A semiconductor device comprises a substrate extending in a horizontal direction and a vertical transistor on the substrate. The vertical transistor comprises: a first diffusion region on the substrate; a channel region on the first diffusion region and extending in a vertical direction relative to the horizontal direction of the extension of the substrate; a second diffusion region on the channel region; and a gate electrode at a sidewall of, and insulated from, the channel region. A horizontal transistor is positioned on the substrate, the horizontal transistor comprising: a first diffusion region and a second diffusion region on the substrate and spaced apart from each other; a channel region on the substrate between the first diffusion region and the second diffusion region; and a gate electrode on the channel region and isolated from the channel region. A portion of a gate electrode of the vertical transistor and a portion of the gate electrode of the horizontal transistor are at a same vertical position in the vertical direction relative to the substrate.