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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'Current-driven' is the traditional concept about biasing a BJT. The transistor contains diodes although that fact rarely comes to mind. Current level might be adjusted externally as the chief means to govern bjt performance.
At the same time the diodes develop varying voltage internally however that fact also rarely comes to mind.

In contrast we speak of jfets/mosfets as being voltage driven.
 

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