3D assembly of GaN power devices

The increase in electrical power density in everyday uses is the result of technological developments including materials and components. The first element to address is the use of a semiconductor material suitable for strong integration and capable of managing high power densities. Since the 2010s, wide bandgap semiconductors such as SiC or GaN have emerged in several applications and are causing a revolution in power electronics design, notably with an increase in the operating frequency and specific power of converters. Concerning Galium nitride (GaN), the increase in switching frequency was made possible thanks to the HEMT (High Electron Mobility Transistor).
The idea of ??this PhD topic is to work on a HEMT GaN cell assembly. The work will involve the an assembly of two components through a common electrode on their backsides in order, making it possible to reduce parasitic inductances and increase the operating frequency. The work will be based on simulation tools such as COMSOL and Synopsys. The thesis will be in collaboration with the GEEPS laboratory at CentraleSupelec and the University of Paris-Saclay.

Development of integrated GaN functions for electrical energy conversion

The integration of GaN (Gallium Nitride) components in power applications requires
take into account the very fast switching speed of these transistors. Furthermore, if we wish
switching currents of the order of several tens of amperes it is essential to bring them as close as possible
the power element control circuit. This rapprochement can be done in two ways: integrating
the control chip and the power chip in the same package or integrate these two elements on the
same chip.
The best option being the second, it is then necessary to carry out logic functions in GaN
making it possible to design a control circuit which will be inserted between the output signals of a
microcontroller and the GaN power transistor(s).
This development will be based on technological bricks produced at Leti using grid components.
buried MIS (Metal, Insulator, Semiconductor) and will make it possible to promote this technology by showing its
advantages (Switching speed (a few nanoseconds), operation at temperatures
(higher than 150°C)) compared to the state of the art.
Firstly, after an in-depth bibliography on the subject, it will be asked to calibrate and use
models (Spice type) of active and passive components to simulate basic logical functions and
to assist in the complete digital design of the control circuit.
A second part will consist of drawing the set of masks, followed by manufacturing the circuit on
chip integrating the control function as well as the power transistors.
In the following, it will be asked to electrically characterize the circuit in an environment close to a
real application. This step will be followed by an improvement pass in order to make it more reliable and increase the
robustness of the circuit in a wide range of temperature and frequency.

Numerical and experimental studies of an ejector designed for a cold or heat production cycle

The ejector has been the research subject in the literature as the main component of refrigeration cycles using “thermal compression” thanks to its simplicity without moving parts. It uses a high-pressure fluid called “primary fluid” to drive and compress a low-pressure fluid, which is called “secondary fluid”. The performance of the ejector is defined by the entrainment ratio, which is the mass-flow ratio between the secondary and primary flows; as well as the critical pressure, which limits the operating range of the ejector. Most of the numerical and experimental studies have been conducted on water vapor ejectors. The studies showed that the geometry optimization is crucial in order improve the ejector performance. Moreover, experiments showed that the flow inside an ejector is often supersonic and highly compressible therefore inducing strong pressure variation. This can induce strong temperature variations and the apparition of liquid water and ice in ejectors have already been witnessed.

Numerical studies carried out previously have shown the importance of accurately modeling the liquid-vapor phase changes in order to establish consistent and accurate numerical models for flows hydrodynamics within the ejector. However, these studies give little or no consideration to the temperature field distribution within the ejector. The main difficulty here are the huge pressure variations that happen inside the ejector which lead to liquid vapor phase changes in a highly compressible flow. In this PhD project, we aim to investigate innovative solutions with ejector integrated into thermodynamic cycles working with natural fluids (ammonia, water, CO2 …) in order to improve the global performances. For this, it is important to understand the local physical phenomena of the flows inside an ejector, especially the impact of liquid-vapor phase change as well as the impact of the operating conditions.

Based on the strong research background of both CEA and INSA Lyon, we will conduct numerical and experimental works about the ejector and the thermodynamic cycles with the following research plan:
* Numerical work:
_ Development of a 1D model and perform the CFD simulations for comparison
_ Modelling and simulations of the identified thermodynamic cycles integrated the appropriate ejector
_ Design of ejector for tests
*Experimental work : fabrication of test ejector and perform measurements for model validation and analysis

For more than 15 years, CEA has conducted extensive research on thermodynamic cycles in order to develop innovative solutions for production of heat, cold and electricity. Recently, we have developed a new model of ejector for integration into a thermodynamic cycle . To bring new insight about the local phenomena of the flows inside an ejector considering the liquid-vapor phase, we have investigated and performed CFD simulations. INSA Lyon has strong research background on the topics related to CO2 such as heat pump cycles, heat exchangers as well as ejector. The test bench of ejector at INSA Lyon together with the INES platform at CEA will be served for the experimental work of this project.

