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 μm) 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-<i>alt</i>-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 (<i>V</i>_th^lin) and a reduction in field-effect charge carrier mobility (μ^lin) were apparently observed during the bias-stress application. When μ^lin and <i>V</i>_th^lin were conventionally extracted from the transfer characteristics around the maximum transconductance, the <i>V</i>_th^lin shift amount and μ^lin reduction depended on the channel length and were smaller in OFETs with short channels. After contact resistance (<i>R</i>_c) correction, the channel length dependence disappeared. Thus, the operational stability in OFETs with short channels: ≤50 (150) μm for the 26 (37) nm-thick active layers, was found to be overestimated without <i>R</i>_c correction. This erroneous evaluation would become more pronounced in short-channel, high-mobility OFETs, because the <i>R</i>_c becomes larger relative to the channel resistance with increasing μ^lin and decreasing channel length. These results suggest that one should pay attention to <i>R</i>_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.