(Invited) Percolation Carbon Nanotube Thin Film Transistors

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The decrease of the gate voltage swing switching a field effect transistor from on to off state and, consequently, the decrease of the power supply voltage is the key challenge of the transistor design. Gate voltage dependent tunneling has been used to achieve this goal by decreasing the slope of the exponential current decay in the subthreshold regime below 60 mV per decade. We describe a new Carbon Nanotube (CNT) Percolating Thin Film Transistor (PTFT) that should allow achieving a steep subthreshold slope. CNT TFTs use mats of CNTs deposited on a substrate and forming a random percolating network. The conduction contacts between the individual CNTs is determined by tunneling and, therefore, could be strongly affected by external agents, such as gases or biological fluids. The asymmetry of the contacts to each CNT enables the rectification of electromagnetic radiation impacted on the CNT mat changing the drain-to-source conduction, with giant relative changes near the percolation point. Our experimental data show that sub-THz radiation shifts the percolation point at high intensity, when the rectified radiation changes the shape of the potential barrier between the CNT in a proximity contact. The CNT PTFT operates near the percolation point. In this regime, the gate bias shifts the percolation point by (a) depleting the CNTs in the subthreshold regime and (b) changing the shape of the tunneling barrier. This ensures the metal-insulator transition resulting in a sharp decrease of the subthreshold current slope. We present a compact model of the percolated CNT PTFT based on our Unified SPICE/ADS model that shows that this effect could reduce the subthreshold slope of the CNT TFs from 340 meV per decade to about 190 meV per decade.AcknowledgmentsThis work at RPI was supported by the U.S. Army Research Laboratory under the Cooperative Research Agreement (Project Monitor Dr. Meredith Reed) and by the US ONR (Project Monitor Dr. Paul Maki).