File:Bjt equilibrium bands v2.svg

Summary
Energy band diagram of a simple NPN bipolar junction transistor under equilibrium showing electron energy versus position. The depletion regions of the emitter-base and base-collector junctions are marked. $$E_c$$ is the conduction band, $$E_f$$ is the device fermi energy level, $$E_i$$ is the intrinsic Fermi level of the undoped semiconductor, and $$E_v$$ is the valence band.