A thin Si layer transfer process for monolithic 3D (M3D) integration is proposed using hydrogen ion (H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) implantation. The upper Si layer was transferred to CMOS circuits fabricated on the lower substrate by H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> implantation, oxide-to-oxide bonding, and a cleavage process at low temperature (< 500 °C). The M3D system comprising the photosensor connected to the CMOS device was demonstrated, where the thickness and roughness of the transferred Si layer were determined by H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> implantation and subsequent processes. The hetero-junctional photosensor was fabricated on the transferred Si layer, which generated the photocurrent ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{I}_{\text {ph}}$ </tex-math></inline-formula> ) by light exposure. The photosensor and ring oscillator circuits of the vertical structure implemented by the M3D process generated the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{I}_{\text {ph}}$ </tex-math></inline-formula> according to the light exposure intensity and showed different frequency behaviors accordingly. Compared with the continuous device scaling approach, M3D may be an alternative scheme for low-power, high-performance, and multi-functional devices.