•Optimal contacts should have smallest size but possess minimum contact effects.•Six times of semiconductor film thickness is found to be optimal for contact length.•It increases in nanoscale channels with more hopping and anisotropic transport.•This optimal contact length is independent of semiconductors and variable mobility.We report on a study seeking an optimized contact configuration for organic transistors that minimizes contact effects but maintains smallest contact size. We begin with the bulk access resistance in staggered transistors which results from the charge transport through the organic semiconductor film. Bulk access resistance is an intrinsic contributor to the contact resistance which has been little understood due to lack of a reliable study tool. In this work, we utilize the inner transported power inside the semiconductor film as a medium to investigate the contact resistance and the relevant contact effects. We examine the influences of the organic film thickness (tSC), the channel length (L), the underlying charge transport and various organic semiconductor materials with variable carrier mobility. A roughly optimal contact length (LC) of LC0 ≈ 6tSC is obtained. The results reveal that besides the device architecture the underlying charge transport should be also taken into account in designing organic transistors for practical application.Graphical abstract

A study of the contact resistance (Rsd) in pentacene-based double-gate transistors is presented. In top-contact transistors, as the negative bias of the additional top-gate bias is increased, Rsd decreases by over five orders of magnitude for small bottom-gate voltages. In bottom-contact transistors, Rsd is reduced by about ten times for all bias values, implying improved charge transport in all operating regimes. The different tunability of Rsd in top/bottom-contact transistors is attributed to different charge injection modulation by the coplanar/staggered top gate. Therefore, double-gate architecture offers a novel and effective approach to limit Rsd and its relevant impacts on organic transistor.Graphical abstractHighlights► Contact resistance (Rsd) can be tuned in the double-gate OFETs. ► In TC transistors, Rsd can be reduced by the negatively increased top-gate bias. ► In BC transistors, Rsd can be effectively reduced over all biases. ► Higher tunability in BC devices is attributed to staggered configuration. ► Double-gate architecture could be a novel and effective manner to reduce Rsd.