a method for fabricating a triple well in a semiconductor device , includes the steps of forming a first well of a first conductive type with a first concentration lower than a first target concentration , wherein the first concentration is the minimum dose capable of isolating neighboring wells each other and forming a second well of a second conductive type with a second concentration higher than a second target concentration , wherein the second well includes a first region surrounded by the first well and a second region isolated from the first region by the first well .