So I understand P and N type semiconductors and the depletion region makes sense.

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html (using this as reference for my question).

So the electrons diffuse across into the acceptor holes in the P region, this makes sense. Which leaves "positively" charged holes in the N region near the depletion region (as shown in the website above).

So 2 questions:

  1. Why don't more electrons (in the N region) keep moving into the new positively charged holes near the depletion region? (from the N region). Is it due to the Negative Ions created? Is the depletion region electric field that strong? or is it because they are not Donor electrons and still in the valence band and aren't at the required energy levels to be excited up to the conduction band?

  2. Same thing I suppose for the other side, why don't the electrons that just diffused over to the P region keep moving around into other holes in the P region (or to the left if you looking at the picture in the link above)


1 Answer 1


Once the PN junction is formed, diffusion of holes from P to N, and electrons from N to P occurs until a depletion region is formed. So a depletion layer is formed on either side of the PN junction, which is devoid of any charge carriers. N side has positively charged donor ions, and P side has negatively charged acceptor ions.

So an electric field is directed from the N to the P side. This field is of the order of $10^6 V/m$. Extremely strong! So it stops the flow of majority carriers. But now the minority carriers flow, due to this field, which are one diffusion length on either side of the junction. The rest recombines before it makes to either P or N side.

But due to thermal agitation small number of majority carries still cross the junction. But that number is very small to have any significant effect at room temperature (300 K).


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