A series of $\mathrm{Pb}{(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x})}_{2}\mathrm{T}{\mathrm{e}}_{4}$ topological-insulator crystals with various Sb molar ratios $x$ was fabricated, and a systematic study of their transport properties was performed, aiming at realizing enhanced bulk insulation in this system. Hall-effect measurements showed that $n$- to $p$-type transition in bulk conduction occurred at $x\ensuremath{\approx}0.80$. Semiconducting behavior with a negative temperature coefficient of resistivity was observed with resistivity values as high as 180 m\textohm{} cm at 2 K around the transition composition. The conduction mechanism in the semiconducting samples is discussed in relation to an impurity band formed within the bandgap.