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We measured the Seebeck coefficient of heavily P-doped silicon-on-insulator layers with P concentrations above 1 ϫ 10 19 cm −3. The coefficient decreased with increasing P concentration, and with a peak of the Seebeck coefficient around 5 ϫ 10 19 cm −3. We calculated the density-of-states ͑DOS͒ of bulk Si based on theoretical models of impurity-band formation, ionization-energy shift, and conduction-band tailing. The calculated impurity-concentration dependence of the energy derivative of the DOS at the Fermi energy also showed a peak. Consequently, the Seebeck coefficient of the heavily doped Si is ruled by the DOS distribution, similar to metallic materials.

We measured the Seebeck coefficient of P-doped ultrathin silicon-on-insulator (SOI) layers with thicknesses of 2-100 nm. The dependence of the coefficient on the impurity concentration was investigated, and was shown to be in good agreement with that of bulk Si for SOI thicknesses above 6 nm. In addition, it was found to decrease with increasing impurity concentration, which is usually observed in semiconductor materials. However, for doping levels above 3.5 × 10 19 cm −3 , the Seebeck coefficient was observed to increase. This is likely to be due to the influence of an impurity band.

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