  The newly-discovered three-dimensional strong topological insulators (STIs)
exhibit topologically-protected Dirac surface states. While the STI surface
state has been studied spectroscopically by e.g. photoemission and scanned
probes, transport experiments have failed to demonstrate the most fundamental
signature of the STI: ambipolar metallic electronic transport in the
topological surface of an insulating bulk. Here we show that the surfaces of
thin (<10 nm), low-doped Bi2Se3 (\approx10^17/cm3) crystals are strongly
electrostatically coupled, and a gate electrode can completely remove bulk
charge carriers and bring both surfaces through the Dirac point simultaneously.
We observe clear surface band conduction with linear Hall resistivity and
well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum
conductivity region. A theory of charge disorder in a Dirac band explains well
both the magnitude and the variation with disorder strength of the minimum
conductivity (2 to 5 e^2/h per surface) and the residual (puddle) carrier
density (0.4 x 10^12 cm^-2 to 4 x 10^12 cm^-2). From the measured carrier
mobilities 320 cm^2/Vs to 1,500 cm^2/Vs, the charged impurity densities 0.5 x
10^13 cm^-2 to 2.3 x 10^13 cm^-2 are inferred. They are of a similar magnitude
to the measured doping levels at zero gate voltage (1 x 10^13 cm^-2 to 3 x
10^13 cm^-2), identifying dopants as the charged impurities.
