 The collector–base junction is reverse-biased, and so negligible electron injection occurs from the collector to the base, but carriers that are injected into the base and diffuse to reach the collector-base depletion region are swept into the collector by the electric field in the depletion region.The thin "shared" base and asymmetric collector–emitter doping are what differentiates a bipolar transistor from two "separate" and oppositely biased diodes connected in series.The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control).