  It is crucial for the studies of the transport properties and quantum effects
related to Dirac surface states of three-dimensional topological insulators (3D
TIs) to be able to simultaneously tune the chemical potentials of both top and
bottom surfaces of a 3D TI thin film. We have realized this in molecular beam
epitaxy-grown thin films of 3D TIs, as well as magnetic 3D TIs, by fabricating
dual-gate structures on them. The films could be tuned between n-type and
p-type by each gate alone. Combined application of two gates can reduce the
carrier density of a TI film to a much lower level than with only one of them
and enhance the film resistance by 10000 %, implying that Fermi level is tuned
very close to the Dirac points of both top and bottom surface states without
crossing any bulk band. The result promises applications of 3D TIs in field
effect devices.
