 The proportion of carriers able to cross the base and reach the collector is a measure of the BJT efficiency.The heavy doping of the emitter region and light doping of the base region causes many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter.A thin and lightly-doped base region means that most of the minority carriers that are injected into the base will diffuse to the collector and not recombine.