An ionic liquid-channel field-effect transistor

The basic theory and first experimental results of a new microfabricated ionic liquid-channel field-effect transistor (ILCFET) are presented. The ILCFET resembles a metal-oxide-semiconductor field-effect transistor (MOSFET) except that the current flowing from the source to the drain is carried in thin channels by ions of a liquid electrolyte. In MOSFETs the gate-induced electric field changes the channel concentration of only one mobile carrier, the holes or the electrons. In ILCFETs the concentrations of both cations and anions are altered. The devices that we have built have channels 0.088 mu m thick and 300, 600, and 900 mu m long. The basic steady-state theory predicts that for a channel thickness of 8 Debye lengths and a gate voltage change from 0 to 25 V (representing a potential change of 150 mV at the solid/liquid interface) the steady-state anion/cation ratio increases by a factor of 17.8. For a glycerol solution of KCl, the channel conductance increases correspondingly by a factor of 5.2. The transient response to a gate voltage step is modeled as a gate capacitance charging current followed by slower ambipolar diffusion. Both processes are described by the diffusion equation. Measurements have been made of conductance vs. gate voltage, gate capacitance charging current, and slow conductance changes due to diffusion for different types of glycerol solutions. Whereas the gate current transient response data agree well with theory, the long-term transient response data indicate that phenomena other than ambipolar diffusion are present. Finally, the slopes of the channel conductance vs. gate voltage curves are less than predicted by the basic theory.