Advanced characterisation and failure analysis of GaN HEMT devices and related technologies

Gallium Nitride (GaN) High Electron Mobility Transistors (HEMT) are among the most promising technologies for high frequency and high power applications due to their excellent switching properties thanks to the formation of a two dimensional electron gas (2DEG). However, their integration into complex systems still faces critical challenges in terms of fabrication defects and reliability. This thesis presents an extensive study on the failure mechanisms and on the fabrication related issues of GaN HEMT devices (chapter 1), with a particular focus on real case scenario and advanced characterization techniques of the full in-line process (chapter 2). The first part of the experimental work (chapter 3) is dedicated to failure analysis of packaged GaN devices integrated in antenna modules. A dedicated analysis protocol was developed to identify the failure origin and distinguish between first and second-generation device designs. The methodology revealed a dramatic reduction in failure rates from approximately 35% to below 10% within single wafers, proving the effectiveness of design and process improvements in mitigating failure mechanisms. A second failure scenario is explored in depth, involving void formation beneath the central via hole of QFN packaged devices. A thermo-mechanical hypothesis is proposed: trapped air during resin curing expands upon heating, forming internal voids that compromise thermal dissipation. This leads to a self-reinforcing degradation process ultimately responsible for catastrophic device failure. Complementing these case studies, the thesis investigates the correlation between growth conditions, defect morphology, and electrical properties on the starting substrate (chapter 4). Advanced microscopic and spectroscopic techniques, including Conductive Atomic Force Microscopy, Raman spectroscopy, Scanning and Transmission Electron Microscopy, were employed to characterize the typical crystal defects such as hexagonal and triangular holes (namely V-pits), growth hillocks, and dislocation related features. Overall, this work underscores the necessity of combining failure analysis protocols with nanoscale material characterization to improve the reliability and scalability of GaN based electronic devices.