Advanced modeling of Gas Diffusion Layers for Fuel Cells: ink impregnation and drying, 3D phase distribution, and effective properties

In the frame of advanced H2 solutions for the energy transition, the Proton Exchange Membrane Fuel Cell (PEMFC) is a relevant solution for the production of low-carbon electrical energy. The European Project DECODE proposes to develop a fully digital chain of design tools, including raw material properties, manufacturing and assembly of the different components, to predict the performance of such ‘virtual’ stack. This will help reducing the development cost and time of improved materials/components suitable for different applications in the future.
The component considered in this thesis is the Gas Diffusion Layer (GDL), which is a combination of a fibrous microporous substrate and of a micro/nano porous layer (MPL for microporous layer). The work will be split into different steps: a) based on (real or virtual) 3D images of the substrate, simulation of the hydrophobic and MPL coating and drying to derive the 3D distribution of the components (fibers, hydrophobicity and MPL); b) simulation of single and two-phase transport properties of the GDL to supply inputs to upper scale performance models; c) sensitivity analysis of the main manufacturing processes (ink properties, drying parameters…)

Raw earth soil, an age-old material with new emerging uses

Raw earth materials, which have found multiple uses for millennia, now offer considerable potential for helping to adapt to the changing climate, thanks to their natural ability to regulate heat and water, as well as their low-CO2 production and shaping. However, scientific advances are still needed to get a more precise understanding of these materials, up to the nanometric scale.

This thesis focuses on the link between the mechanical properties of raw earth soil materials and their nanostructure, emphasizing the roles of confined water, ions and organic substances. Two approaches, based on the expertise on nanoporous media developed at CEA, Saclay and Marcoule, will be followed: the analysis of old materials using spectroscopic and radiation scattering methods, and the development of a screening protocol to identify physicochemical parameters important for durability. This research, which ultimately aims to optimize the formulation of raw earth materials, will be carried out in collaboration with architects specialists in the field.

Non-invasive characterization of power circuits by near-field probes

The optimization of power modules is made complex by the parasitic elements of the circuits (inductances, capacitances, resistance) which, when subjected to switching of high currents and voltages with high speeds (di/dt, dV/dt ), cause overvoltages or current oscillations which can be damaging to the system and components (accelerated aging, early failures, breakdown, thermal runaway, etc.)

The interest of the proposed PhD is to go beyond the usual methodology aimed at using probes or sensors (invasive and therefore which disrupt the circuit that we seek to characterize) by developing a non-invasive method ("Near Field Scanning » or NFS) making it possible to map the electric and magnetic fields near circuits and components (resolution GHz). This field mapping is thus an image of the currents and voltages of the circuit, which will be necessary to process by inverse physical modeling, in order to go back to the real currents and voltages in the circuit. The PhD work therefore aims to develop and implement a hybrid approach which aims to couple a near-field characterization to 2D or 3D electromagnetic simulations in order to de-convolve the measured signal and provide information allowing the different current paths to be evaluated.
The longer-term prospects are to set up a non-invasive characterization tool coupled with simulation, in order to be able to characterize power modules and more generally power circuits by measuring their currents and voltages, even electromagnetic emissions (EMI for “Electro-magnetic Interferences”) which is another major problem in power circuits.

This PhD will take place at CEA Grenoble, within a mixed team from CEA-LETI and CEA-LITEN bringing together experts in power electronics.

Références :
- C. Lanneluc, P. Perichon and D. Bergogne, "DC-Bus capacitors influence in a GaN Motor Drive Inverter," PCIM Europe 2019; International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management, Nuremberg, Germany, 2019, pp. 1-8.
- S. Serpaud, A. Boyer, S. B. Dhia and F. Coccetti, “Performance Charaterisation of the Dec Capa Network using the NFS Measurement”; EMC-Europe, September 2023, Krakow, Poland.

Deployment strategy for energy infrastructures on a regional scale: an economic and environmental optimisation approach

The CEA develops a software to optimize dimensioning and control of energy systems, in order to conduct tec-eco studies, including an environmental part, for industry and territories. The optimization is run by a MILP solver.

