Methods and devices for forming both high-voltage and low-voltage transistors on a common substrate using a reduced number of processing steps are disclosed. An exemplary method includes forming at least a first high-voltage transistor well and a first low-voltage transistor well on a common substrate separated by an isolation structure extending a first depth into the substrate, using a first mask and first implantation process to simultaneously implant a doping material of a first conductivity type into a channel region of the low-voltage transistor well and a drain region for the high-voltage transistor well.