Ex-situ doping is used to introduce a constant dose of P atoms into Si nanosheets with thickness ranging from 70 to 6 nm. Full activation of P atoms is demonstrated in Si nanosheets with nm. Conversely, progressive deactivation of dopant impurities is observed when decreasing the thickness of the Si nanosheets below 30 nm. Si nanosheets with thickness nm exhibit electron mobility values higher than the ones reported for bulk Si at the same doping concentration. Data provide information for the design of ultra-scaled GAAFET devices.