In active operation of NPN bipolar transistor, electrons move form the emitter to collector, and the transport mechanism changes, depending on where they are:
- in the quasi neutral region of emitter (n-type), electrons are majority carriers, and move by drift (in a very weak electric field)
- from emitter to the base, the transport process is thermionic injection
- in the p-base, electrons are minority carriers, and move by diffusion, or, in the case of a built-in electric field (from dopant gradient, or from SiGe Ge dose grading) - by drift. If base is very thin, electrons can move ballistically or quasi ballistically.
- in the depletion region of base-collector junction, electric field is large, so electrons move by drift, with saturation velocity
- in quasi neutral collector, the transport is again a drift of majority carriers in a weak electric field.