Reliability and dynamic properties of GaN devices

Nowadays power electronics market is increasingly having the need for high effciency power conversion systems. Due to its outstanding properties in terms of high temperature, high voltage and high frequency capability, GaN material seems to be a valid candidate to provide the solution to the market requirements. In particular, GaN High Electron Mobility Transistors (GaN HEMTs) are promising devices suitable for high voltage and high power applications. In the last years, many works about GaN power HEMTs devices have been published and this confirms the huge interest in this emerging technology. Despite the outstanding performance already demonstrated of GaN HEMTs, market is still wary of this technology due to some still open reliability issues. In particular, many works have been published regarding the reliability of such devices but only few are able to predict the lifetime of such devices working in the final real application with reasonable accuracy. Many failure modes have been investigated in laboratory direct current (DC) and pulsed conditions but only few works try to analyze the behavior of GaN HEMTs transistors in the real application environment. The main goal of this thesis is to investigate reliability of GaN power HEMT devices during dynamic operation in real life conditions. This thesis try to identify the most important aspects that limit reliability of GaN HEMT devices in real life conditions and try to identify the failure modes during real switching operation. Many test concepts to assess performance and reliability of GaN HEMTs working in application conditions are also reported. The thesis starts by explaining the traditional state-of-the art approach used until now to assess HEMTs reliability, then introduce novel measurement concepts that are used to test devices in real switching operation close to the operative conditions of a real power converter. First of all, a novel measurement system able to assess dynamic performance and reliability of GaN devices is presented. As a result, some novel measurements concepts and the related results are reported showing that testing GaN HEMTs in real life conditions is of fundamental importance to assess performance and reliability of such devices. At the end of the thesis a methodology capable to carry out accelerated stress test in dynamic operation conditions has been found, using an external additional capacitor in parallel with the HEMT under test. We think that the concepts introduced in this thesis enable a novel test approach that can lead to the definition of a lifetime model for GaN power HEMTs able to predict the time to failure of devices working in real life conditions.