one embodiment of the present invention is a method of simultaneously forming high - voltage and low - voltage devices on a single substrate , the method comprising : forming a thin oxide layer on the substrate , the thin oxide layer having a desired thickness for a gate oxide for the low - voltage device ; selectively forming a gate structure for the high - voltage device , the thin oxide is situated between the gate structure and the substrate ; and selectively thickening the thin oxide under the gate structure while keeping the thin oxide layer utilized for the low - voltage device at the desired thickness .