Electron travelling against electric field in NPN transistor

[XT.5155]

When NPN transistori is forward biased, electron in BASE region would travel against electric field in BASE-COLLECTOR junction.

I read also that it is due to diffusion process which I still don't understand because it is still swimming against electric field.

Kindly explain about it, also how does electron travel in neutral zone inside Base-Collector junction.

Thank you in advance
XT

 

[WimRFP]

Diffusion is a thermally driven proces (see it as vibration driven movement). Even in steady air, bad smelling gas will spread across the room (so is able to travel certain distance without air flow). It may even go upstream a very slow airflow.

So if you connect P material with N material, electrons from the N region will diffuse into the P region (for now I ignore the holes). Electrons that travel from the N region to the P region (diffusion), leave a positive charge behind (because of the charge of nucleus). When more electrons leave the N region, the field strength (pointing from the N region towards the P region) increases and there will establish a return flow of electrons. This leads to equilibrium between electron flow due to diffusion and electron flow due to the electric field strength in the depletion layer (N area where the material is depleted of electrons).

When someone reduces this electric field by applying an external voltage (P material positive w.r.t. N material), the return flow is reduced, but the diffusion flow continues, as we didn't change the temperature. Now we have a net current flowing across the junction.

Now to the transistor.
The base has relatively low doping (w.r.t. the emitter region), the base is also thin. When an electron enters the P base (from the N emitter), it will move erratically across the base. When, after some time, it meets a hole it recombines and will add to the base current. However when it reaches the Collector Base junction, it will be contribute to the collector current.

When the base is too thick it will recombine before it has moved sufficiently to reach the collector base depletion. A BJT works because of the thin base layer where charge carriers can cross the base via diffusion in short time (less then the carrier lifetime in the base).

Note that there will also be a flow of holes (as the hole concentration in the P material is higher then in the N material). So the total current is made up of electron and hole flow.

 

[XT.5155]

Thank you Wim.

My 4-bit brain still need time to digest the dual of your explanation that is the flow of hole.

 

[XT.5155]

Note that there will also be a flow of holes (as the hole concentration in the P material is higher then in the N material). So the total current is made up of electron and hole flow.

what device can we use to measure current which generated ONLY by holes.