  We report on the circular and linear photogalvanic effects caused by
free-carrier absorption of terahertz radiation in electron channels on
(001)-oriented and miscut silicon surfaces. The photocurrent behavior upon
variation of the radiation polarization state, wavelength, gate voltage and
temperature is studied. We present the microscopical and phenomenological
theory of the photogalvanic effects, which describes well the experimental
results. In particular, it is demonstrated that the circular (photon-helicity
sensitive) photocurrent in silicon-based structures is of pure orbital nature
originating from the quantum interference of different pathways contributing to
the absorption of monochromatic radiation.
