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I have read in my book that The barrier potential of a diode junction cannot be measured by a voltmeter only. It says that the thermal equilibrium is disrupted. Besides , the contact potential is also mentioned.

But I don't understand it. What does it mean actually?

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One way to see this is that an ordinary voltmeter is a load and requires some current to show a value. But a diode cannot output energy to a load without input of energy. Other contact voltages will balance the built-in potential over the $pn$ junction.

However, some light absorption will cause a photovoltaic current. The open-circuit voltage is close to the built-in potential.

For a real measurement, one can use a (scanning) Kelvin probe, which measures the difference in work function.

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  • $\begingroup$ @Pieter...Well,the book also says that "even if current passed,the junction would be cooled and wire would be hot.This disrupts thermal equilibrium".What's this? $\endgroup$ – user182687 Feb 24 '18 at 7:58
  • $\begingroup$ And also , what do you mean by "contact potential"? $\endgroup$ – user182687 Feb 24 '18 at 7:58
  • $\begingroup$ @UltraInstincts There will be contact potentials between the metal leads and the semiconductor on both the $p$- and the $n$-doped side, so that the built-in potential over the junction is cancelled. As for what the book means..., that is a bit of a mystery to me too. The wires should not get really warm. And maybe they are alluding to a Peltier effect, but that does not actually need a $pn$-junction. $\endgroup$ – Pieter Feb 24 '18 at 14:48
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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.

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