  Few layer graphene systems such as Bernal stacked bilayer and rhombohedral
(ABC-) stacked trilayer offer the unique possibility to open an electric field
tunable energy gap. To date, this energy gap has been experimentally confirmed
in optical spectroscopy. Here we report the first direct observation of the
electric field tunable energy gap in electronic transport experiments on doubly
gated suspended ABC-trilayer graphene. From a systematic study of the
non-linearities in current \textit{versus} voltage characteristics and the
temperature dependence of the conductivity we demonstrate that thermally
activated transport over the energy-gap dominates the electrical response of
these transistors. The estimated values for energy gap from the temperature
dependence and from the current voltage characteristics follow the
theoretically expected electric field dependence with critical exponent $3/2$.
These experiments indicate that high quality few-layer graphene are suitable
candidates for exploring novel tunable THz light sources and detectors.
