  Fondazione Bruno Kessler (FBK, Trento, Italy) has recently delivered its
first 50 $\mu$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 $\phi \sim$ 10$^{15}$ n$_{eq}$/cm$^2$. In this paper, we present the
characterisation, the timing performances, and the results on radiation damage
tolerance of this new FBK production.