We want to go further by optimizing the deployment of infrastructure over time and space. Indeed, changes in demand, economic environment, and technological performance need to be taken into account from the beginning of an energy system deployment. The spatial dimension is also important, to make the good choice between centralizing production to make economies of scale, or dispatch the production resources across a territory and ensuring transportation.

Addressing these broader issues leads to more complex calculation with higher times of resolution.

The goals of the PHD will therefore be as follows:
- Establish a generic formalism to describe this type of problem and make it easily modelable, taking into account economic and environmental aspects, as well as the associated uncertainties.
- Compare, select and improve methods of optimization and artificial intelligence allowing to deal with the complexity of the problem.
- Apply this algorithm on concrete case studies.
We are looking for a candidate with a background in applied mathematics. They should be interested in the energy transition.

Innovative electrical characterization of diamond power components for power electronics applications

In a context where the demand for clean and efficient energy is constantly growing, power components are essential. Diamond transistors, with their wide bandgap, offer significant advantages over traditional silicon-based technologies, including superior efficiency, increased power density, and better performance at high temperatures. They are ideal for energy converters, battery management systems, and high-frequency switching devices. The objective of this thesis is to characterize the electrical properties of diamond as a function of temperature to anticipate its behavior in future power applications. This research, utilizing the platforms of CEA Occitanie and LAAS-CNRS, aims to evaluate component losses, control techniques, and self-heating effects, while addressing thermal management and packaging through 3D ceramic structures. The results could transform energy conversion technologies and systems requiring robust and efficient components at high temperatures.

Study of innovative MOS gate stack for energy efficient SiC power transistors.

Silicon carbide (SiC) components represent the future of power electronics, surpassing silicon technologies in terms of temperature tolerance and power handling capability. At the heart of this evolution, CEA Leti plays a key role in the development of these new generation components, essential for applications such as electric vehicles, charging systems or photovoltaic installations. Our teams provide support to major manufacturers European countries by establishing pilot production lines for GaN and SiC components, as well as by developing SiC substrates with our industrial partners
This thesis aims to develop innovative technological approaches for the design of SiC MOS transistor gates and to evaluate their environmental impact to inform our technological choices. You will deepen the understanding of the physics of SiC MOSFETs by examining different gate architectures. and in
identifying the physical processes that restrict electron mobility in a SiC MOS channel. Questions such as the influence of stress on mobility and the possibility of separating carrier mobility from threshold voltage will be at the heart of your research.

Electromagnetic compatibility converter optimization with wide band gap devices for frequency increase

Power Electronics systems aims at converting one shape of Energy into another. From very low power, such as USB power delivery (hundreds of watts) to high power, like electric vehicle drivetrain (hundreds of kW, power scale is large. Power Electronics systems are everywhere, this is why such systems must be optimized to be efficient, compact, profitable and with an optimized environmental footprint.
The emergence of "wide bandgap components" (WBG) revolutionize the efficiency of power converters along with their power density. However, WBG components generate more disturbances in the grid, and could possibly affect even degrade the operation of systems in the immediate environment. There are standards to regulate the amount of disturbances allowed to go outside of the converter environment. The committee in charge of the electromagnetic compatibility aspect is called "the International Special Committee on Radio Interference" (CISPR).
On the contrary, filtering technologies did not evolve as fast as WBG components and the main difficulty today to turn a proof of concept into an industrial product is to meet the electromagnetic compatibility standards.

The objectives of the PhD are the followings:
- Topology Study of different types of filter (passive and active)
- Study and characterization of magnetic material, their design,, efficiency in function of the frequency etc.
- Propose filtering solutions (materials, integration, modulation etc.)
- Skills improvement on EMC modelling with a given "use case", and take into account, parasitic elements, couplings by radiation along with material performances.

The PhD should give answer about frequency increase, depending on the use case, based on WBG components and relevancy of such technology on applications with strong constraints (EMC, cost, volume, and efficiency).

This PhD will take place at CEA Grenoble, in the System Department, in the laboratory of Electronics for power and Energy. CEA environment is very transversal; the candidate will be able to feed his researches with engineers from the system department. The candidate, after electrical habilitation, will have access to the laboratories of power Electronics to make his experimental testing.

The candidate must have skills in power Electronics, in particular the topologies of power converter, components (active and passive) and attractions for modelling complex systems thanks to finite elements software and experimental testing. The candidate must have analytical and critical skills and propose scientific experiment during the PhD.