A semiconductor device structure is provided. A first well region with a first type of conductivity is formed over a semiconductor substrate. A second well region with a second type of conductivity is formed over the semiconductor substrate. A well region is formed over the semiconductor substrate and between the first and second well regions. A first gate structure is disposed on the well region and partially over the first and second well regions. A drain region is in the first well region. A source region and a bulk region are in the second well region. The drain region, the source region and the bulk region have the first type of conductivity. A second gate structure is disposed on the second well region, and separated from the first gate structure by the source region and the bulk region.