Collector Base Junction controlling reverse or forward bias

Discussion in 'General Electronics Chat' started by donut, May 28, 2012.

  1. donut

    Thread Starter Member

    May 23, 2012
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    Im missing a concept regarding saturation, active mode of a npn transistor.


    for a bjt npn to be in saturation mode the collector base junction is forward biased and the base emitter junction is forward biased.

    If you look at the bjt as a diode model you see that the collector base junction diode has its cathode pointing to the Vcc of the collector and the anode pointing to the base. so current flow creates a reverse biased situation.

    My question is what will ever make the collector base junction looking at it as a diode model go into a forward biased situation?

    To me it always seems as if the collector base junction is always reversed biased?

    Is a forward biased situation in the collector base junction caused by increasing the amount of current flowing through it regardless of the diode direction?
     
  2. ActivePower

    Member

    Mar 15, 2012
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    Well, to start with the mode of transistor operation is controlled by the input current.

    Suppose, you have a npn transistor in Common Emitter configuration. The input in this case is the base current.

    In its usual (active) mode of operation, the collector-base junction is kept reverse-biased to suck out the minority carriers that are injected continuously into the base via the emitter. Thus, the base potential is higher than the emitter potential but lower than the collector potential.

    Consider decreasing the base current a little (by varying external resistance). This may cause the base potential to decrease and become less positive compared to the emitter potential and thereby putting both the "diodes" in reverse-bias. This is called transistor cutoff.

    If on the other hand, you increase the base current such that VBase becomes more than the collector potential, it results in both the "diodes" becoming forward biased and thus a large current flowing through the device. This is called saturation state of the transistor.

    Any intermediate state is the active state of the device.
     
  3. donut

    Thread Starter Member

    May 23, 2012
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    So it sounds like the bias state (reverse or forward) of the collector base junction depends on the voltage potential of the base and collector?

    If collector potential is greater than base potential the collector base junction is reversed biased and if the base potential is greater than the collector we have then forward biased? And you can control the current of the base which determines the voltage at the base with the Rb (base resestor)

    Please confirm
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Essentially true (note comment below about Vbe). If you have a resistor load from the collector to power, for example, and slowly increase the base current, the collector voltage will decrease as it conducts more and more current, dropping more voltage across the collector resistor. Eventually the collector emitter voltage will drop below the value of the base emitter voltage and the transistor enters saturation.

    Note that the base-emitter junction looks like a diode so Rb is just a small portion of its forward voltage. Vbe follows the logarithmic current-voltage relationship of a diode with Rb in series.
     
  5. donut

    Thread Starter Member

    May 23, 2012
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    0
    Sorry to beat this subject to death but what i am struggling with is the diode representation of the NPN BJT which is to say.

    The base to collector diode representation of the bjt shows the anode side connected to the base and the cathode connected to the collector.

    So if base voltage is greater than collector voltage i see how it is forward biased but what i dont see is even if the collector current increases due to the relationship between ic = beta*ib how are you able to have current flow through the collector when it is connected to the cathode instead of the anode?

    please help again.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    If the collector were the anode then it would conduct current all the time. The collector junction is normally reverse biased so it can block the current when the base current is zero. When the base-emitter current is not zero then carriers are generated at the base collector junction which allows a much larger current to flow across the reverse biased base-collection junction and through the emitter. That's the basis of bipolar transistor operation.
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Hi donut,

    Whilst the two diode model of a BJT might be somewhat helpful it contributes little to the understanding of the physical mechanisims at work in the transistor. Such as - how one can have charge carrier flow across a reverse biased junction.

    There is an abundance of more comprehensive & enlightening explanations to be found on the web.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I understand the physics of the BJT in the active region (Ebers & Moll equation), and I also understand saturation from an electronics point of view, but I think our OP and I are struggling with the physics of how this happens (think NPN):

    Conventional wisdom says that when a diode is forward biased, charge flows through it. When Vce < Vbe (saturation), the CB diode is forward biased. Charge should flow from collector to base. However, charge is still flowing from emitter to collector (with a little leaking off to the base). These two currents would be opposition to each other. I think that maybe charge does not actually flow from collector to base. Instead, the excess base current comes from the emitter. That makes sense, but if that is the case, what keeps charge from flowing from collector to base, in light of the fact that Vce < Vbe?
     
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    There are probably several factors that come into play. For instance the relative forward bias values would be important from the perspective of charge carrier injection from either junction into the base. The collector-base forward bias would be somewhat less than the base-emitter bias. Also one would need to know how efficient each forward biased junction is in the generation of charge carriers - the emitter-base junction is presumably a more efficient region & specifically doped with that expectation in the fabricator's mind. The conclusion would be that the base-emitter region produces the greater proportion of minority carriers in the base region and these diffuse according to the relative charge density distribution in the base.
     
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