Understanding how a NPN transistor works
Electrons (majority carriers) travel from the n-type emitter region to the p-type base region because of the forward biased BE junction. Once in p-type base region these electrons become minority carriers. Because the base is thin and lightly doped only a few of these electrons recombine to produce the base current, the majority are swept across the reverse biased CB junction attracted by the positive potential at the collector. It is because the electrons are minority carriers in the base region that they can travel across the reversed biased CB junction as it is quite possible for minority carriers to travel across a reversed biased PN junction.