Transistors with low-power operation and sufficient signal processing speed have been widely required especially for mobile applications. To meet these requirements, we propose a novel tunneling- and diffusion-current hybrid MOSFETs which utilize both a small S-factor of tunneling-current and a high current drivability of diffusion-current. On the basis of the device concept and the working principle that we propose, device structures and parameters of the hybrid MOSFET were examined in detail with device simulation. We investigated the optimum structure of the tunneling and diffusion parts, and the effects of various structural parameters. In particular, we found that the threshold voltage adjustment of each current and the suppression of leakage current were the most important to maximize the characteristics of the hybrid MOSFET. From the result of structural parameter optimization, a minimum S-factor of 20mV/decade and a high current drivability of 500 μA/μm were obtained simultaneously. In addition, a process flow idea to fabricate three-dimensional structure hybrid MOSFETs with tunneling-current side channels and diffusion-current top channel is presented.