A semiconductor device includes high-voltage (HV) and low-voltage (LV) MOS's formed in a substrate. The HV MOS includes a first semiconductor region having a first-type conductivity and a first doping level, a second semiconductor region having the first-type conductivity and a second doping level lower than the first doping level, a third semiconductor region having a second-type conductivity, and a fourth semiconductor region having the first-type conductivity. The first, second, third, and fourth semiconductor regions are arranged along a first direction, and are drain, drift, channel, and source regions, respectively, of the HV MOS. The LV MOS includes the fourth semiconductor region, a fifth semiconductor region having the second-type conductivity, and a sixth semiconductor region having the first-type conductivity. The fourth, fifth, and sixth semiconductor regions are arranged along a second direction different from the first direction, and are drain, channel, and source regions, respectively, of the LV MOS.