We explain variability observed for the resonant tunnelling transport through donors in scaled silicon nanowires by the influence of charge configuration changes at the edges between the channel and the source-drain regions. This charge configuration is remarkably robust with respect to ageing effects, thermal cycling and the associated Id-Vg characteristics at low temperature constitute a real ldquoelectro-fingerprintrdquo for the samples. This stability is prerequisite for applications based on the gate control of single donor orbitals in nanoscale CMOS devices.