By comparing the changes in pi-pi* absorption with the transconductance in PEO-LiClO4 electrolyte-gated FETs, we have demonstrated that the high channel currents obtained at low gate voltages result from reversible electrochemical doping of the semiconducting polymer film. At low temperatures, the conductivity of the electrochemically doped poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT-C14, is nonlinear with a crossover from d sigma(T)/dT > 0 to d sigma(T)/dT approximate to 0 as a function of the source-drain voltage. High current densities, up to 106 A/cm(2) at 4.2 K, can be sustained in the electrochemically doped PBTTT-C14 films.