Possible Error In Aac Page!!!

Discussion in 'Feedback and Suggestions' started by steeve_wai, Sep 25, 2007.

  1. steeve_wai

    Thread Starter Active Member

    Sep 13, 2007
    i have doubts about the figure showing only holes an electrons in the following page:


    the arrow in blue shows the electron(s) flowing out of the base.i am not really sure if base current happens that way.what i had heard was that the holes of base(of npn bjt) that are lost in recombination are regenerated by the breaking of a covalent bond in the p material.a hole and an electron are generated.this electron(s) flows out from the base wire and the hole stays in the base...please comment
  2. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
    In any semiconductor (even undoped intrinsic semiconductor) electron hole pairs are being continuously formed, and lost due to recombination. If I were asked to measure this in the base, I would remove the collector battery in /images/03413.png (above the 03414.png figure in question). I would reverse the base battery to reverse bias the emitter-base junction. I would expect to measure a leakage current (due to thermal electron hole formation--breaking the covalent bond) of well under a microAmp in silicon at room temperature. This is the extent of the current due to breaking of covalent bonds. This appears to be a 5th component of current that we do not account for. I think that it could cancel some of the base current flow when B-E is forward biased.

    However, if we go back to the original circuit with the base battery forward biasing the emitter-base junciton, the forward bias current in the base is about 100 microAmp in a small signal transistor assuming beta=100 and collector current=10 milliAmp as an operating point. The point to be made is the leakage current due to thermal electron-hole formation is small (well under a uA) compared to the 100microAmp base current.

    Why might this current due to thermal electron-hole formation be very small compared to the base current? The emitter is heavily doped compared to the base. That means that the emitter will have many free electrons to diffuse into the base, compared with the meager formation of thermal electron-hole pairs. The forward bias of the base emitter junction will actively force these electrons into the base.

    Now, I would not expect 99% of this (well under a uA) current to flow out the base. I would expect 99% of the electrons to diffuse (very thin base) into the base-collector junction where the high electric field would collect them In other words, 99% does not affect base current flow.

    To summarize, at first glance, one might expect breaking of covalent bonds, thermal electron-hole formation, in the base to cancel the electrons at (a) which fall into holes. However, there are so many more electrons entering from the emitter, that more fall into holes than are newly formed electron-hole pairs.