 A discrete transistor has three leads for connection to these regions.Typically, the emitter region is heavily doped compared to the other two layers, and the collector is doped much lighter than the base (collector doping is typically ten times lighter than base doping ).By design, most of the BJT collector current is due to the flow of charge carriers (electrons or holes) injected from a heavily doped emitter into the base where they are minority carriers that diffuse toward the collector, and so BJTs are classified as minority-carrier devices.