Organic thin‐film transistors (OTFTs), based on two n‐type structures of fullerene (C60) and N,N′‐dioctyl‐3,4,9,10‐perylenedicarboximide (PTCDI‐C8), are investigated. The two OTFT devices reveal a negative differential resistance (NDR) behavior in the saturation region of the output curves. The OTFT based on C60 shows a more pronounced NDR than that based on PTCDI‐C8. The main origin of the NDR is ascribed to the trapping states. In addition, the contact resistance (Rc) of the two devices, extracted using the developed voltage transition method (Vtr method), as well as the obtained values of Rc, are in fair agreement with the experimental values extracted using the total resistance. The device based on C60 exhibits a higher Rc than that based on PTCDI‐C8. OTFT parameters, such as transconductance (gm), mobility (μmax), threshold voltage (Vth,), turn‐on voltage (Von), ratio current Ion/Ioff, and subthreshold slope (SS), are obtained in the linear and saturation regimes. The extracted value of SS for the OTFT based on C60 is very small compared with that of the device based on PTCDI‐C8, which indicates an increase in the density of trapping states in the C60 film.