Organosilane self-assembled monolayers, embedded in organic single-crystal field-effect transistors, significantly affect subthreshold properties in the transfer characteristics. The monolayer of either polarized or nearly unpolarized molecules is deposited on a SiO2/doped Si substrate before softly attaching a rubrene thin single crystal to form the "laminated crystal" transistors. As a result of effective passivation of the SiO2 surface, the device has achieved a subthreshold swing as low as 0.11 V/decade. It is also demonstrated that threshold gate voltage is shifted by polarization of the monolayers, indicating that threshold of the device is adjustable by the choice of silane materials. Both of the functions are closely associated with low-power applications such as logic-circuit components.