A bipolar junction transistor (BJT) includes a semiconductor substrate and a first isolation structure. The semiconductor substrate includes a first fin structure disposed in an emitter region, a second fin structure disposed in a base region, and a third fin structure disposed in a collector region. The first, the second, and the third fin structures are elongated in a first direction respectively. The base region is adjacent to the emitter region, and the base region is located between the emitter region and the collector region. The first isolation structure is disposed between the first fin structure and the second fin structure, and a length of the first isolation structure in the first direction is shorter than or equal to 40 nanometers. An effective base width of the BJT may be reduced by the disposition of the first isolation structure, and a current gain of the BJT may be enhanced accordingly.