Intersubband resonances are investigated in hole space-charge layers on Si(111), Si(100), and Si(110), and in electron space-charge layers on Si(110) and Si(111). The resonances are excited by far-infrared radiation with the electric field vector polarized parallel to the interface. We report on frequency domain studies covering a wide range of frequencies (50---1000 ${\mathrm{cm}}^{\ensuremath{-}1}$) and charge densities [$(1\ensuremath{-}14)\ifmmode\times\else\texttimes\fi{}{10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}2}$]. We have investigated resonance positions, line shapes, excitation strengths, polarization dependence, and the influence of depletion fields. Results are compared with available theories on hole and electron subband systems.