Overestimation of\nOperational Stability in Polymer-Based\nOrganic Field-Effect Transistors Caused by Contact Resistance

The bias-stress effects of bottom-gate top-contact polymer-based\norganic field-effect transistors (OFETs) with different channel lengths\n(50–500 μm) were evaluated by repeating cycles of prolonged\non-state gate-bias application and transfer characteristics measurements\nin the linear regime. The thicknesses of poly(didodecylquaterthiophene-<i>alt</i>-didodecylbithiazole) active layers were 26 and 37 nm.\nAll OFETs exhibited nonlinear (nonideal) transfer characteristics\nwith a maximum transconductance within the gate-source voltage sweep\nrange. Both a shift in threshold voltage (<i>V</i><sub>th</sub><sup>lin</sup>) and a reduction in field-effect charge carrier mobility\n(μ<sup>lin</sup>) were apparently observed during the bias-stress\napplication. When μ<sup>lin</sup> and <i>V</i><sub>th</sub><sup>lin</sup> were conventionally extracted from the transfer\ncharacteristics around the maximum transconductance, the <i>V</i><sub>th</sub><sup>lin</sup> shift amount and μ<sup>lin</sup> reduction depended on the channel length and were smaller in OFETs\nwith short channels. After contact resistance (<i>R</i><sub>c</sub>) correction, the channel length dependence disappeared. Thus,\nthe operational stability in OFETs with short channels: ≤50\n(150) μm for the 26 (37) nm-thick active layers, was found to\nbe overestimated without <i>R</i><sub>c</sub> correction.\nThis erroneous evaluation would become more pronounced in short-channel,\nhigh-mobility OFETs, because the <i>R</i><sub>c</sub> becomes\nlarger relative to the channel resistance with increasing μ<sup>lin</sup> and decreasing channel length. These results suggest that\none should pay attention to <i>R</i><sub>c</sub> in the\nfundamental research into the origin of operational instability and\nin evaluating the effects of active layers, gate dielectrics, and\nactive layer/gate dielectric interfaces on operational stability.