Integrated circuit devices including a bipolar junction transistor (BJT) and/or a P-N junction diode are provided. The integrated circuit devices may include a first stack including first and second semiconductor regions that are spaced apart from each other in a horizontal direction and have a first conductivity type and a plurality of nano-semiconductor layers that are stacked in a vertical direction and are between the first and second semiconductor regions. The plurality of nano-semiconductor layers each have a second conductivity type, and the first semiconductor region may include a side surface facing the plurality of nano-semiconductor layers. The integrated circuit device may also include a vertical semiconductor layer having the second conductivity type and a conductive contact that contacts the plurality of nano-semiconductor layers. The vertical semiconductor layer may contact the side surface of the first semiconductor region and the plurality of nano-semiconductor layers.