High-frequency performance projections for ballistic carbon-nanotube transistors

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 (f_T) with gate voltage. The circuit is shown to reduce to the expected forms in the so-called "MOS" and "bipolar" limits. The f_T is shown to approach a maximum value of v_F/2πL≈130 GHz/L (μm) at high gate voltage, where v_F 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 f_T is also discussed. Numerical simulations on a "MOSFET-like" or "bulk-switched" carbon-nanotube transistor are shown to support the conclusions.