Ex-situ doping of ultra-thin silicon-on-insulator (SOI) substrates is performed by using polymers terminated with a doping containing moiety. The injection of P impurity atoms is investigated confining the same P dose of ∼ 1 x 1013 cm-2 in a progressively thinner device layer, with thickness values (HSOI) from 6 to 70 nm. The dopant concentration is determined by Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) measurements. Sample resistivity (ρ), carrier concentration (ne) and mobility (μ) are determined combining sheet resistance and Hall measurements in van der Pauw configuration. Almost complete activation and full ionization of the injected dopants is observed at room temperature in the samples with HSOI ≥ 30 nm. The ionization fraction progressively drops to 5 % when reducing the thickness of the device layer. Dopant incomplete ionization is accompanied by an increase in the electron …