Overestimation of Operational Stability in Polymer-Based Organic Field-Effect Transistors Caused by Contact Resistance

The bias-stress effects of bottom-gate top-contact polymer-based organic field-effect transistors (OFETs) with different channel lengths (50-500 mu m) were evaluated by repeating cycles of prolonged on-state gate-bias application and transfer characteristics measurements in the linear regime. The thicknesses of poly(didodecylquaterthiophene-alt-didodecylbithiazole) active layers were 26 and 37 nm. All OFETs exhibited nonlinear (nonideal) transfer characteristics with a maximum transconductance within the gate-source voltage sweep range. Both a shift in threshold voltage (V th lin) and a reduction in field-effect charge carrier mobility (mu lin) were apparently observed during the bias-stress application. When mu lin and V th lin were conventionally extracted from the transfer characteristics around the maximum transconductance, the V th lin shift amount and mu lin reduction depended on the channel length and were smaller in OFETs with short channels. After contact resistance (R c) correction, the channel length dependence disappeared. Thus, the operational stability in OFETs with short channels: <= 50 (150) mu m for the 26 (37) nm-thick active layers, was found to be overestimated without R c correction. This erroneous evaluation would become more pronounced in short-channel, high-mobility OFETs, because the R c becomes larger relative to the channel resistance with increasing mu lin and decreasing channel length. These results suggest that one should pay attention to R c in the fundamental research into the origin of operational instability and in evaluating the effects of active layers, gate dielectrics, and active layer/gate dielectric interfaces on operational stability.