PN Junction Physics

Discussion in 'Physics' started by Digin8918, Sep 13, 2010.

  1. Digin8918

    Thread Starter New Member

    Sep 5, 2009
    I have read so much about PN junctions, but I simply still don't get all of it. Every description I ever read when they are explaining Forward Biased says something about the depletion region, barrier voltage etc, and I believe I get most of that, but what I do not understand is how there is ever any net current in either direction when electrons and holes continue to constantly recombine with one another. Either the holes or the electrons must be able to continue on in some way for their to be any sort of useful current no?

    In short, I need these couple of sentences from Wikipedia explained on PN Junctions with my comments of what I understand and do not understand included.

    "With forward bias, the depletion region is narrow enough that electrons can cross the junction and inject into the P-type material( got it). However, they do not continue to flow through the P-type material indefinitely, because it is energetically favorable for them to recombine with holes( makes sense..but then what continues on to create current flow?). The average length an electron travels through the P-type material before recombining is called the diffusion length, and it is typically on the order of microns.[2]

    Although the electrons penetrate only a short distance into the P-type material, the electric current continues uninterrupted( What?! How? I thought electric current WAS flow of electrons or holes?), because holes (the majority carriers) begin to flow in the opposite direction"

    I'd appreciate any help that anyone with knowledge of the subject would care to offer. Thank you for your time.
  2. lendo1

    Active Member

    Apr 24, 2010
    The last sentence may be the answer to your confusion:

    "because holes (the majority carriers) begin to flow in the opposite direction"

    The flow of holes is technically just the illusion of positive charge carriers moving against the flow of electrons, although in reality no positive charge carriers are moving. In the depletion region of the PN junction, current flows because the applied potential difference is enough to overcome the natural resistance to current which builds - in a way it's like electrical breakdown. So when sufficient voltage is applied, electrons will naturally begin to drift through the barrier, however they will encounter positive holes. Some of the electrons will recombine into these holes, as wikipedia points out, and some will continue relatively unimpeded. Also, some complete bonds may be broken by the applied voltage as other electrons take their place, giving the appearance of holes traveling in the opposite direction of electrons. Hope that helps; I'm still working on the concepts myself so bear with me.
  3. neonstrobe


    May 15, 2009
    Hi Digin8198
    You nearly got it, I think. Electrons in the P material recombine with holes, and the holes convey the current. So at the contact to the p material, holes are created which move into the p material while electrons (which actually move) flow in the wire.
    So the current starts in the N type as electrons, moves through the depletion region and enter the P. As they recombine with the holes moving towards them in the opposite direction the current becomes made up from the holes.

    The point is that in semiconductors, current can consist of electrons and holes and usually both.

    The number of electrons entering the p region depends on the applied voltage. FOr a given number the density falls exponentially as they recombine. The diffusion length represents the distance over which they recombine - as it is exponential, it is basically a characteristic length indicating where the concentration has fallen to 1/e (about 30%) of the density at the depletion edge.

    Hi Lendo1
    There is a state of flux in an unbiased diode. Electrons try to diffuse into the P region (due simply to the concentration difference) and holes from the P into the N.
    As they try to move they leave a fixed charge behind (due to the doping which makes the N and P regions N and P, such as phosphorus and boron). This produces a counter flux caused by charges (electric field) and with no overall bias the currents all balance. Any external bias even a small one changes the balance. If it reduces the internal potential the diffusion current wins (normal forward current) which increases exponentially with voltage; if it increases the internal potential the drift current or electric field component wins, which basically means that the diffusion component is suppressed more strongly and the overall current reduces (reverse bias). This current is quite small and essentially is leakage current.