Tobias Kristensen, Microwave Electronics Laboratory

Main supervisor: Dan Kuylenstierna, PhD, Associate Professor, MEL, MC2
Assist Supervisors: Mattias Thorsell, PhD, MEL MC2 and Saab AB, Torbjörn Nilsson, PhD, Saab AB
Examiner: Christian Fager, PhD, Full Professor,  MEL, MC2
Discussion leader: Peter Melin, PhD, Ericsson AB

Abstract:
Gallium nitride (GaN) monolithic microwave integrated circuits (MMICs) are attractive for compact high-power microwave front-ends. Thermal modeling is vital for these systems to ensure reliable operation and predict static and dynamic device behavior. To be able to push the integration density of microwave front-ends further, it is important to understand how the heat generated in a device interacts with other devices integrated on the same MMIC. This thermal coupling can considerably increase the thermal impedance of a device and be a source of low-frequency dispersion effects. Fortunately, the thermal coupling can be reduced by optimizing the layout and packaging of the MMIC, as will be discussed in this seminar.
The thermal coupling is investigated through transient measurements with a small temperature sensor integrated on an MMIC close to a controlled source of dissipated power. This gives a controlled experiment to study how thermal coupling changes with separation and temperature, or due to interaction with the boundary of the MMIC. The experimental results have been replicated in finite element method (FEM) simulations, which are an important tool for designing MMICs. Uncertain material properties in this FEM model need to be calibrated against experimental results to ensure reliable results. Here, it is found that the transient measurement of the thermal coupling is valuable for this purpose, and a calibration method is proposed to isolate the contribution of the epitaxial layers, the substrate, and the die attach layer on the measurement results. Further, a test structure resembling the output stage of an MMIC power amplifier is investigated, and it is shown that thermal coupling is important to describe the dynamic response of a power amplifier. A linear thermal model of the power amplifier is discussed and implemented in a circuit simulator to enable fast simulations with complex modulated signals. The results provide valuable guidelines and tools for designing thermally optimized and reliable high-power GaN MMIC front-ends.

Appended papers 
T. Kristensen, T. M. J. Nilsson, A. Divinyi, J. Bremer and M. Thorsell, "Numerical Modeling of Dynamic Thermal Coupling in GaN HEMTs Calibrated by Transient Measurements," in IEEE Transactions on Electron Devices, doi: 10.1109/TED.2024.3478180.
T.  Kristensen, T. M. J. Nilsson, A. Divinyi, J. Bremer and M. Thorsell, "Dynamic Thermal Coupling in GaN MMIC Power Amplifiers," in IEEE Transactions on Microwave Theory and Techniques, doi: 10.1109/TMTT.2024.3458189.
T. Kristensen, T. M. J. Nilsson, A. Divinyi, J. Bremer and M. Thorsell, "Characterization and Modeling of Dynamic Thermal Coupling in GaN MMIC Power Amplifiers," 2024 IEEE/MTT-S International Microwave Symposium - IMS 2024, Washington, DC, USA, 2024, pp. 1053-1056, doi: 10.1109/IMS40175.2024.10600290.
A. Divinyi, T. M. J. Nilsson, N. Rorsman, T. Kristensen, H. Hultin, S. E. Gunnarsson, M. Thorsell, "On-Chip Sensors for Temperature Monitoring of Packaged GaN MMICs," in IEEE Transactions on Components, Packaging and Manufacturing Technology, vol. 14, no. 5, pp. 891-896, May 2024, doi: 10.1109/TCPMT.2024.3387736.
T. Kristensen, A. Divinyi, J. Bremer, T. M. J. Nilsson and M. Thorsell, "Thermal Transient Measurements of GaN HEMT Structures by Electrical Measurements," 2023 18th European Microwave Integrated Circuits Conference (EuMIC), Berlin, Germany, 2023, pp. 293-296, doi: 10.23919/EuMIC58042.2023.10288814.