Physical Origin of The Thermoelectric Coefficients
A material's temperature, crystal structure, and impurities influence the value of the thermoelectric coefficients. The Seebeck effect can be attributed to two things: charge-carrier diffusion and phonon drag. Typically metals have small Seebeck coefficients because of partially filled bands, with a conductivity that is relatively insensitive to small changes in energy. In contrast, semiconductors can be doped with impurities that donate excess electrons or electron holes, allowing the value of S to be varied over a large range (both negative and positive). The sign of the Seebeck coefficients can be used to determine whether the electrons or the holes dominate electric transport in a semiconductor or semimetal.
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