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Hi CircuitLab people!

I have a question about the junction built-in potential (V_J) in DIODE 1N4148 and in all the diodes in CircuitLab.

I see that V_J in 1N4148 is...20V!! I thought that "junction built-in potential" and the common "threshold voltage" were the same thing. Moreover, I have always considered the threshold voltage approximately 0.6V - 0.7V in diodes, in all my studies... However, not only this... If I simulate a simple circuit composed by a voltage source and my diode, I obtain the correct graph I_D - V_D, showing a threshold voltage of 0.7V.

So, the question is: what are the differences between the JUNCTION BUILT-IN POTENTIAL (the parameter used on CircuitLab) and the common used THRESHOLD VOLTAGE? If they are not the same thing, how can I calculate the threshold voltage knowing the V_J?

I hope I have been clear.

Thank a lot to everyone!

Bye!

Please see:

https://www.circuitlab.com/forums/modeling-and-simulation/topic/4jex93sw/importing-transistor-from-datasheet/#comment_2309

which also points to:

https://www.circuitlab.com/forums/basic-electronics/topic/9dg62j8d/help-for-beginners-hobbyists/#comment_2019

:)

It's ok, but from what I've read, I've understood that V_J in CL is the same of the common "THRESHOLD VOLTAGE"... But from the datasheet of the diode 1N4148, I dont' see that it has a built-in voltage of 20V as the CL default parameter for it... Not only I don't see it but it's impossible for a diode to have a similar junction built-in potential...

"but from what I've read, I've understood that V_J in CL is the same of the common "THRESHOLD VOLTAGE"..."

Sorry but you have either misunderstood what you have been reading or what you have been reading has been written by someone who does not understand spice and spice-like diode simulation models.

Whilst VJ (V_J in CL) does represent the "knee" or "threshold" voltage of approximately 0.55V to 0.7V for a silicon diode, it is not used to represent this voltage at DC.

VJ is used only in the calculation of the dynamic behaviour of the junction junction capacitance based on the zero bias capacitance CJ0 as part of the Large Signal Transient part of the diode model.

This is described in Section 3.2 of:

http://www3.imperial.ac.uk/pls/portallive/docs/1/7292572.PDF

If you run a DC sweep of a diode model with a value of 1 and again with a VJ value of 20 or even 20k, you will see no difference in the I vs. V curve.

What you will see is a difference in the transient or switching behaviour because the junction capacitance has been changed.

Whilst it is true that the 1N4148 model in CL has an apparently unrealistic V_J value it is not possible to say if this is a mistake or if it is deliberate on the part of the authors at CL.

This is because (a) CL is not spice and (b) the authors give no information out about their modelling and model verification.

Ok, now it's clear! Thank you!

So, to know the threshold voltage of the diode I want to use, I have to simulate it in DC looking at the I vs. V curve... correct? And so, I have to consider, for example, that the diode 1N4148 has a threshold voltage of approximately 0.7V, or the 1N5819 has a threshold voltage of about 0.5V.

Yup.

Make yourself a sim like this:

with as many diodes as you want in parallel across V1 and then plot the currents through each diode (put a probe directly on each diode anode pin).

That will plot the current through each diode against the same V axis so you can easily compare forward I/V characteristics.