Analysis of contact effects in fully printed p-channel organic thin film transistors

Contact effects have been analyzed in fully printed p-channel OTFTs based on a pentacene derivative as organic semiconductor and with Au source–drain contacts. In these devices, contact effects lead to an apparent decrease of the field effect mobility with decreasing L and to a failure of the gradual channel approximation (GCA) in reproducing the output characteristics. Experimental data have been reproduced by two-dimensional numerical simulations that included a Schottky barrier (Φb=0.46eV) at both source and drain contacts and the effects of field-induced barrier lowering. The barrier lowering was found to be controlled by the Schottky effect for an electric field E<105V/cm, while for higher electric fields we found a stronger barrier lowering presumably due to other field-enhanced mechanisms. The analysis of numerical simulation results showed that three different operating regimes of the device can be identified: (1) low |Vds|, where the channel and the Schottky diodes at both source and drain behave as gate voltage dependent resistors and the partition between channel resistance and contact resistance depends upon the gate bias; (2) intermediate Vds, where the device characteristics are dominated by the reverse biased diode at the source contact, and (3) high |Vds|, where pinch-off of the channel occurs at the drain end and the transistor takes control of the current. We show that these three regimes are a general feature of the device characteristics when Schottky source and drain contacts are present, and therefore the same analysis could be extended to TFTs with different semiconductor active layers.