Air-stable bottom-gate, top-contact n-channel organic transistors based on a naphthalene diimide exhibiting electron mobilities up to 0.8 cm(2)/Vs at low voltages were fabricated. Transistors with channel lengths of 1 mu m show a transconductance of 60 mS/m, but are significantly limited by the contact resistance. Transmission line measurements in combination with contact resistance models were applied to investigate this influence. Both contact resistance and contact resistivity are proportional to the inverse gate overdrive voltage. Organic complementary ring oscillators were fabricated on a flexible plastic substrate showing record signal delays down to 17 mu s at a supply voltage of 2.6 V. (C) 2013 AIP Publishing LLC.