(Invited) Enhancing the Performance of Polymer Thin-Film Transistors

Abstract

One of the important challenges in the field of thin-film transistors is to improve designs that result in performance speeds. With polymer semiconductors, non-quasistatic circuits such as rectifiers in which the maximum frequency depends on the carrier transit time, have been demonstrated to work to a few 10’s of MHz. The challenge is to realize clocked sequential circuits that operate as speeds much larger than the few 10’s of kHz that have been demonstrated so far. This talk will describe several approaches to increase the performance characteristics of polymer thin-film transistors. The metrics that we seek to enhance include mobility, on/off current ratio, and short channel length performance. Using a combination of experimental data and theoretical calculations and simulations, we describe device architectures and fabrication methods that improve performance. This includes nanostructured geometries, gate dielectric engineering, and new materials combinations. We present a detailed theoretical model of charge transport in such polymers that provides considerable insight into the operation of such TFTs. There have been many reports of thin-film transistors based on semiconductors with mobilities in the range 1-20 cm2/(Vs) in the past few decades. The mobilities and mean free paths are simply too low for the application of conventional semiconductor transport theories based on solutions to the Boltzmann transport equation (BTE). There have been some material-specific treatments that have tried to circumvent the basic difficulty of the mean free path being less than, or at best comparable to, the lattice constant or the minimum transport lengths required for extended state transport. We will present a very general solution based on the statistical nature of charge transport and introduction of a factor related to the probability of mean free path exceeding the minimum transport length. We are then able to apply the BTE with appropriate scattering mechanisms and obtain very good results that agree well with experiment. This approach is very well suited to thin-film transistors based on polymer and organic semiconductors with room temperature mobilities in the range 1-20 cm2/(Vs).