Forward biasing action and direction of current details of NPN transistor - original source below

  • The beauty of transistor structure lies in its different areas of each of the layers. Also, the base layer is kept between collector and emitter. We can say that the base acts just like a ‘door’ for the transfer of electrons from emitter to collector through it. When base voltage increases, this ‘door’ opens wider and more number of electrons are allowed to flow through it.
  • The area of base layer is deliberately kept small because of two reasons. First, it creates strong repulsive forces among the crowded electrons in base layer. When electrons from emitter layer come into base layer, they get trapped, as they cannot go back, due to repulsive force of negative ions of base layer near B-E junction. In base layer they find very small number of holes to recombine. The base voltage is also small. So very small number of electrons are attracted towards positive terminal of Vbe battery.
  • Eventually, all these electrons crowded in base layer have but one option, to surmount the ‘gap’ of C-B junction and enter into collector layer. But this junction is already reverse biased (note the polarity connections of Vcb battery). Now how they will jump over this ‘large gap’ of C-B junction? They will jump this gap because, the strong positive electric field of collector layer is ‘inviting’ them or attracting them. So with two forces: one is mutual repulsive force among them in base layer and one is the strong attractive force from collector layer, they succeed to surmount this large gap of C-B junction and enter into collector layer.

(Original source for diagram and text above)

I am just a beginner in Semiconductors so I just had some doubts when I read the above statement, which include:

  • 1) What the base voltage really is Vcb or Vbe and also base current?

  • 2) Why does increasing base voltage increase the number of electrons?

  • 3) What really happens in collector region? I am confused about how they explained above.

This is a common base configuration

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    \$\begingroup\$ Please add a reference for the original source of the diagram and text. Did they come from here? \$\endgroup\$ – SamGibson Feb 16 '18 at 13:11
  • \$\begingroup\$ @SamGibson yeah from there but it would have been too long \$\endgroup\$ – Scáthach Feb 16 '18 at 13:15
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    \$\begingroup\$ "but it would have been too long" Sorry, but I don't understand: What would have been too long? || Although the site rules here, which explain how to correctly reference external information, are written with the expectation that people would copy external sources in answers, it is always good practice to reference sources, so that you don't get accused of plagiarism. \$\endgroup\$ – SamGibson Feb 16 '18 at 13:29
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    \$\begingroup\$ @SamGibson I will remember that.Sorry for the inconvenience \$\endgroup\$ – Scáthach Feb 16 '18 at 13:31

1) Talking about "base voltage" is indeed confusing and I think it actually means nothing as a voltage is always between two points. If the emitter was grounded then it could make sense as then the "base voltage" could be the potential on the base. But there's no ground in the circuit! So "base voltage" means nothing.

I'm quite sure the writer means \$V_{be}\$ when writing "base voltage". This is because "more electrons flow" when the "base voltage" is increased. It would be better to write:

When \$V_{be}\$ is increased, the voltage across the Base-Emitter PN junction increases causing more current to flow.

The base current is simply the current flowing into the base contact through the \$I_b\$ current meter.

2) Increasing \$V_{be}\$ means more voltage across the Base-Emitter PN junction. That PN junction behaves as a diode. Look up the behavior of a PN diode and remember that the relation is exponential so even a small increase in voltage causes an very large increase in current.

3) The collector just "catches" the electrons that made it through the base (which is most of them since the base is very short) and these electrons from the collector current \$I_c\$.

This is a common base transistor No it is not.

You can use a bipolar transistor (NPN or PNP) in a common base circuit configuration. This relates to using a transistor as a signal amplifier. Other configurations are Common emitter and Common collector.

A transistor is never a common .... transistor. Any transistor can be used in any of the configurations I mentioned. But this is circuit design and not device physics/ how a transistor works, those are really separate subjects. Related though but still separate.

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  • \$\begingroup\$ One thing I want to make sure is that whether Ib is the electrons leaving the base junction and collector current is the electrons leaving the collector? \$\endgroup\$ – Scáthach Feb 16 '18 at 14:29
  • \$\begingroup\$ @harambe This is an NPN in forward mode, then Ib flows into the base meaning the electrons come out. The same is true for the collector, Ic flows into the collector so the electrons flow out of the collector. If beta (current gain) of an NPN is 99 then you could say that of 100 electrons going into the emitter, one goes out at the base, the other 99 come out at the collector. \$\endgroup\$ – Bimpelrekkie Feb 16 '18 at 14:44
  • \$\begingroup\$ @Bimpelrekkie Why is Vcb>>Vbe for active state...is there a particular reason?Also is Vce=Vbc+Vbe \$\endgroup\$ – Hydrous Caperilla Feb 17 '18 at 2:04
  • \$\begingroup\$ also what is this positive electric field thaat they have metioned in collector region ...is that from the holes? \$\endgroup\$ – Hydrous Caperilla Feb 17 '18 at 9:00
  • \$\begingroup\$ @HydrousCaperilla Actually Vcb and Vbe are independent. For active state it is only needed that the CB-junction is reversed biased. If Vcb = 0 that's already the case. If Vcb < 0 you start to enter saturation mode. The positive field in the collector comes from the collector being connected to a higher positive voltage. OK, there would be no field in the case that Vcb = 0 but then still any electrons ending up in the collector would leave via the collector contact. They will not go back to the base. \$\endgroup\$ – Bimpelrekkie Feb 17 '18 at 10:46

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