Impact of Contact Placement on Subthreshold Characteristics of Organic Thin-Film Transistors

The impact of placing source/drain contacts at the bottom or on top of semiconductor film on subthreshold characteristics of organic thin-film transistors is investigated using 2-D numerical simulations. It is shown that placement of contact on the top allows gate to have more influence over the potential in the semiconductor, especially close to the top where most of current in OFF-state flows. As a result, subthreshold swing and ON-OFF current ratios are best when both contacts are on the top and worst when both are at the bottom. Asymmetric structures with one contact on top and the other at bottom show intermediate performance. However, the use of a stack of two dissimilar metals with appropriate properties can allow an asymmetric structure with bottom source and top drain contact to have OFF-state characteristics, which are even superior to those observed in top-contact transistors.