The large maximum of thermopower as a function of temperature observed for Bismuth Antimony alloy is studied theoretically based on the Boltzmann transport theory of single carrier. The Fermi statistics is fully taken into account, with the chemical potential calculated self-consistently at each temperature from experimental Hall coefficients. It is almost constant at lower temperatures, and begins to increase at T=70 K, where the maximum of the thermopower is observed. The temperature dependence of the calculated thermopower agrees well with the measurements. The occurrence of the maximum is thus shown to be associated with the transition between extrinsic conduction and intrinsic conduction, which provides theoretical confirmation to the known empirical rule.
