Question

Although there is a physical, electrostatic potential across a junction when no net current is flowing at equlibrium, this voltage does not show up with a VOM or DVM measuring instrument. Why not?

Answer 1

In a closed circuit loop in thermal equilibrium (i.e. constant temperature and and no EMF) all contact potentials, i.e. voltage differences at junction between different conductors, must sum up to zero. Otherwise a current would flow which is against the 2nd law of thermodynamics. This includes also diode junctions (both metal-semiconductor and p-n junctions) which also have contact potentials which are usually called built-in voltages. A consequence of this is that you cannot measure a built-in voltage of a diode with a voltmeter. All contact voltages including the contacts to the voltmeter give a net voltage zero and therefore no current necessary for functioning of the voltmeter.