# How to find threshold voltage of an NMOS with BSIM3v3 model in LTSPICE

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?

• 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. Oct 22 '16 at 16:42
• 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. Oct 23 '16 at 3:28

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.

simulate this circuit – Schematic created using CircuitLab

• 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 ! Oct 22 '16 at 14:16
• I think I didn't fully understand. How do we specify the current source value? And do we measure the voltage at the gate? Oct 22 '16 at 15:22
• 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. Oct 22 '16 at 16:05
• 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. Oct 22 '16 at 23:13

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):