The field effect devices prepared using active channels fabricated from doped conducting polymers, such as PEDOT/PSS (poly-(3,4-ethyldioxythiophene/poly(styrene sulfonic acid)), polypyrrole/Cl-, and polyaniline/Cl- with various dopants are reported. Normally in the "on" state, the devices have a sharp switch off at a small gate voltage. The current ratio I<sub>on</sub>/I<sub>off</sub> can exceed 10<sup>4</sup> at room temperature. The temperature dependence of the dc conductivity of the PEDOT/PSS follows the variable range hopping law both before and after application of the gate voltage. The activation energy, T<sub>0</sub>, increases even for on/off ratios as small as 1.07 demonstrating that the electric field effect has changed the bulk charge transport in the active channel despite the expected screening due to mobile charge carriers. Based on these transport and optical studies we propose that the conducting polymer is near the metal-insulator transition and that the field effect in the transistor is related with electric field modulating this transition in the region underneath of gate through field induced ion motion. The time dynamics of current with the gate modulation strongly supports our proposal. Application of the Doped Polymer Field Effect Devices (DPFEDs) to form circuit components has been demonstrated.