PN Junction open conditions conservation of energy

[SpartanG345]

When a PN juction is in the open circuit condition, my textbooks says the following.

"The voltage measured between the terminals is zero. That is the junction Voltage Vo does not appear across between the diode terminals.This is because of the contact voltages existing at the metal-semiconductor junctions at the diode terminals, which counter and exactly balance the barrier voltage."

Otherwise the there would be a violation of the conservation of energy.

I am not sure what this means

if the anode is at 0v and the cathode is at 0v what are the "contact voltages existing at the metal-semiconductor junctions at the diode terminals"?

 

[uart]

There's a contact potential when you make electrical contact between any two materials which have a different work functions (affinity for electrons). Normally (in thermal equilibrium) you can't measure this potential as they always cancel each other out and thus add to zero around a closed loop.

If you don't have thermal equilibrium however then the voltages don't necessarily cancel and you get a net potential around a closed loop. This is how a thermo-couple works and is also how a photovoltaic cell (solar cell) works, disturbing the thermal equilibrium by adding heat or light to one of the junctions.

 

[SpartanG345]

so wait this contact potentials occur right next to the depletion region?

I still can't visualise the potential graph

 

[uart]

so wait this contact potentials occur right next to the depletion region?

Yes the depletion region coincides with the electric field that produces the contact potential.

 

[SpartanG345]

This is a picture of my interpretation so far.

The contact charges occur in the boundary of the depletion region. The region where the contact charges occurs is not between the boundary of to different materials.
I am wondering what causes these contact charges.

I think it may be localised build up of positive wholes and electrons (minority charge carriers). Is this correct?

Also is there a limit to the magnitude of the contact charge? If you have 2 very large positive and negative charges which were separated somehow you would expect there to be a measurable potential between the two terminals.

 

[SpartanG345]

edit...

I think it may be localised build up of majority charge carriers electrons in the n material ect

 

[marcusl]

so wait this contact potentials occur right next to the depletion region?

I still can't visualise the potential graph

No. The barrier or built-in voltage exists across the junction, while the contact potentials occur where the metal contacts connect to the bulk n+ and p+ semiconductors. Thermodynamics conspires that the contact potentials exactly counterbalance the built-in potential in a closed circuit, preventing you from drawing current from the barrier.

I looked around online and found a consicse discussion with diagrams here:
"ocw.mit.edu/courses/electrical-engineering.../lecture5annotat.pdf"[/URL]
go to p. 17-18

 

[SpartanG345]

http://ocw.mit.edu/courses/electric...s-fall-2005/lecture-notes/lecture5annotat.pdf

'h'ttp://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-012-microelectronic-devices-and-circuits-fall-2005/lecture-notes/lecture5annotat.pd
full link

is this the link you were referring to?

 

[marcusl]

Yes it is.