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What exactly does a diode do?

What happens in a diode to make current only flow one way around a circuit? My physics teacher said it was something to do with the metals in it and the impurities in them. Can anyone give me a detailed explanation. Any help much appreciated.

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marked as duplicate by Leon Heller, Dave Tweed, Anindo Ghosh, Brian Carlton, Nick Alexeev Jan 14 '13 at 20:10

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Some demonstration of basic research would be nice. – Phil Frost Jan 14 '13 at 14:07
up vote 6 down vote accepted

This is probably better off asked on the physics stack, but essentially:

You have two differently doped semiconductors, one P-type and one N-type. The N-type has excess electrons, and the P-type has excess "holes" (which means the absence of an electron in an electron shell)

When you bring the P and N type materials together, you form a P-N junction. At the junction, some of the excess electrons cross over and fill the holes in the P-type material, forming a depletion zone (essentially a built in potential):

Depletion zone

This depletion zone is (almost) non-conductive, as it has no free charge carriers. Now if we apply a potential (voltage) across the junction, depending on the polarity, the depletion zone either widens or narrows. If the region widens, the non-conductive region increases and no current can flow through the diode.

Widening zone

If it narrows, at a certain potential (~0.7V for a silicon diode) the diode will begin to conduct. This (hopefully) explains why the diode acts in the way it does.

Narrowing zone

Above images from this introduction to diodes.

Also, this image (from the Wikipedia P-N Junction link above) shows the P-N junction with a little more information than the above images:

Wiki P-N Junction

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