Injection Length in Staggered Organic Thin Film Transistors: Assessment and Implications for Device Downscaling

In staggered thin film transistors, the injection length is the fraction of the gate to contact overlap that is effectively involved in current injection. Its assessment is important to properly downscale device dimensions. In fact, in order to increase transistor operation speed, the whole device footprint should be downscaled, which means both the gate to contact overlap and the channel length, as they affect the relative weight of gate to contact parasitic capacitances and the carrier transit time along the channel respectively. Nevertheless, it is not advisable to make the gate to contact overlap smaller than the injection length, because this negatively affects contact resistances. Suitable figures of merits are introduced to quantify these aspects, and a method is proposed to extract the injection length from electrical measurements. As an example of application, transistors based on the prototypical n-type polymer poly[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) (P(NDI2OD-T2) are analyzed. When the channel length is scaled while driving voltages are kept constant, in P(NDI2OD-T2) the injection length decreases as well, thus proving that the downscaling of the whole device footprint is feasible. The physical origins of this finding are analyzed and traced back to material properties, in order to suggest general guidelines for a successful transistor downscaling.