A semiconductor device includes a first and second transistor. Each of the first and the second transistors includes a well of a first conductivity type, a band-shaped region provided on the well, a drain region of a second conductivity type provided on the well, and a gate electrode. The band-shaped region, the drain region and the gate electrode extend in a first direction. The band-shaped region includes a back gate region of the first conductivity type and a source region of the second conductivity type. The back gate region and the source region are arranged alternately along the first direction in the band-shaped region. A ratio of a length of the source region to a length of the back gate region along the first direction of the first transistor is greater than the ratio of the second transistor.