  Electronic transport experiments involving the topologically protected states
found at the surface of Bi2Se3 and other topological insulators require fine
control over carrier density, which is challenging with existing bulk-doped
material. Here we report on electronic transport measurements on thin (<100 nm)
Bi2Se3 devices and show that the density of the surface states can be modulated
via the electric field effect by using a top-gate with a high-k dielectric
insulator. The conductance dependence on geometry, gate voltage, and
temperature all indicate that transport is governed by parallel surface and
bulk contributions. Moreover, the conductance dependence on top-gate voltage is
ambipolar, consistent with tuning between electrons and hole carriers at the
surface.
