a method for forming a transistor comprising the steps of : forming a trench in a substrate ; filling an insulating layer in the lower portion of said trench except for the upper portion thereof ; filling a conductive layer in the upper portion of said trench and on said insulating layer for a channel of the said transistor ; forming a gate oxide layer on the resulting structure ; and forming a gate electrode on said gate oxide layer ; and implanting impurity ions into said substrate to form a source / drain region .