# P-n junction

Hello.

I have a question regarding the p-n junction. When I look at figure B here:

http://en.wikipedia.org/wiki/P-n_junction

it looks completely wrong to me. Take the E-field. It is going to be positive outside the depletion region, which in turn will make the voltage tend towards zero for x→±∞. Right ??

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mfb
Mentor
The figure looks good.
It is going to be positive outside the depletion region
No.
(explain more what you don't understand to get more specific answers)

The figure looks good.
No.
(explain more what you don't understand to get more specific answers)
Well the depletion region is a dipole, isn't it? So the e-field will look like this

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/edip2.gif

So if the field is negative "inside" the dipole, it is positive outside (but of small magnitude, of course).

mfb
Mentor
Well the depletion region is a dipole, isn't it? So the e-field will look like this

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/edip2.gif
It is not, you have two "infinite" sheets, and infinite sheets have a constant field around them - two sheets with opposite charge of the same magnitude lead to fields that cancel everywhere outside.

It is not, you have two "infinite" sheets, and infinite sheets have a constant field around them - two sheets with opposite charge of the same magnitude lead to fields that cancel everywhere outside.
Well, clearly the depletion region is not infinite in extent. Also, the field is not constant in the figure that you said looks good.

mfb
Mentor
Well, clearly the depletion region is not infinite in extent.
That is a very good approximation for most transistors.
Also, the field is not constant in the figure that you said looks good.
The field is zero outside the regions with net charge, so the potential is constant. The field is not constant in the region with net charge, as there is net charge on the sides.