is the postive voltage to PN junction that allow the current to pass through it?

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TicTek

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Hi folks,

I was led to confusion by an experienced electronic engineer who told me that for a diode (PN Junction), it is not only that if apply a voltage across its terminal that will make it conduct but also if we force a current into it, it will generate a differential voltage on its terminal!!

So, here I am confused of cause/effect. Is the voltage that causes the current, or also a forced current causes a voltage?
All books I read they specify applied voltage leads to a diode conducting current!!
 

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'Forcing' current doesn't sound right, it will conduct a current when sufficient voltage is applied across the terminals. I think the confusion may be that when a PN junction is conducting (passing a current), there will be a voltage dropped across it, much as there would be if a current passed through a resistor. Different junction materials and particular device types may result in different voltages but unlike a resistor, the voltage stays relatively constant as the current changes.

Brian.
 
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Hi,

there are (constant) voltage sources ... and (constant) current sources.
Both feed the "value" to a load.

For a (constant) voltage source ... no load is needed. The voltage exists and can be measured and regulated.

This is different with a current source. It needs a load for a current to flow.
Indeed a constant current source usually regulates the voltage in a way that the expected current will flow.

***
Now what your electronics engineer means (most probably):
You can apply a constant current to a PN ... and then you can measure a voltage across the PN.
But for sure - as said above - it needs a voltage for a current to flow.

***
For sure one can bring up extreme examples where this seems not to be true.
Like a ring of supra conductor ... when an external magnetic field is applied.
There surely will be current flow within the supra conductor ... but is there also a voltage, too?

Klaus
 
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Thanks
--- Updated ---

No, he clearly told me that from the relationship of Id=Is*exp(Vd/Vt) that Vd=Vt*ln(Id/Is) which means the physical relationship does not behave on primary cause of Vd but rather vice versa
--- Updated ---

ok, lets say I have a current source that is connected to a diode in forward mode. How do I know if this diode will conduct as I know only there is a current source?
 

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The model of a MOSFET has a capacitance between each node. we can expect a Miller effect from the Cdg to create an exponential current response to Vgs in addition to Cgs and Cds. However I am not familiar with drain current lagging source. But I suppose that is possible with internal circulating currents from capacitance, briefly. Not all FETs have the same ratios of each of the 3 internode capacitance yet comparing all devices RdsOn*Coss tends to be an important Figure of Merit FoM which are generally constant with many exceptions due to Vds max and chip size. But C tends to increase with lower RdsOn or Rs of all PN junctions include power diodes but varies widely with Si, GaAs, SiC
 

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TicTek said:
ok, lets say I have a current source that is connected to a diode in forward mode. How do I know if this diode will conduct as I know only there is a current source?
Click to expand...
you can measure the current through PN
You can measure the voltage across PN

Klaus
 

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No, he clearly told me that from the relationship of Id=Is*exp(Vd/Vt) that Vd=Vt*ln(Id/Is) which means the physical relationship does not behave on primary cause of Vd but rather vice versa
Click to expand...

Your question was about a PN junction. Please explain where Id, Vd, Vt and Is come into the equation.

Brian.
 

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