a semiconductor device has at least one high - voltage and low - voltage transistor on a single substrate . the reliability of the high - voltage transistor is enhanced by performing a ldd implantation in only the high - voltage transistor prior to conducting an oxidation process to protect the substrate and gate electrode . after the oxidation process is performed , the low - voltage transistor is subjected to an ldd implantation process . the resultant semiconductor device provides a high - voltage transistor having a deeper ldd region junction depth than the low - voltage transistor , ensuring reliability and performance .