  Transport due to spin-helical massless Dirac fermion surface state is of
paramount importance to realize various new physical phenomena in topological
insulators, ranging from quantum anomalous Hall effect to Majorana fermions.
However, one of the most important hallmarks of topological surface states, the
Dirac linear band dispersion, has been difficult to reveal directly in
transport measurements. Here we report experiments on Bi2Te3 nanoribbon
ambipolar field effect devices on high-k SrTiO3 substrates, where we achieve a
gate-tuned bulk metal-insulator transition and the topological transport regime
with substantial surface state conduction. In this regime, we report two
unambiguous transport evidences for gate-tunable Dirac fermions through {\pi}
Berry$'$s phase in Shubnikov-de Haas oscillations and effective mass
proportional to the Fermi momentum, indicating linear energy-momentum
dispersion. We also measure a gate-tunable weak anti-localization (WAL) with 2
coherent conduction channels (indicating 2 decoupled surfaces) near the charge
neutrality point, and a transition to weak localization (indicating a collapse
of the Berry$'$s phase) when the Fermi energy approaches the bulk conduction
band. The gate-tunable Dirac fermion topological surface states pave the way
towards a variety of topological electronic devices.
