Inversion layer instability in a P(VDF/TriFE)/silicon transistor structure : effect of oxygen exposure/polymer porosity, interface, and bulk trapping

A copolymer of vinylidene fluoride and trifluoroethylene is solution cast onto a p-type silicon wafer to produce an enhancement field-effect transistor structure. A strong inversion is induced at the silicon surface when an aluminum gate electrode is suitably biased. When the gate is left floating, the inversion layer decays with time. Both the initial strength of this inversion and its rate of decay are found to depend on the charging history of the device, as well as the oxygen concentration in the surrounding gas ambient. The oxygen sensitivity is due to the large gas permeability of the cast film as compared to commercially extruded film. A novel method for directly measuring these permeabilities is presented. All aspects of device behavior may be treated in a single model where the inversion layer decay is shown to be a function of interface and bulk trapping in the polymer as well as ambient oxygen concentration. Excellent agreement between theory and results is obtained. The physical significance of all model parameters and their relation to device behavior under different experimental conditions is discussed.