Simple pn junction / diode question
A PN junction creates a local potential field.  The high concentration of majority carriers (holes in P, electrons in N) diffuse accross the junction because the concentration is much lower on the other side.  However, that displacement of the carriers causes a electric field.  A equillibrium is reached where the electric field pushes the carriers back to where they came from exactly opposite to the diffusion pressure.  This has the side effect of clearing the junction region of minority carriers, which make the diode "off".

If a external E field is applied to counter the diffusion, then the carriers move back to their respsective sides, allowing the minority carriers to return, causing the junction to conduct again.  This is how a diode turns "on".

Anyway, you can't measure the voltage caused by the local E field in the junction using a closed circuit because you don't have a way of doing so without additional carrier gradients that then make their own reverse E field.  The net diffusion-caused E field will be zero in a loop.

However, it seems to me that it could be possible to measure the PN junction static E field another way.  Suppose it is used in a vaccuum tube to deflect a electron beam.  I haven't tried that, but I think it should work.

In any case, you can't get power from this without putting some in somehow.  That is exactly how solar cells work.  A solar cell is a big diode with the junction layer close enough to the surface so that ordinary light photons can penetrate it.  A photon hitting the junction in the right place causes a loose electron, which is then accellerated by the field and can show up between the ends of the device as a current.