The power source Ub is applied to the emitter junction through the resistor Rb, the emitter junction is forward biased, and the majority carriers (free electrons) in the emitter region continuously cross the emitter junction and enter the base region, forming the emitter current Ie. At the same time, the majority carriers in the base region also diffuse to the emitter region, but since the majority carrier concentration is much lower than that in the emitter region, this current can be ignored, so it can be considered that the emitter junction is mainly electron current.