P-n junction

1. Nov 27, 2013

daudaudaudau

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 ??

2. Nov 27, 2013

Staff: Mentor

The figure looks good.
No.
(explain more what you don't understand to get more specific answers)

3. Nov 28, 2013

daudaudaudau

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).

4. Nov 29, 2013

Staff: Mentor

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.

5. Nov 29, 2013

daudaudaudau

Well, clearly the depletion region is not infinite in extent. Also, the field is not constant in the figure that you said looks good.

6. Nov 29, 2013

Staff: Mentor

That is a very good approximation for most transistors.
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.