The post-doctoral subject focuses on studying thermal effects and electrical performance in FDSOI MOSFET transistors down to very low temperature for cryogenic applications, such as quantum computers and space applications. The goal is to model and characterize STMicroelectronics' 28FDSOI technology down to 4K and below, concentrating on self-heating and its impacts on circuit performance. The work includes DC and RF measurements, studying the back bias effect, exploring thermal couplings, and associated modeling. The project also aims to integrate these models into a 4K-valid Process Design Kit (PDK) to optimize circuits operating at very low temperatures. This work is part of the IRT Qloop project, in collaboration with STMicroelectronics. The results will contribute to advancing Cryo-CMOS electronics and the development of high-performance quantum computers.