The influence of bulk traps on the subthreshold characteristics of an organic field effect transistor

The subthreshold characteristics of fabricated organic field effect transistors based on regioregular poly (3-dodecylthiophene) (P3DDT) as the active layer and poly-4-vinylphenol (P4VP) as the gate insulator have been investigated. The transistor turn-on occurs at a threshold voltage of around V-th = 0 V. The (hole) mobility of 0.002-0.005 cm(2)/(V s) has been estimated from the linear region of the transfer characteristics. As usually observed for organic transistors, the inverse subthreshold slope is very high, in our case S approximate to 7 V/dec. Furthermore, the subthreshold current depends on the drain voltage although the transistor is a long channel device. One possibility to explain these peculiarities are interface traps, as demonstrated recently by Scheinert et al. [J. Appl. Phys. 92 (2002) 330]. In this paper, the influence of bulk traps is shown. It turns out that both the high inverse subthreshold slope and the drain voltage dependence can be explained also by recharging of bulk traps. Therefore, other frequency and temperature dependent dynamic measurements have to be applied to distinguish between the different possible influences. (C) 2003 Elsevier Science B.V. All rights reserved.