Today, the continued miniaturization of field effect transistors (FETs) results in major scaling issues that curtail further voltage reduction. The resultant increase in power consumption density limits the overall performance. Therefore, alternative materials and devices are required that support steep sub-threshold slopes and low-voltage operation. The tunnel FET (TFET) is regarded as the most promising candidate because it is based on gate-controlled band-to-band tunneling in a p-i-n+ structure and thus can break the 60 mV/dec limit of conventional FETs . Implementing the TFET principle in the nanowire (NW) geometry provides optimum electrostatic control. Here we demonstrate controlled in-situ doping of silicon (Si) NWs, the effect of scaling on the active number of doping atoms in the NW and the implementation of a Si NW TFET.