Amplification of electrical signals with molecule-based transistors: Power amplification up to a kilohertz frequency and factors limiting higher frequency operation

Polyaniline- and poly(3-methylthiophene)-based transitors are demonstrated to amplify electrical signals at frequencies exceeding 100 Hz. The amplication of sinusoidal signals to the gate is established by showing the ratio of the average power in the drain circuit to that in the gate circuit exceeds unity. For the devices measured the amplification factor falls to unity between 100 and 1000 Hz. Measurements have been carried out at 298 K for the microelectrochemical transistors immersed in aqueous or nonaqueous electrolyte solutions. The transistors consist of a pair of Au (or Pt) microelectrodes (∼50 μm long x ∼2 μm wide x ∼01 μm high) connected by the redox polymer. The two microelectrodes serve as "source" and "drain" and the polymer serves as the "channel" of the device. The potential difference between source and drain is the drain potential, V_D, and Current I_D, flows between source and drain for gate potentials, V_G, where the polymer is conducting. The gate current, I_G, is associated with the charge needed to oxidize and reduce the polymer to change its conductivity. The amplification factor, A, is determined at a given frequency by measuring I_D (at fixed V_D) and I_G vs. time for sinusoidal variation in V_G between a potential where the device is off and a potential where the device is on. I_D and I_G are measured as a function of frequency to determine the frequency dependence of the average power in the drain and gate circuits. The maximum frequency for A ≥ 1 is limited by (1) the conductivity changes of the polymer as a function of the state of charge (2) the volume of redox polymer used to make the device, and (3) the charge transport properties of the polymer. The results suggest that power amplification at frequencies that significantly exceed 10³ Hz is possible for microelectrochemical devices.