Limitations of the Intrinsic Cutoff Frequency to Correctly Quantify the Speed of Nanoscale Transistors

The definition of the intrinsic cutoff frequency (f
_T
) based on the current gain equals to one (0 dB) is critically analyzed. A condition for the validity of the quasistatic estimation of f
_T
is established in terms of the temporal variations of the electric charge and electric flux on the drain, source, and gate terminals. Due to the displacement current, an electron traversing the channel generates a current pulse of finite temporal width. For electron devices where the intrinsic delay time of the current after a transient perturbation is comparable with such a temporal width, the displacement currents cannot be neglected and the quasi-static approximation becomes inaccurate. We provide numerical results for some ballistic transistors where the estimation of f
_T
under the quasi-static approximation can be one order of magnitude larger than predictions obtained from time-dependent numerical simulations of the intrinsic delay time (including particle and displacement currents). In other ballistic transistors, we show that the gate current phasor amplitud can be smaller than the drain one at all frequencies, giving no finite value for f
_T
.