Abstract Fondazione Bruno Kessler (FBK, Trento, Italy) has recently delivered its first 50 μ m thick production of Ultra-Fast Silicon Detectors (UFSD), based on the Low-Gain Avalanche Diode design. These sensors use high resistivity Si-on-Si substrates, and have a variety of gain layer doping profiles and designs based on Boron, Gallium, Carbonated Boron and Carbonated Gallium to obtain a controlled multiplication mechanism. Such variety of gain layers will allow identifying the most radiation hard technology to be employed in the production of UFSD, to extend their radiation resistance beyond the current limit of ϕ∼ 10 15 n e q/cm 2. In this paper, we present the characterisation, the timing performance, and the results on radiation damage tolerance of this new FBK production.