1. A method for fabricating a semiconductor device, the method comprising: forming a step-type recess pattern in a substrate, wherein the step-type recess pattern includes a first pattern and a second pattern connected to a bottom of the first pattern and having the width smaller than the width of the first pattern; forming a gate electrode filling the recess pattern, wherein a gap is disposed between the gate electrode and upper sidewalls of the recess pattern; forming an insulation layer filling the gap; and forming a source region and a drain region by doping impurities into the substrate.