  Combining the ability to prepare high-quality, intrinsic Bi$_2$Te$_3$
topological insulator thin films of low carrier density with in-situ protective
capping, we demonstrate a pronounced, gate-tunable change in transport
properties of Bi$_2$Te$_3$ thin films. Using a back-gate, the carrier density
is tuned by a factor of $\sim 7$ in Al$_2$O$_3$ capped Bi$_2$Te$_3$ sample and
by a factor of $\sim 2$ in Te capped Bi$_2$Te$_3$ films. We achieve full
depletion of bulk carriers, which allows us to access the topological transport
regime dominated by surface state conduction. When the Fermi level is placed in
the bulk band gap, we observe the presence of two coherent conduction channels
associated with the two decoupled surfaces. Our magnetotransport results show
that the combination of capping layers and electrostatic tuning of the Fermi
level provide a technological platform to investigate the topological
properties of surface states in transport experiments and pave the way towards
the implementation of a variety of topological quantum devices.
