Formation energies of native defects in Bi₂(Te_xSe_3−x), with comparison to ideal Bi₂Te₂S, are calculated in density-functional theory to assess transport properties. Bi₂Se₃ is found to be n type for both Bi- and Se-rich growth conditions, while Bi₂Te₃ changes fromn to p type going from Te- to Bi-rich conditions, as observed. Bi₂Te₂Se and Bi₂Te₂S are generally n type, explaining observed heavily doped n-type behavior in most samples. A (0/−) transition level at 16 meV above valence-band maximum for Bi on Te antisites in Bi₂Te₂Se is related to the observed thermally active transport gap causing a p-to-n transition at low temperature. Bi₂(Te_xSe_3−x) with x>2 are predicted to have high bulk resistivity due to effective carrier compensation when approaching the n-to-pcrossover. Predicted behaviors are confirmed from characterization of our grown single crystals.