A method includes forming a first dielectric layer over the substrate and covering first, second, third, fourth, fifth and sixth protrusion regions; forming first, second, and third gate conductors over the first, fourth, and fifth protrusion regions, respectively; performing a first implantation process to form a second source region and a second drain region in the fourth protrusion region; performing a second implantation process to form a first source region and a first drain region in the first protrusion region, and to form a third source region and a third drain region in the fifth protrusion region; forming a metal layer over the third protrusion region; patterning the metal layer to form an inner circular electrode and an outer ring electrode encircling the inner circular electrode; forming a word line; and forming a bit line.