A method for manufacturing a semiconductor device includes forming a device isolation layer in a substrate to define an active region, forming a gate insulating layer covering at least a portion of the active region, forming a gate electrode on the gate insulating layer, and forming an interlayer insulating layer on the gate electrode. The gate insulating layer includes a first portion overlapping with the active region and a second portion overlapping with the device isolation layer. The forming of the gate insulating layer includes etching at least a part of the second portion of the gate insulating layer to thin the part of the second portion of the gate insulating layer.