Due to the large reverse voltage applied to the collector junction, the electric field force generated by this reverse voltage will prevent the electrons from the collector region from diffusing to the base region, and at the same time, the electrons diffused to the vicinity of the collector junction will be pulled into the collector region to form the main collector of the collector. current Icn. In addition, the minority carriers (holes) in the collector region will also produce drift motion, and flow to the base region to form a reverse saturation current, which is represented by Icbo, which is very small in value, but is extremely sensitive to temperature.