Analysis of parasitic effects and reliability issues of Gallium Nitride (GaN) -based devices

The increasingly widespread use of electricity as an energy source with low environmental impact, involves the need to optimize the production, distribution and conversion of electricity with the aim of minimizing waste. About 10% of the electricity is wasted due to conversion losses. In many cases, energy conversion is achieved through switching circuits (converters, inverters...), where transistor performance plays a crucial role in defining overall system efficiency. Silicon has been widely used in electronic circuits since the 50s and this is mainly due to its high availability in nature at low cost. However, devices based on this technology have reached the performance limits dictated by the intrinsic properties of the material, and the challenge has become to find innovative materials for efficient energy conversion. The most promising material as an alternative to Silicon is Gallium Nitride (GaN), a wide energy gap semiconductor epitaxially grown on substrates mainly in Silicon; this guarantees excellent scalability (mass production) and reduced costs, to the disadvantage of the quality of the material. Despite the numerous advantages of this material, the technology based on this is still under development and presents reliability and stability issues. In order, for the Gallium Nitride transistors to be effectively available in international trade and used in the main high-power applications and not only, it is necessary to obtain a clear and deep characterization of the main properties and limits of these devices. This work is based on the detailed study of the physical mechanisms that govern the operation and degradation of devices, with the aim of explaining and modelling the phenomena that limit both their performance and lifetime under the thermal and bias conditions of the final application. The study is conducted through the combined use of experimental characterization techniques and physical model definition. As a result, an understanding of the main issues that afflict GaN-HEMTs and possible improvement strategies from both the materials and structure point of view are provided in order to obtain a performing and reliable technology.