The preparation of 20 +/- 5 nm diameter Si nanowires and the electrical characterization of Si nanowire devices are presented. The nanowires were grown at 450-500 degreesC on solid substrates via the vapor-liquid-solid mechanism using Au or Zn nucleation catalysts and SiH4 as the silicon source. The wires were investigated by high-resolution transmission electron microscopy. Two types of wires were found, as characterized by different growth directions ([11 (1) over bar] and [211]). Several types of devices, including crossed nanowire devices, four- and six-terminal devices, and three-terminal (gated) devices, were fabricated. For certain devices, various electrode compositions were also studied. The measured resistivity of these nanowires was separated from the contact resistance and could be varied from >10(5) Ohm cm to similar to 10(-3) Ohm cm. The wide variation in resistivity was related to the nature of the electrical contact to the wires (Schottky or Ohmic) and to the doping level of the wires. Doping of the nanowires was performed by the thermal diffusion of metal catalyst into the nanowires at 750-850 degreesC. Au nucleated nanowires exhibited resistivity values much lower than those of similarly treated Zn nucleated nanowires. This result is attributed to the much larger relative solid solubility of gold in silicon.