Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help on understanding NPN transistor

  1. Oct 13, 2008 #1

    I would like help understanding this scenario:

    (1) base-collector voltage, V_BC, is reverse-biased
    (2) base-emitter voltage, V_BE, is forward-biased
    (3) collector current, I_C, is negative

    Normally, if I_C was positive, I would assume the transistor to be in the forward-active region. However, can you still consider it in the forward-active region if the collector current is negative? Does this scenario make sense?


  2. jcsd
  3. Oct 14, 2008 #2
    Never heard of a negative current.. It is only a convention wrt some current flow direction.

    Basically electrons fire across the forward biased emitter-base and then about 99% of them carry on across the reverse biased base-collector. A small base current controls a much larger collector current.
  4. Oct 14, 2008 #3


    User Avatar
    Homework Helper

    Did you instead meant to say that the collector current is flowing out of the collector junction? Hmm, that sounds as though the BJT is working in the reverse active mode. The gain for this is much for smaller though due to disproportionate doping for the collector and emitter regions. Check your textbook for details on that mode of operation.
  5. Oct 14, 2008 #4
    If the BE junction is forward biased, and the CB junction is reverse biased, shouldnt Ic be positive? The transistor should work in the active region.
  6. Oct 14, 2008 #5


    User Avatar
    Science Advisor

    Well Mark that is the one "quadrant" of collector operation that is physically impossible to obtain. All three other possiblities at the collector can indeed occur. I'll summarize them (NPN).

    1. I_c +ive, BC junction reverse biased = active region

    2. I_c +ive, BC junction slightly forward biased = saturation region.

    3. I_c -ive, BC forward biased = reverse operation.

    In modes 1 and 2 above the BE junction should be forward biased. In mode 3 it either operates as an inefficient reversed NPN (that is, with C and E functions interchanged) if the BE junction is reverse biased, otherwise it pretty much degenerates into two diodes with a common annode (that being the base).

    BTW. I should note that the definition of +ive collector current is when the current flows into the device. This is a standard definition. Perhaps you were unaware of this and were mistakenly thinking negative Ic corresponded to the case where Ic crosses the BC junction in the direction from N to P? In this case then that's actually the positive direction for Ic and you would be looking a simple active region operation.
  7. Oct 14, 2008 #6


    User Avatar
    Science Advisor
    Gold Member

  8. Oct 15, 2008 #7
    hey, thx everyone. it helped improve my conceptual understanding of bjt transistors.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Help on understanding NPN transistor
  1. Pnp npn transistors (Replies: 25)