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Homework Help: How does a transistor work?

  1. Jun 2, 2012 #1
    I know this question is very vague, but my textbook, amongst various websites/videos, tend to discuss what a transistor does (amplify/control current) rather than how it does it. I know current IC and IE (collector and emitter current) are controlled/proportional to a base current that flows through the base of a transistor, but what is most confusing is that my textbook says a transistor amplifies current, how is this so? From what it seems to me, a transistor looks like a gateway that splits, in one way or another, emitter current into both base current and collector current, but isn't that to say that collector current is the deduction of base current from emitter current, rather than an amplification?

    Any help would be appreciated.
  2. jcsd
  3. Jun 2, 2012 #2


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    Changing the EB potential V_BE reduces the potential barrier across the EB diode junction so that current flows. The charges in the B region are minority carriers that, if B is thin enough, diffuse to the BC junction before recombination. The back-bias potential V_BC then sweeps the charges to the collector contact. To understand it fully you should understand band gaps, valence bands, minority carriers, pn junctions, etc. Take a look at any standard text, like Sze's Semiconductor Physics.

    You are on the right track with your current "deduction." The claim that a transistor amplifies current arises from the fact that only a small fraction (called alpha) of the injected charges across the EB diode junction are lost to recombination, so the base current needed to make up for recombination is small compared to the EC current. Although it is actually a potential that controls the current if you look at the band diagram, from outside the device it appears that a small base current "controls" the large collector current.

    EDIT: Here's an online text
    Last edited: Jun 2, 2012
  4. Jun 2, 2012 #3
    2 circuits here, base-emitter and emitter-collector(more voltage supply than base-emitter).
    The resistance in the emitter-collector can be controlled by the amount of current passing thru base-emitter.
    Less resistance in emitter-collector means more current.
    Thus any changes in base-emitter current follow by bigger changes in emitter-collector current due to more voltage.
  5. Jun 2, 2012 #4


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    azizlwl, your view of transistor action is too simplistic to address the OP's question.
  6. Jun 3, 2012 #5
    OK the BJT we are talking about here is basically a current controlled current amplifier. What this means is that the collector current depends on the base current(when dealing with the CE configuration). The actual 'amplification' of the current will be easier to understand if you are familiar with the basics of semiconductor physics, such as depletion regions and junctions.
    When the emitter-base junction is forward biased sufficiently, electrons are able to pass through the depletion region, as it is now narrowed down. Some of these electrons combine with the holes in the base region, but most of them go on to be 'collected' by the collector. If you go on to analyse the voltage and current relationships BJT, you will find that the base current has been amplified, and this is the collector current.
    Hope this helps.
  7. Jun 4, 2012 #6
    Ok, I think this has clarified the things I needed to know.
    Thanks for the help everyone
  8. Jun 5, 2012 #7

    I beg to differ. A BJT is a voltage controlled current amplifier. It appears that Ic is being controlled by Ib, but that is an illusion. Ib is an indication of what Ic is, but Vbe controls both Ib and Ic in a exponential relationship.

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