Negative output differential resistance effect in organic thin film transistors based on pentacene: Characterization and modeling

Abstract A series of p-channel organic thin film transistors (OTFTs) based on small molecule pentacene semiconductor was fabricated to characterize for developing analytical modeling. In this present work, the effect of channel length variation (2.5–20 μm) on the electrical parameters of the OTFTs was emphasized. As the channel length (L) of the pentacene-TFTs is decreased, some effects such as negative output differential resistance, the threshold voltage shift, an increase in the drain current (transconductance) and an improvement in other parameters may be observed. The highest device performance with the mobility value of 8 × 10 −3  cm 2  V −1  s −1 was obtained from a short channel device (L = 2.5 μm). Total resistances including contact and channel resistances were extracted and discussed in detail. The results showed that the contact resistance significantly affected the performance of the OTFTs with channel lengths of 2.5 μm and 5 μm. A negative differential resistance (NDR) behavior was obtained from the saturation region of the output characteristics for each device in the case while channel length was decreased from 20 μm to 2.5 μm. This NDR effect is attributed to the trapping and de-trapping mechanism of the mobile charges at the pentacene–metal electrode interface. Finally, an analytical compact model of organic thin film transistor was developed including the effect of contact resistance and gate voltage dependent mobility, simultaneously. The proposed model was validated by comparing the results obtained from the model with those measured. The results show that the proposed model is a good agreement with the experimental transfer data for devices with the channel length value of 2.5 μm, 5 μm, 10 μm and 20 μm.