Looking at a simple electric circuit: battery, resistor and diode.
The applied voltage of the battery (V_a) drops on both the resistor and the diode. Thus, at a closed loop the voltage is zero.
In the PN diode there is a voltage development due to the difference between the Fermi-levels of the two semiconductors (N-type, P-type).
SO, why does this voltage is not the same as that of the battery. Why does it has an effect only to the diode itself?
Dear collegue,
I suppose that, removing any source like your battery, ie disconnecting all external circuits from diode, the built-in potential remains... It is a delicate question because I can't prove it ! To do it, I should measure that potential, so add again some connections, and even if I measure it by indirect means, (electric field), I take off energy, so affect the phenomenon. But there are some theoretical correlations which tend to make me believe of it: inside a doped conductor, you could examine either standard theory and/or make ab initio simulations. I guess that one should find that electric field is generated by particular disposition of crystal, regarding (as)symetry. You have also to consider the mechanical "equilibrium", so perhaps there compensation between mechanical energy - crystal distorded by dopants- and waveform distortion. Is that later distortion characterized by "surface waves" ? Some other collegues from solid state physics could confirm or infirm my point of view.
JLuc
The potential development is space limited to the region of the PN contact area. The large P and N areas remain at the same potential if an external voltage is not applied (experimental evidence: a voltmeter will measure 0.0 V on a diode in absence of an external emf).
A good (superb) battery is like a fountain of power for a specific (quite low power) Load; it functions, also, as a (near ideal) Voltage Source; so, it has an extremely high (-->oo) ratio r (= RLoad/RinternalOfTheBattery). An analog in cars might be an extreme scenario planning to set a ... Ferrari car "fountain" engine in a Citroën 2CV (French:"deux chevaux", fetish) car, Load.
A very bad battery has a zero (-->0) ratio r (= RLoad/RinternalOfTheBattery). An analog in cars might be an extreme scenario planning to set a 2CV engine fetish car in a Ferrari fetish car. Our case (the built-in potential in a PN diode) might be, even, a more extreme scenario; say a script (for kids scenario) joining a 2CV engine in a ... large oil tanker ship.
In reality, now; the "built-in potential[1] in a PN diode" is supported by a minimal model of a capacitor C with a parallel (discharging) Rp resistor. So, a common (using a low pass filter) voltmeter will measure, always, 0.0V, as Cornel Radu, said. However, an "extreme" EIS (extended to low[2] Freq. bands) supports this simulation circuit; it is applying very simple (but parametric[3]) lumped elements, in a simulation based on a "C//Rp" model[4].
1. Physicists call it, also, electrostatic potential, as well as, Chemists call it "galvanic potential".
2. The Low Freq. F[Low]
An estimation[1] of the parallel (discharging, or the near "diffusion current"[7] as the origin's link of the) Rp resistor (see above) can be found, also, from some classic/typical i-V characteristics[2] for an example of a common (to say one: 1N4001) Si-diode[3].
1. It depends, also, on the case of the packaging (plastic, etc.) for the commercial Si-diodes, and the temperature[4,5,6].
2. V-I CHARACTERISTICS OF Diode [see page 4, for 1N4001] http://www.cmi.ac.in/~ravitej/lab/3-diode
3. General Semiconductor / Vishay 1N4001-E3/54 https://www.alliedelec.com/general-semiconductor-vishay-1n4001-e3-54/70217642/
4. Junction Diodes http://www3.imperial.ac.uk/pls/portallive/docs/1/32737697.PDF
5. Diode Characteristics https://www.electronicshub.org/diode-characteristics/
6. Temperature Effects in Semiconductors https://link.springer.com/chapter/10.1007/978-1-4614-0748-5_2
7. Module - 1 Lecture - 2 PN Junction Diodes https://cheapstery.com/review/module-1-lecture-2-pn-junction-diodes-115893
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