A semiconductor device includes a first transistor, a second transistor, a third transistor, and a fourth transistor. The first and second transistors operate under a lower gate voltage than the third and fourth transistors. The first transistor has a first active gate structure and the second transistor has a second active gate structure. The first and second active gate structures are separated by a first gate isolation structure along a first direction. The third transistor has a third active gate structure and the fourth transistor has a fourth active gate structure. The third and fourth active gate structures are separated by a second gate isolation structure along the first direction. The variation of a first distance between respective sidewalls of the first gate isolation structure is equal to the variation of a second distance between respective sidewalls of the second gate isolation structure along the first direction.