We demonstrate organic field-effect transistors (OFETs) with an ion-dispersed polymer for the gate dielectrics. By applying external electric field (V(ex)), the dispersed ions can migrate by electrophoresis and separated ion pairs form space charge polarization in the gate dielectrics. After V(ex) was applied, the drain current is increased over 7 times and threshold voltage is decreased from - 12.9 V to - 2.9 V. The shift direction Of V(th) is controllable by the polarity of the V(ex). Results of ultraviolet/visible differential absorption study reveal that the active layer of OFETs is charged not only electrostatically but electrochemically with increasing the time after V(ex) was applied. (C) 2009 Elsevier B.V. All rights reserved.