A method for manufacturing a semiconductor device includes forming a first CPODE dummy poly gate and a second CPODE dummy poly gate on a semiconductor substrate; removing the first CPODE dummy poly gate and a portion of the semiconductor substrate therebelow to form a first trench extending into the semiconductor substrate; filling the first trench with a first dielectric material to form a first isolation structure to isolate the first and second transistors from each other; removing the second CPODE dummy poly gate and a portion of the semiconductor substrate therebelow to form a second trench extending into the semiconductor substrate; and filling the second trench with a second dielectric material having a dielectric composition different from that of the first dielectric material to form a second isolation structure to isolated the third and fourth transistors from each other.