A semiconductor and a method for manufacturing the semiconductor device are provided. A semiconductor substrate is provided. A first oxide layer is formed over an active region. A first STI is formed to adjoin a first side of the active region, and a second STI is formed to adjoin a second side of the active region. A gate layer is formed over the first STI, the second STI and the first oxide layer. A masking element is formed over the gate layer. The gate layer is etched using the masking element to form a first gate electrode over the first oxide layer, a first dummy gate electrode over the first STI, and a second dummy gate electrode over the second STI. A width of the first gate electrode is smaller than a width of the first dummy gate electrode and a width of the second dummy gate electrode.