Thesis - Study of the reliability and robustness of GaN power devices under hard and soft switching conditions

The IRT Saint Exupéry* accelerates science, technological research and transfer to the aeronautics and space industries for the development of reliable, robust, certifiable and sustainable innovative solutions.

At our sites in Toulouse, Bordeaux, Montpellier, Sophia Antipolis and Montreal, we offer an integrated collaborative environment composed of engineers, researchers, experts and PhD students from academia and industry for research projects and R&T services supported by technological platforms around 4 axes: advanced manufacturing technologies, greener technologies, smart technologies and methods & tools for the development of complex systems.

Our developed technologies meet the needs of industry, integrating the results of academic research.

* We are a private research foundation, supported by the French government, which funds projects in proportion to the industrial contribution and defines the regulatory framework of the foundation.

Context :

We offer a challenging PhD thesis in a dynamic and multidisciplinary environment. The thesis is supported in the frame of the GANRET (Gallium Nitride Reliability for Trasnsport) project, The aim of the project is to enable the insertion of GaN based power electronics into severe environmental constrains applications. GANRET gathers several partners across different fields from space (ADS, Safran Electronics & Defense, TAS) to aeronautic with (Safran Electronics & Defense, Safran Tech and AIRBUS), including automotive with Vitesco Technology.

The industrial needs are complemented by a rich ecosystem of academic laboratories of test and technology investigation expertise and facilities. In particular the project integrates the national best-in-class scientific offer represented by four public laboratories (AMPERE à Lyon, IES à Montpellier, LAAS in Toulouse, SATIE in Paris) and a private academic laboratory (ICAM in Toulouse).

Description :

Gallium Nitride (GaN) power devices present higher electron mobility, lower capacitance values and lower on-state resistance than their silicon counterparts, leading to overall performances improvement particularly in higher frequencies and higher efficiency. This technology aims to enable the transition toward better efficiency in power electronics conversion. However, the characterization and fine understanding of the reliability of these components remains one of the last obstacles.

The aim of the proposed study is to analyze the impact of hard and soft-switching conditions on reliability (ageing, robustness) of GaN power transistors. The analysis of hard switching could be based on the Double Source Test (DST) circuit. The Ampere laboratory has developed DST benches allowing characterization of power transistors up to 1 kV and 100 A. Different experiments will be performed varying the current, the voltage and the frequency to understand the effect of each stressor. To have a better understanding of the trapping mechanism, the laboratory also has a great experience in Multiphysics modelling and the use of simulation software like ANSYS, COMSOL or ADS. Simulation will be useful to comprehend the component behavior and the related failure mechanisms.

A second part of the thesis activities concerns the investigation of the reliability under soft-switching conditions. In particular, the main objective is to develop a test bench dedicated to soft-switching, allowing to characterize the Device Under Test (DUT) on a wide range of current and voltage stresses and to identify physical phenomena related to this switching mode.

In the literature, a test bench has already been proposed and could be the starting point for this task. Ideally, the proposed test bench will be modular to permit hard- and soft-switching characterizations of the DUT.

To complete the understanding of the switching behavior, other activities will be hold at CEA and at IRT. In particular:

• At CEA, dynamic tests will be performed with the objective to stress the device with multiple switching and therefore extrapolate the aging laws.
• At IRT, the Modified Opposition Method (MOM) will be used to evaluate switching losses with a test vehicle that is close to the real use application (full bridge configuration)

Thus, the collaboration between the three laboratories will make possible to link the static, dynamic, frequency and losses aspects. This will allow an in-depth analysis of the components and a better understanding of failure phenomena.

PhD Tasks :

• Comparison of different testing and characterization methods
• Development of the test benches to achieve the different measurement campaigns
• Design of Experiment (DOE) to compare device performance before and after stress and extrapolate related degradation laws
• Comprehension of the physics of failure

• MSc degree in a relevant area of electrical engineering, or applied physics
• Interest in experimental work and simulation
• Ability to work with experts from a broad range of scientific and technology backgrounds

Applicants may also advantageously have experience in one or more of the following areas:

• A first experience in power devices characterization
• Fluent spoken and written English
• Semiconductor reliability

Involved partners :

IRT Saint-Exupery is one of the eight Institutes of Technology in France aiming to enhance R&D synergy between academia and industry. It fosters innovation by gathering partners around common roadblocks and builds projects on private-public funding basis to tackle them. This is a unique environment where industrial practical needs are directly matched to the scientific body of knowledge available in academia and research laboratories. The IRT Saint Exupery in Toulouse, in particular target this strategic industrial competitive edge in the following key areas: Reliability, green technologies, power electronics.

AMPERE Laboratory is a CNRS joint unit, shared by three other universities in Lyon area: École Centrale de Lyon, Lyon 1, and INSA Lyon. Power semiconductor device studies belong to the Electrical Energy Department. The staff includes 6 academics, an average of about 15 PhD students and 3 Postdoctoral positions. The Ampere laboratory has always had a significant research activity on power semiconductor devices and particularly on Wide-Band Gap Semiconductor, with numerous modelling and characterization tools.

CEA Toulouse has been working for almost 10 years on testing power GaN components and has developed test bench that allowed accurate characterization of dynamic characteristics like dynamic Ron, trise, switching energies. The CEA has also developed industrial tools for testing GaN components in representative quantities that could be very useful for reliability studies.