Two-dimensional (2D) materials have been considered to hold promise for transistor ultrascaling, thanks to their atomically thin body immune to short-channel effects. The lower channel size limit of 2D transistors is yet to be revealed, as this size is below the spatial resolution of most lithographic techniques. In recent years, chemical approaches such as chemical vapor deposition (CVD) and metalorganic CVD (MOCVD) have been established to grow atomically precise nanostructures and heterostructures, thus allowing for synthetic construction of ultrascaled transistors. In this review, we summarize recent developments on the precise synthesis and defect engineering of electronic nanostructures/heterostructures aiming for transistor applications. We demonstrate with rich examples that ultrascaled 2D transistors are achievable by finely tuning the “growth-as-fabrication” process and could host a plethora of new device physics. Finally, by plotting the scaling trend of 2D transistors, we conclude that synthetic electronics possess superior scaling capability and could facilitate the development of post-Moore nanoelectronics.