Method for predicting fT for carbon nanotube FETs

A method based on a generic small-signal equivalent circuit for field-effect transistors is proposed for predicting the unity-current-gain frequency f_T for carbon-nanotube devices. The key to the useful implementation of the method is the rigorous estimation of the values for the components of the equivalent circuit. This is achieved by numerical differentiation of the charges and currents resulting from self-consistent solutions to the equations of Schrodinger and Poisson. Sample results are presented, which show that f_T can have a very unusual dependence on the gate-source bias voltage. This behavior is due mainly to the voltage dependence of the transconductance and capacitance in the presence of quasi-bound states in the nanotube.