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A quasi-static approach is combined with a theory of ballistic nanotransistors to assess the high-frequency performance potential of carbon-nanotube field-effect transistors. A simple equivalent circuit, which applies in the ballistic limit of operation, is developed for the intrinsic device, and then employed to determine the behavior of the unity-current-gain frequency (fT) with gate voltage. The circuit is shown to reduce to the expected forms in the so-called "MOS" and "bipolar" limits. The fT is shown to approach a maximum value of vF/2πL≈130 GHz/L (μm) at high gate voltage, where vF is the nanotube's Fermi velocity and L is the channel length, and to fall at low gate voltage due to the presence of source and drain electrostatic capacitances. The impact of the gate electrostatic capacitance on the fT is also discussed. Numerical simulations on a "MOSFET-like" or "bulk-switched" carbon-nanotube transistor are shown to support the conclusions.
Date of Publication: Jan. 2006