We report on the observation of the circular photogalvanic effect in Si-metal-oxide-semiconductor fieldeffect transistors with inversion channel excited by terahertz radiation. We demonstrate that in spite of the fact that the photocurrent is caused by transfer of the photon angular momentum to free carriers, it is not due to spin orientation but has a pure orbital origin. It results from the quantum interference of different pathways contributing to the free-carrier absorption of monochromatic radiation. DOI: 10.1103/PhysRevB.79.121302 PACS number͑s͒: 78.40.Fy, 72.40.ϩw, 73.40.Qv, 78.20.Ϫe The spin of electrons and holes in solid-state systems is an intensively studied quantum mechanical property showing a large variety of interesting physical phenomena. One of the most frequently used and powerful methods of generation and investigation of spin polarization is optical orientation with circularly polarized light. 1 Besides purely optical phenomena such as circularly polarized photoluminescence, the optical generation of an unbalanced spin distribution in a semiconductor may lead to helicity-dependent photocurrents, e.g., circular photogalvanic effect ͑CPGE͒. 2-4 CPGE current is excited only by light of nonzero helicity and reverses its direction upon switching the sign of circular polarization. So far, the CPGE has only been detected in materials with strong spin-orbit coupling and described by microscopic mechanisms based on spin-related processes. [2][3][4] Here we report on the observation of the CPGE caused by absorption of terahertz radiation in Si-metal-oxidesemiconductor field-effect transistors ͑Si-MOSFETs͒. The experimental demonstration of the existence of a helicitysensitive photocurrent in Si-based structures is of particular interest. Silicon is characterized by a vanishingly small spinorbit coupling which makes spin-related mechanisms of the CPGE ineffective and, therefore, cannot account for the observed helicity-dependent photocurrent. Thus, an access in explaining the CPGE is required, involving mechanisms of pure orbital ͑spin-unrelated͒ origin. Here, we show that the CPGE in our structures is due to quantum interference of different pathways contributing to monochromatic radiation absorption. This effect has been predicted theoretically 5 ͑see also Refs. 6 and 7͒ but not observed so far. Quantum interference plays an important role in various transport and optical phenomena. It has also been shown that it can induce photocurrents. Here, however, in contrast to the well-known photocurrents caused by quantum interference of one-and two-photon absorption processes in two color light, 8-11 the photocurrent is due to quantum interference in the elementary one-photon absorption process.We study n-type MOSFETs prepared on miscut Si surfaces to reduce spacial symmetry and enable photocurrents at normal incidence. The surfaces of our samples are tilted by the angle = 9.7°͑sample 1͒ or = 10.7°͑sample 2͒ from the ͑001͒ plane around x ʈ ͓110͔. We note, that the point group describing the miscut trans...