The electronic properties of isolated single-walled carbon nanotubes (SWNTs) adsorbed onto n- and p-doped hydrogen-passivated Si(100) surfaces are studied by ultrahigh vacuum scanning tunnelling spectroscopy and ab initio density-functional methods. SWNTs identified as semiconductors (s-SWNTs) have well-defined conduction and valence band edges separated by a ≈1 eV gap, with the mid-gap Fermi level implying that the s-SWNTs are undoped. Relative s-SWNT/H-Si(100) band alignments inferred from dI/dV plots are sensitive to the polarity of the substrate doping. Band structure calculations for a (12,4) s-SWNT corroborate experimental data: n-type (p-type) doping of the substrate leads to a shift of the surface bands lower (higher) in energy relative to those of the s-SWNT. The adsorption energy and charge transfer calculated for the (12,4) s-SWNT physisorbed onto H-Si(100) are considerably less than values reported for the same tube on unpassivated Si(100) and are registration independent. The atomistic results presented here have critical implications to hybrid electronic and photonic devices that rely upon a direct interface between a SWNT and a technologically relevant semiconductor such as Si or GaAs.