  We report on the observation of magnetic quantum ratchet effect in
metal-oxide-semiconductor field-effect-transistors on silicon surface
(Si-MOSFETs). We show that the excitation of an unbiased transistor by ac
electric field of terahertz radiation at normal incidence leads to a direct
electric current between the source and drain contacts if the transistor is
subjected to an in-plane magnetic field. The current rises linearly with the
magnetic field strength and quadratically with the ac electric field amplitude.
It depends on the polarization state of the ac field and can be induced by both
linearly and circularly polarized radiation. We present the quasi-classical and
quantum theories of the observed effect and show that the current originates
from the Lorentz force acting upon carriers in asymmetric inversion channels of
the transistors.
