A method of forming a field effect transistor includes forming a vertical channel protruding from a substrate including a source/drain region junction between the vertical channel and the substrate, and forming an insulating layer extending on a side wall of the vertical channel toward the substrate to beyond the source/drain region junction. The method may also include forming a nitride layer extending on the side wall away from the substrate to beyond the insulating layer, forming a second insulating layer extending on the side wall that is separated from the channel by the nitride layer, and forming a gate electrode extending on the side wall toward the substrate to beyond the source/drain region junction.