NPN BJT with forward-biased:

An NPN transistor can be referred as two diodes along with a shared anode. In common operation, the base-emitter junction is forward biased and the base-collector junction is reverse biased. In an NPN transistor, for instance, while a positive voltage is applied to the base-emitter junction, the equilibrium among the thermally generated carriers and the repelling electric field of the depletion region turns unbalanced, permitting thermally excited electrons to inject into the base region. These electrons wander or "diffuse" via the base from the region of high concentration near the emitter in the direction of the region of low concentration near the collector. The electrons in the base are termed as minority carriers since the base is doped p-type that would make holes the majority carrier in the base.

To minimize the percentage (%) of carriers which recombine before arriving at the collector-base junction, the base region of transistor must be thin enough which carriers can diffuse across it in much less time than as compared to the semiconductor's minority carrier lifetime. Particularly, the thickness of the base has to be much less than diffusion length of the electrons. The collector-base junction is reverse-biased, and thus the little electron injection takes place from the collector to the base, but electrons that diffuse by the base towards the collector are swept into the collector via the electric field in the depletion region of the collector-base junction. The thin shared base and asymmetric collector-emitter doping is what distinguishes a bipolar transistor from two separate and oppositely biased diodes that are connected in series.