Even though Si is the most cost efficient and extensively used semiconductor in modern optoelectronics, it is not considered to be an effective THz emitter due to its low carrier drift velocity and small saturated built-in electric field from the inversion layer. Herein, we present an effective way to enhance THz generation using a graphene/Si Schottky junction (GSSJ) excited with a femtosecond laser under electrical gating without rapid saturation and with high carrier drift velocity. This mixed-dimensional van der Waals interface demonstrates large saturation pump fluence with an invalid inversion layer by removing the native oxide on the Si surface. The THz emission amplitude from GSSJ effectively increases with the gate voltage. The THz emission from GSSJ under the same excitation conditions is stronger than that from the surface of InAs (100) and GaAs (100). The results not only show an efficient THz emission from GSSJ but also demonstrate the ability of THz generation for probing the mixed-dimensional van der Waals interface.