Temperature-dependence (T: 5−300 K) of the thermoelectric (TE) properties of n-type Si_0.97Sn_0.03 epitaxial layers with the different electron concentrations have been firstly investigated. A huge Seebeck coefficient due to the phonon-drag effect was observed in the Si_0.97Sn_0.03 layers with high electron concentration above 10¹⁹ cm⁻³, which is not observed in the heavily doped Si bulk. This leads to a superior power factor of 1.1×10³ μWcm⁻¹K⁻² at 25 K. Additionally, we found that some phonon-drag effect remains even at 300 K, unlike Ge_1−xSn_x layers grown on GaAs substrates.