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This might be a stupid question but I am either having a mental block or I'm getting something. I'm learning about excess minority charge carriers.

So, when an electric field is applied across a pn diode (in such a way as to decrease the depletion layer width, i.e. forward biased), the carriers move from their "majority" sides to their opposite, "minority" sides.

We call this excess of the minority charge the excess minority charge concentration.

As the length travelled increases, more and more charges recombine with their opposites. So in short diodes, the charges go straight across to the "wire"/contact with only a few of them recombining.

Question:

But in (say very) long diodes, the excess charge carriers recombine with their opposites (almost) fully. So then, how is charge carried across the pn diode?

It feels like I'm missing something here. I don't fully understand the significance of the excess minority charge carriers? My book says that the initial gradient of the decay of minority charge carriers is the current. I don't understand this at all.

Can anyone shed any light on where I'm going wrong?

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  • \$\begingroup\$ If you don't get an answer here, you might try on physics SE. \$\endgroup\$
    – Phil Frost
    Jun 15, 2013 at 23:37

1 Answer 1

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As the injected minority carriers recombine, they have to recombine with something. What they recombine with is some of the majority carriers. So majority carriers near the junction are being "used up" by the recombination process.

As the majority carriers are taken up by recombination, new majority carriers flow in from the area farther away from the junction. This majority carrier flow is what carries the current in the "far away" part of the device.

I found a diagram showing this here:

enter image description here

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  • \$\begingroup\$ Another site with some explanation: nptel.iitm.ac.in/courses/Webcourse-contents/IIT-Delhi/… For your question, see the area around eqn 39 or so. \$\endgroup\$
    – The Photon
    Jun 16, 2013 at 1:27
  • \$\begingroup\$ Cheers for that! So, is there any external difference in terms of external characteristics if a diode is long or short? (i.e. how do you tell without knowing its length) Does the length matter? \$\endgroup\$ Jun 16, 2013 at 5:16
  • \$\begingroup\$ I think (but my memory isn't perfect) that short base means that injected carrier recombination isn't significant --- essentially all of the minority carriers make it to the contact surface...so that pretty much makes your question not apply... \$\endgroup\$
    – The Photon
    Jun 16, 2013 at 5:21
  • \$\begingroup\$ Yeah I agree with that! But what I meant was, if I arbritarily increase the length, will it matter? I researched here and there and I think what changes is the amount of charge you have to slosh back and forth each time you switch a diode. i.e. Delay. \$\endgroup\$ Jun 16, 2013 at 6:08
  • \$\begingroup\$ If you increase the length beyond the minimum for a "wide base" diode, I don't think it changes the analysis. The charge sloshing back and forth, IIRC, is more to do with the depletion region. \$\endgroup\$
    – The Photon
    Jun 16, 2013 at 13:53

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