Analysis and extraction of contact resistance in pentacene thin film transistors

We have fabricated pentacene organic thin film transistors (TFTs) on p-doped Si substrate serving as the gate electrode with good yield and uniformity. These transistors have excellent electrical characteristics, with carrier field-effect mobility as large as 0.5 cm²/Vs, on/off current ratio of 10⁶, subthreshold slope of lV/decade, and near-zero threshold voltage. The devices consisted of metal source and drain electrodes contacting a 30 nm-thick pentacene film thermally deposited on SiO₂ dielectrics . We have applied a simple model to analyze and extract the resistance of the source and drain contacts in these transistors. The model was done based on the dependencies of the channel resistances on the gate length and gate voltage. We found that the contact resistance is typically greater than the channel resistance. This suggests that the electrical performance of organic TFTs, in which reliable contact doping is difficult, may be dictated by the contacts, rather than by the intrinsic carrier mobility of the organic semiconductor.