*Q: Why can't we measure the potential barrier of a pn-junction by simply using a voltmeter?* A: The potential barrier in a p-n junction cannot be directly measured using a voltmeter because the voltmeter measures the net voltage across the terminals, which is influenced by the metal-semiconductor contact potentials. When a voltmeter is connected to the p-n junction, it forms metal-semiconductor junctions at both ends. These junctions develop their own contact potentials due to differences in work functions between the metal and semiconductor. The contact potentials typically oppose and cancel out the built-in potential of the p-n junction, leading to a net voltage reading of zero in an equilibrium state. This is because the built-in potential is an internal property of the junction and does not produce an external electric field measurable by the voltmeter.