  We present a fabrication process for high quality suspended and double gated
trilayer graphene devices. The electrical transport measurements in these
transistors reveal a high charge carrier mobility (higher than 20000 cm^2/Vs)
and ballistic electric transport on a scale larger than 200nm. We report a
particularly large on/off ratio of the current in ABC-stacked trilayers, up to
250 for an average electric displacement of -0.08 V/nm, compatible with an
electric field induced energy gap. The high quality of these devices is also
demonstrated by the appearance of quantum Hall plateaus at magnetic fields as
low as 500mT.
