LTSpice netlist: plotting Vth vs length
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 &ndash; Schematic created using CircuitLab