1
\$\begingroup\$

I want to determine the threshold voltage of an NMOS from a bvsim3v3 model in LTSPICE(Not a specific one I'm asking generally for this model). I can't get it directly from the parameters so I want to know how to. It doesn't have to be the exact value, I know it can be changeable but a good approach would be sufficient for me.

One way I thought of is to obtain it from the drain current equation. I set Vgs, W and L myself and measure the drain current with simulation. So this leads to a sub-question: How can I obtain oxide capacitor(Cox) and mobility from the model parameters?

\$\endgroup\$
  • 1
    \$\begingroup\$ There's a Vto parameter, specified as "zero bias threshold voltage". But, what Vgs are you referring to? Do you mean the one where it is specified as IC=VGS? If so, that's only to be used as initial condition for .uic. \$\endgroup\$ – a concerned citizen Oct 22 '16 at 16:42
  • \$\begingroup\$ Ok, I think Vto is the answer to me, thank you. According to this equation: wikimedia.org/api/rest_v1/media/math/render/svg/… second addend goes zero because source and bulk are shorted right? Then we have Vt=Vto left. About Vgs; I only mentioned that as an extra information about the drain current Id equation. I referred to regular gate-to-source voltage, nothing significant. Doesn't relate to the solution. \$\endgroup\$ – packt Oct 23 '16 at 3:28
2
\$\begingroup\$

Ground the source, connect the drain to the gate, and feed the drain with a constant current source set to the current value at which you want to know the threshold voltage.

For example:

In Circuitlab this shows a threshold voltage of 4.01V which is actually on the high side of allowable spec.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
  • 1
    \$\begingroup\$ This is indeed what I would do also. For a very large powerfet like the IRF530, that Id = 250 uA is very appropriate. For a smaller MOSFET like used deep inside a chip, I would use a much smaller current like 100 nA even ! \$\endgroup\$ – Bimpelrekkie Oct 22 '16 at 14:16
  • \$\begingroup\$ I think I didn't fully understand. How do we specify the current source value? And do we measure the voltage at the gate? \$\endgroup\$ – packt Oct 22 '16 at 15:22
  • \$\begingroup\$ You measure it with the SPICE simulation by reading the voltage at the gate/drain net. The current is what you define as the drain current at the threshold voltage. As @FakeMoustache says, on a chip, you might use 100nA. I picked 250uA because that is the drain current at which Vt is defined on the particular IRF530 MOSFET datasheet I was comparing it against. \$\endgroup\$ – Spehro Pefhany Oct 22 '16 at 16:05
  • \$\begingroup\$ So this method is valid if the drain current at threshold voltage is known right? I was actually asking about a transistor, of which you only know the spice model, not a commercial one that you have a datasheet about. \$\endgroup\$ – packt Oct 22 '16 at 23:13
1
\$\begingroup\$

As @a concerned citizen mentioned, the parameter to be looked is Vto at first. According to equation of Vt, it gives the basic threshold voltage where source and bulk are shorted(VSB=0):

Vt

Additionally, Bsim3v3 User's Manual provides more detailed information about this topic:
http://ngspice.sourceforge.net/external-documents/models/bsim330_manual.pdf At the end of Appendix A(nI-1. and nI-2.) and beginning of Appendix B the necessary equations can be found.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.