We investigate the effect of potassium (K) doping on the transport properties of aligned single-walled carbon nanotube fibers. The temperature dependence of the electrical resistance, the current-voltage characteristics, and the magnetoresistance vs external magnetic field of the fibers consistently show that doping enhances the metallic character of the fibers and that the response of the samples can be quantitatively explained in two thermal regimes separated by a characteristic temperature T

more »

... * . At temperatures higher than T * , the data are interpreted in the framework of variable range hopping theory, suggesting that the increased conductance with potassium doping is due to the increase of the density of states, which enhances carriers hopping. For temperatures below T * , experimental evidence of fluctuation temperature-induced tunneling mechanism suggests that the doping by K atoms affects the potential barriers established between adjacent carbon nanotubes, enhancing the metallic properties of the fibers.