M. Itoh, H. Noguchi, H.Kitagawa, Y. Noda

The large maxima of the Seebeck coefficients as functions of the temperature observed for Bi-Sb semiconducting alloys are studied theoretically based on the Boltzmann transport theory. The systems are characterized by the very small effective mass and very narrow band gap, providing typical examples of strongly degenerate n-type semiconductors. The Fermi statistics is fully incorporated in the calculation, and the chemical potential is calculated self-consistently at each temperature from the Hall constant data. As increasing the temperature the calculated chemical potential stays almost constant at lower temperatures but starts to rise abruptly at certain temperature, where the maximum of the absolute Seebeck coefficient is observed, as well as the anomalies of other transport data (resistivity and Hall effect). The calculated temperature dependence of the coefficients agrees remarkably well with the measurements for the annealed samples, and the occurrence of the maxima is thus shown to be associated with the extrinsic-to-intrinsic transition. The thermoelectric characteristics of Bi-Sb alloys are therefore expected to be controllable, and improvable, by introducing various imperfections.