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A question about the working of BJT

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Magnethicc

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Hi everyone, I was wondering if you could answer a question I have:
In the case of a NPN Transistor in the active/saturation region - what is the primary reason for the collector to emitter current?
is it diffusion or is it drift?
 

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.
 
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