  We report on microwave operation of top-gated single carbon nanotube
transistors. From transmission measurements in the 0.1-1.6 GHz range, we deduce
device transconductance gm and gate-nanotube capacitance Cg of micro- and
nanometric devices. A large and frequency-independent gm of about 20
microSiemens is observed on short devices, which meets the best dc results. The
capacitance per unit gate length of 60 aF/micrometer is typical of top gates on
a conventional oxide with a dielectric constant equal to 10. This value is a
factor of 3-5 below the nanotube quantum capacitance which, according to recent
simulations, favors high transit frequencies . For our smallest devices, we
find a large transit frequency equal to 50 GHz with no evidence of saturation
in length dependence.
