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-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 (Vthlin) and a reduction in field-effect charge carrier mobility (μlin) were apparently observed during the bias-stress application. When μlin and Vthlin were conventionally extracted from the transfer characteristics around the maximum transconductance, the Vthlin shift amount and μlin reduction depended on the channel length and were smaller in OFETs with short channels. After contact resistance (Rc) 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 Rc correction. This erroneous evaluation would become more pronounced in short-channel, high-mobility OFETs, because the Rc becomes larger relative to the channel resistance with increasing μlin and decreasing channel length. These results suggest that one should pay attention to Rc 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.
Keywords: bias-stress effects; contact resistance; modified transmission line method; operational stability; polymer-based organic field-effect transistors.