Contacts are made to all three regions, the two outer regions called the emitter and collector and the middle region called the base. The base and collector current are positive if a positive current goes into the base or collector contact. The emitter current is positive for a current coming out of the emitter contact.
Electrons diffuse from the emitter into the base and holes diffuse from the base into the emitter.
This carrier diffusion is identical to that in a p-n junction. However, what is different is that the electrons can diffuse as minority carriers through the quasi-neutral region in the base. Once the electrons arrive at the base-collector depletion region, they are swept through the depletion layer due to the electric field. These electrons contribute to the collector current. In addition, there are two more currents, the base recombination current and the depletion layer recombination.