We report the electrical characterization of field effect transistors based on regioregular poly(3-hexylthiophene) (RRP3HT) nanofibers fabricated using nanostencil shadow masks. Mobility values were 0.02 cm(2)/VS With on/off current ratios of 10(6). Current densities of similar to700 A/cm(2) were achieved in single nanofibers. A series of Soxhlet extractions was employed to separate RRP3HT into narrow molecular weight fractions. Nanofibers made from the THF fraction exhibited superior electrical properties in terms of increased current levels and decreased activation energy. The lowest activation energies in the nanofibers were achieved by using top contacts (i.e., vapor-deposited metal on top of the nanofibers) and material purified by Soxhlet extraction. Contact effects were eliminated from bottom contact devices (i.e., nanofibers on top of metal electrodes on a dielectric substrate) with a four-probe geometry. Temperature-dependent measurements reveal two distinct regimes of transport. The high-temperature regime (355-245 K) is characterized by activation energies of 62-145 meV depending on contact geometry and RRP3HT purity with a Meyer-Neldel energy of 33 3 meV. The low-temperature regime (235-85 K) has lower activation energies of 31-112 meV. Shifts in the turn-on gate voltage with temperature indicate similar to4.8 x 10(12) acceptor-like states/cm(2) (or similar to1 per nm of fiber length) and 3.2 x 10(11) donor-like states/cm(2) exist in the nanofibers. We propose that transport can be explained in terms of the multiple trap and release (MTR) or variable range hopping (VRH) formalisms of transport in a bimodal, exponential distribution of shallow and deep donor-like states.