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Based on a careful physical description, the RF performance of ultrathin-body (down to 3 nm) silicon-on-insulator transistors is investigated. While the mobility reduction in a thin Si film slightly degrades the peak cutoff frequency and the maximum frequency of high-performance cross-coupled pair-based RF oscillators, the changes in feedback capacitance improve the low operating power and high-performance wideband and power operation of RF circuits. Also, the influence of various gate stacks on the benefits of downscaling is investigated. Fully silicided gates will enable to benefit from gate-length downscaling from an RF perspective down to 9 nm if the finger width is kept below 6 μm, and deposited metal gates have the potential to provide advantages if the total interface resistivity is below 6-7 Ω·μm2. Finally, the effect of series resistance at the source/drain is quantified. The device RF performance decreases by 10% per 100 Ω·μm of series resistance.