NPN Transistor

Discussion in 'General Electronics Chat' started by Computer Engineer, Aug 28, 2009.

  1. Computer Engineer

    Thread Starter New Member

    Aug 25, 2009
    7
    0
    Hi my friends in electricity
    I have a question about NPN Transistor , why the Base - Collector part is connected in reverse biased , what is the reason ?

    Thank you
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  3. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    Computer Engineer,

    For a NPN, you need to make the collector more positive than the base so the electrons emitted from the emitter will pass through the base and get attracted and injected into the collector. This positive voltage on the collector makes it reversed biased, but it needs to be that way, otherwise the transistor will not operate in the active region.

    Ratch
     
  4. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    Also note that, when the transistor is operating in saturated
    mode, such as usually happens when you are using it as a
    switch, the base-collector junction is forward biased. If you
    want to make an amplifier rather than a switch, reverse
    biasing puts you in a situation (active mode) where the
    collector current can be effectively controlled by what is
    going on at the base-emitter junction.
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
  6. Computer Engineer

    Thread Starter New Member

    Aug 25, 2009
    7
    0
    Thank you my friends ,, I get it now :)
     
  7. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    Jony130,

    Your answer and explanation to the OP has got me scratching my head.

    What does the reverse diode current of a photodiode have to do with the need to reverse bias the base-collector of a BJT so as to keep it in the active region?

    The above certainly shows that the Ib is an indicator of what the Ic will be, but it doesn't do anything to explain why the base-collector of a BJT must be reverse biased to keep it in the active region.

    Ratch
     
  8. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    I can see a vague parallel; in both a photodiode and transistor, one key part of the function is injecting charge carriers to allow a reversed-biassed junction to conduct.

    In a photodiode, this is caused by photons creating electron-hole pairs, in a bipolar transistor it is direct current injection.

    The photon injection effect also occurs in general semiconductors, that is one reason they are usually in lightproof enclosures. Cut the top off a metal can transistor, or the lid off a dynamic RAM chip and you have a photosensor or even a crude CMOS camera sensor - there were a few DIY projects based on this some years ago, when 'real' solid-state cameras were things needed a lottery win to buy.
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    The inventors of a bjt wanted that bjt has a large voltage gain.
    So "output circuit" of a bjt should have a properties of a current source.
    And accidentally reverse biased p-n junction has the properties of a current source.
    And figure 4 is a example of a reverse biased p-n junction with properties of a current source.
    And this current can be regulated by change the amount of a minority carriers.
    And this is a job for forward bias p-n junction which injected minority carriers to the base (electrons for NPN). And this minority carriers take part in a current of a reverse biased junction.
     
  10. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    rjenkins,

    Hmm, I don't see any parallel at all. The first way you mentioned was direct injection. The second way was to break apart bonds within the semiconductor to create charge carriers. Two different mechanisms.

    Yes, I was aware that radiation (light and x-rays) besides heat can cause bonds to break and release charge carriers.

    Ratch
     
  11. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    Jony130,

    Then they failed, because gain depends on the configuration and the external components. For instance, a common collector configuration has a gain of less than 1.

    Yes, that is true.

    The pn junction does not give the collector the properties of a current source. If you could injected majority carrier electrons from a external source into a single slab of N-type semiconductor material, and connect a voltage to each end of the slab, you would have a current source. That is because the current depends on the injected electrons, not on the voltage applied to N-type material. The P-type material of the base is next to the collector because it is geometrically necessary to be there to collect the electrons injected from the emitter-base junction. So is is wrong to compare the base-collector of a transistor to a regular diode. For that and other reasons, you cannot make a transistor with 2 diodes connected back to back.

    It is an example of any BJT, whose collector acts as a current source for the reason given above, not because it is a reversed PN junction.

    The current is regulated by changing Vbe.

    You mean the emitter ejects majority carriers (electrons for the emitter) through the base and into the collector where the positive voltage attacts them into the external voltage source. The small base current is a unfortunate side effect of the Vbe necessary to lower the junction barrier voltage so the emitter can send out electrons. This base current is proportional to the current injected into the collector, and is useful for modeling and calculation purposes.

    Ratch

     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    No, he means that the transistor conduction is controlled by the base current. Your view -
    may work nicely as a model, but there are also other models. From where most of us stand, base current is the active and immediate control agency.

    This is the same music in a different dance. Let's not immediately jump into the most abstruse possible interpretations of transistor operation at the earliest possible moment.

    As to "control", that is probably a topic that needs some further discussion.
     
  13. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    beenthere,

    He would be wrong.

    Actually, the assumption of base current controlling collector current works much better in a model because of its linearity. But, as I pointed out before, a model does not necessary show what is actually happening.

    It is not abstruse to point out what is really happening in a BJT.

    Not the control describing the physics of the BJT. There is no way to change that.

    Ratch
     
  14. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Ratch
    Apparently you didn't understand the core of my statement.
    And I don't change my point of view.
    In NPN the "first junction" (base-emitter) "control" the amount of injected majority carrier to the collector. And "second junction" (base-collector) the reverse biased, gives the bjt properties of a current source. Because the current that is flow through collector is a mostly majority carriers current. And this is why the designers gives BJT such a structure.

    It is obviously, that it is impossible.

    And for this, maybe you can read this book
    "Analogue Design: The Current-Mode Approach"
    Particularly chapter 2, by Barrie Gilbert.
     
  15. Ratch

    New Member

    Mar 20, 2007
    1,068
    3
    Jony130,

    I agree, that is certainly true.

    You are repeating the same statement you made before, so I will repeat and rephrase my answer. The current source property of the collector does not depend on a PN junction, or the fact that the junction happens to be reversed biased. I showed that a single N-type semiconductor slab with a voltage across it could act as a current sourse if it was injected with electrons by some kind of electron gun. That was because the electron gun controls the current, and the voltage across the N-type semiconductor slab does not. I also pointed out that a positive voltage was needed to attract the electrons from the emitter, and that the position of the collector was geometrically necessary to be in the path of the emitted electrons. It is only conincidental that the collector forms a PN junction with the base, and it is reversed biased with respect to the base. If you disagree with the above discourse, please tell me where and why, but it does not do any good to repeat what you said before. I do understand what you said, and have evaluated it.

    Ratch
     
Loading...