LTSpice netlist: plotting Vth vs length

Question

I'm using LTspice on Mac, and trying to only use netlists, no GUI. I'm trying to follow berkeley ee240. They use a model that I don't think is available online so I'm just using default mosfet models.

So I need to sweep length, and plot the resulting Vth parameter. I cannot plot V(vout) without manually selecting the trace, it will not show up using the '.plot DC V(vout)' command.

I cannot access the Vth parameter. I have tried various combinations of lv9 and vth but can't get anything to appear on the plot. V(Vout) only appears on the plot if it is selected from the GUI. Vth is not an option in the GUI. Netlist:

.model nm1 nmos(level=14)
.param length=1u

* netlist

Vgs vgs 0 1.2
Vds vds 0 1.2

M1 vout vgs gnd gnd nm1 L={length} W=1e-6
R1 vds vout 1k

* analysis
.op
.DC lin param length 90nm 500nm 50nm
.plot DC V(vout)

.end

Answer 1

I'm not familiar with LTSpice, I use a different simulator (Cadence Spectre) but there I can also not plot Vt directly. The reason for this is that Vt is an internal model parameter. In Spectre I can get a value for Vt at a certain operating point, for that I need to do a DC operating point simulation so not a DC sweep ! If I do multiple of these operating point simulations I could draw a curve from that.

A simpler way is to emulate the way that you would measure Vt on a bench. What you do is force a very small current (1 uA) for Id and measure Vgs. Since Id is very small Vgs will be almost equal to Vt. A simple circuit for this is here:

^{simulate this circuit – Schematic created using CircuitLab}