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Why is the voltage across a PN junction 0 v when it is in open circuit conditions ?

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It is not. The current is zero (because it is in open-circuit condition, no path for the flow.)

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  • \$\begingroup\$ Ok but my text book says that it has to be 0 because the contact voltage at the metal semiconductor junction terminal will exactly counter act the barrier potential.I don't understand the concept of the voltage between the metal semiconductor terminal. Thanks. \$\endgroup\$ – Timmy Sep 14 '14 at 2:08
  • \$\begingroup\$ Well, now it is different. Are we talking about the external or internal voltage (the built in)? As I understand the book is explaining the mechanism in which an electric field is established immediately to counterpart any charge accumulation, which will result in equilibrium. How about if you increase the contact reverse bias voltage to a value much greater than the built in voltage? Think about these terms: reverse bias current, break down, zener diode. \$\endgroup\$ – Meshal Sep 14 '14 at 2:21

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