Subthreshold characteristics of field effect transistors based on poly(3-dodecylthiophene) and an organic insulator

The properties of field effect transistors with organic insulator and semiconducting regions, fabricated with a top-gate architecture, have been investigated. Thin films (dapproximate to30 nm) of regioregular poly(3-dodecylthiophene) were employed as the active semiconductor and the gate insulator was formed by a 500-nm-thick layer of poly(4-vinylphenol). Both were solution-processed on top of poly(ethylenetherephthalate) films, which were used as substrates. The output characteristics show a pronounced saturation behavior with an unconventional nonquadratic saturation current dependence on the gate voltage. Hence the (hole) mobility of 0.002-0.005 cm(2)/Vs has been estimated from the linear region of the transfer characteristics. The transistor turn-on occurs at a threshold voltage of approximately V-th=0 V, and the device can be operated with a supply voltage of between 15 and 20 V. As is usually observed for organic transistors, the inverse subthreshold slope (S) is very high, in our case Sapproximate to7 V/dec, by contrast with Sapproximate to200 mV/dec obtained for the similar material poly(3-octylthiophene) (P3OT) with silicon dioxide (SiO2) as an insulator. Furthermore, the subthreshold current depends on the drain voltage even though the transistor is electrically a long channel device with L=2 mum, notwithstanding the fact that this channel length is rather small for the present organic devices. To clarify these peculiarities numerical simulations have been carried out with a systematic variation of the relevant material parameters and assuming the existence of interface or bulk trap states. It turns out that both the high inverse subthreshold slope and the drain voltage dependence can be explained by recharging of trap states either at the interface or in the bulk. Considering the difference to the P3OT device with SiO2 as insulator it is proposed that interface traps are responsible for these effects, although one excludes the possibility that the film formation either on an organic substrate or on SiO2 leads to different bulk properties. (C) 2002 American Institute of Physics.