulsed HTRB. Here, especially the resulting degradation of the device parameters caused by it as opposed to a constant HTRB was considered. To this end, the parameter degradation was compared for both stress methods using statistical tools. Additionally, the influence of HTRB on the device threshold voltage was studied. It was found that the degradation caused by the constant HTRB cannot be reproduced using the pulsed method. This result was also analysed with respect to the underlying physical degradation mechanism. For the parameter degradation during HTRB, several physical models were considered. A good agreement was found between the data and an existing model based on the drift of ionic charges. The thesis was completed at Infineon Technologies Austria AG in Villach with scientific supervision provided by KAI (Kompetenzzentrum für Automobil- und Industrieelektronik) GmbH personnel.