  The nonlinear capacitance in doped nanotube junctions is calculated self
consistently. It decreases as a function of the applied bias when the latter
becomes larger than the pseudogap of the nanotube. For this device, one can
deduce a relaxation time of about 0.1 femtosecond. Because of its negative
differential resistance (NDR), a switching time of less than a fs can also be
deduced.
