  We report the effect of Sn doping on the transport properties of the
topological insulator Bi_{2}Te_{2}Se studied in a series of
Bi_{2-x}Sn_{x}Te_{2}Se crystals with 0 \leq x \leq 0.02. The undoped
stoichiometric compound (x = 0) shows an n-type metallic behavior with its
Fermi level pinned to the conduction band. In the doped compound, it is found
that Sn acts as an acceptor and leads to a downshift of the Fermi level. For x
\geq 0.004, the Fermi level is lowered into the bulk forbidden gap and the
crystals present a resistivity considerably larger than 1 Ohmcm at low
temperatures. In those crystals, the high-temperature transport properties are
essentially governed by thermally-activated carriers whose activation energy is
95-125 meV, which probably signifies the formation of a Sn-related impurity
band. In addition, the surface conductance directly obtained from the
Shubnikov-de Haas oscillations indicates that a surface-dominated transport can
be achieved in samples with several um thickness.
