A method for manufacturing a semiconductor device includes forming a first gate electrode on a semiconductor substrate in a first transistor region; forming a channel dose region; and forming a first source extension region, wherein the channel dose region is formed by using a first mask as a mask and by ion-implanting a first dopant of the first conductivity type, and the first mask covering a drain side of the first gate electrode and covering a drain region, and the first source extension region is formed by using a second mask and the gate electrode as masks and by ion-implanting a second dopant of a second conductivity type that is a conductivity type opposite to the first conductivity type, the second mask covering the drain side of the first gate electrode and covering the drain region.