A bit line driver device, comprising: a semiconductor substrate; and at least one isolation structure disposed in the semiconductor substrate for defining active regions in the semiconductor substrate, wherein each of the active regions is elongated in a first direction, two of the active regions are disposed adjacent to each other in a second direction, and each of the active regions comprises: a first portion and a second portion; and a third portion disposed between the first portion and the second portion in the first direction, wherein a width of the third portion is smaller than a width of the first portion and a width of the second portion; the bit line driver device further comprising: a gate structure disposed on the semiconductor substrate, wherein the gate structure is elongated in the second direction, and the gate structure partially overlaps each of the active regions; drain regions disposed in the active regions, wherein at least a part of each of the drain regions is disposed in one of the first portions; and source regions disposed in the active regions, wherein at least a part of each of the source regions is disposed in one of the second portions, wherein the drain region and the source region disposed in the same active region are disposed at two opposite sides of the gate structure in the first direction, wherein the gate structure partially overlaps the second portions in a thickness direction of the semiconductor substrate, and characterised in that the gate structure partially overlaps the third portions in a thickness direction of the semiconductor substrate. a semiconductor substrate; and at least one isolation structure disposed in the semiconductor substrate for defining active regions in the semiconductor substrate, wherein each of the active regions is elongated in a first direction, two of the active regions are disposed adjacent to each other in a second direction, and each of the active regions comprises: a first portion and a second portion; and a third portion disposed between the first portion and the second portion in the first direction, wherein a width of the third portion is smaller than a width of the first portion and a width of the second portion; the bit line driver device further comprising: a gate structure disposed on the semiconductor substrate, wherein the gate structure is elongated in the second direction, and the gate structure partially overlaps each of the active regions; drain regions disposed in the active regions, wherein at least a part of each of the drain regions is disposed in one of the first portions; and source regions disposed in the active regions, wherein at least a part of each of the source regions is disposed in one of the second portions, wherein the drain region and the source region disposed in the same active region are disposed at two opposite sides of the gate structure in the first direction, wherein the gate structure partially overlaps the second portions in a thickness direction of the semiconductor substrate, and characterised in that the gate structure partially overlaps the third portions in a thickness direction of the semiconductor substrate. a first portion and a second portion; and a third portion disposed between the first portion and the second portion in the first direction, wherein a width of the third portion is smaller than a width of the first portion and a width of the second portion; the bit line driver device further comprising: a gate structure disposed on the semiconductor substrate, wherein the gate structure is elongated in the second direction, and the gate structure partially overlaps each of the active regions; drain regions disposed in the active regions, wherein at least a part of each of the drain regions is disposed in one of the first portions; and source regions disposed in the active regions, wherein at least a part of each of the source regions is disposed in one of the second portions, wherein the drain region and the source region disposed in the same active region are disposed at two opposite sides of the gate structure in the first direction, wherein the gate structure partially overlaps the second portions in a thickness direction of the semiconductor substrate, and characterised in that the gate structure partially overlaps the third portions in a thickness direction of the semiconductor substrate. a gate structure disposed on the semiconductor substrate, wherein the gate structure is elongated in the second direction, and the gate structure partially overlaps each of the active regions; drain regions disposed in the active regions, wherein at least a part of each of the drain regions is disposed in one of the first portions; and source regions disposed in the active regions, wherein at least a part of each of the source regions is disposed in one of the second portions, wherein the drain region and the source region disposed in the same active region are disposed at two opposite sides of the gate structure in the first direction, wherein the gate structure partially overlaps the second portions in a thickness direction of the semiconductor substrate, and characterised in that the gate structure partially overlaps the third portions in a thickness direction of the semiconductor substrate.