There are two main categories of organic transistors: electrochemical transistors (ECT) and field-effect transistors (FET).
Typically, in an ECT the transistor channel is formed of an electrochemically active layer, and the conductivity of the transistor channel is altered by changing its electrochemical redox state (electrochemical doping). In other words, one oxidation state of the channel corresponds to low channel conductivity (the transistor is OFF), while the other oxidation state corresponds to high channel conductivity (the transistor is ON). The transistor channel is normally defined by the bulk of the electrochemical active layer located between the source and drain electrodes, which is in ionic contact with an electrolyte layer. The active section in the channel is defined by the thickness of the electrochemically active layer. In order to switch the electrochemically active layer between the two oxidation states, ions need to migrate between the electrolyte layer and the bulk of the electrochemically active layer. This migration is controlled by a voltage applied to the gate electrode. Hence, there is a transport of active ions, i.e. ions which participate in the electrochemical reaction of the switch channel, between the electrolyte and the bulk of the electrochemical active layer. A more detailed description can be found in U.S. Pat. No. 6,806,511.
Traditional OFETs have an organic semiconductor film (the channel) that is separated from an electrode (the gate) by a thin film insulator, made of e.g. silicon oxide, metal oxide etc. This gate-insulator-organic semiconductor sandwich is analogous to a capacitor that causes field-effect current modulation in the channel (between said source and drain electrodes which contact the semiconductor film). Hence the current between the source and drain electrodes can be adjusted, by tuning the voltage applied to the gate electrode.
Organic Field-Effect Transistors (OFET) along with other “plastic” electronic devices are a platform for printed, flexible and fully-integrated electronics. Ideally, these systems should be fast, operate at low voltage, and be robust enough to be manufactured using standard printing techniques.
Tremendous effort has been devoted to reach high capacity (per unit area) Ci between the gate and the channel to allow transistors to operate at low voltage. Various kinds of inorganic and organic high-permittivity insulators have been explored. Alternatively, molecular assembly and self-organisation techniques have been utilised to manufacture gate dielectric layers only a few nanometers thick. These latter approaches sacrifice ease-of-manufacturing for low-voltage operation.