A semiconductor structure includes an array of unit metal-oxide-semiconductor (MOS) devices arranged in a plurality of rows and a plurality of columns is provided. Each of the unit MOS devices includes an active region laid out in a row direction and a gate electrode laid out in a column direction. The semiconductor structure further includes a first unit MOS device in the array and a second unit MOS device in the array, wherein active regions of the first and the second unit MOS devices have different conductivity types.