A bit line driver device includes a semiconductor substrate and at least one isolation structure disposed in the semiconductor substrate. Active regions are defined in the semiconductor substrate by the at least one isolation structure. Each of the active regions is elongated in a first direction, and two of the active regions are disposed adjacent to each other in a second direction. Each of the active regions includes a first portion, a second portion, and a third portion. The third portion is disposed between the first portion and the second portion in the first direction. A width of the third portion is smaller than a width of the first portion and a width of the second portion. The distance between the two adjacent active regions may be increased by the third portions accordingly.