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Can we find potential difference of a P-N junction by connecting voltmeter across diode terminals? I know the answer is no but i don't know how.

hood
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2 Answers2

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The correct way to measure a diode is with a I-V curve tracer. The barrier potential occurs at the "knee" voltage in the forward bias. This is the point at which current increases greatly compared to the applied voltage.

curve tracer

I found a video of how to do this cheaply. 2 If you are student your electronics lab may have something like a Tektronix 370A somewhere you can use.

If you are wondering "why" this is you are asking a physics question and the answer is that the barrier potential is not really a "voltage". A diode is still a passive device like a resistor. You have to apply a voltage/current to get a meaningful output. 3

Community
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checkback
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  • Nice, Of course there is no real "knee" voltage in a forward biased diode. There's an exponential relation between current and voltage from nA to mA, at the high currents the diode gets a resistive term, which makes the current increase less with voltage. – George Herold Jan 18 '16 at 20:34
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I think you are referring to the diffusion voltage across the pn junction, correct? No - it is not possible to measure or even to use this voltage because each electrical contact at the collector resp. emitter terminals will produce the opposite effect (voltage with opposite sign) because each of these contacts acts like a Schottky junction (metal-semiconductor).

LvW
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    _contacts_ in a semiconductor process are rarely meant to be rectifying, i.e. Schottky. There are many metal-semiconductor systems that form ohmic contacts that are not rectifying. Your statement is false. – placeholder Jan 18 '16 at 16:41
  • http://electronics.stackexchange.com/questions/106496/why-isnt-there-a-potential-difference-across-a-disconnected-diode – LvW Jan 18 '16 at 18:01
  • @placeholder, so what is the correct answer? (Stating that something is not correct, one should know the correct answer). – LvW Jan 18 '16 at 18:03
  • Off topic, but that's a fallacy. You don't have to be able to lay an egg to be able to spot a rotten one. – user207421 Jan 19 '16 at 00:24