Circuit simulation with NMOS behaves weird

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SUMMARY

The discussion focuses on unexpected behavior in a circuit simulation involving an NMOS transistor with a threshold voltage of 2V. The user reports that the circuit does not enter saturation mode at the anticipated resistance value of 1kΩ, but rather at approximately 1.8kΩ. Simulation results show significant discrepancies between expected and actual voltages, particularly highlighting a calculated voltage of 6.30V instead of the expected 12V. The issue is attributed to the specific transistor model used, which begins to conduct at around 1V and enters saturation at approximately 7V, contrary to the user's initial assumptions.

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Sesse
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I have got this circuit.
http://img225.imageshack.us/img225/4369/circuithf1.jpg
Threshold voltage of the n-channel MOSFET is 2Volts. RVAR varies from 0.5k[itex]\Omega[/itex] to 2k[itex]\Omega[/itex].

When I simulate the circuit, I get:
http://img132.imageshack.us/img132/2779/simulationuw4.jpg

It should go into saturation mode when RVAR is about 1kOhms, but it doesn't. I nstead, it does it around 1.8kOhms.

Here are some values for comparison:
1/k[itex]\Omega[/itex] | Expected| Simulation
0.500| 1.1540V | 0.606V
1.000| 2.1100V | 1.154V
2.000| 3.5800V | 2.105V

Here is another weird thing, say you use the simulation value of 0.606V to calculate the voltage above R1.
[tex]\frac{0.606}{0.5} \times (4.7+0.5) = 6.30V[/tex]
Even though, it should be 12V.

If anyone could shed some light on this?
 
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The thing is that the specific transistor model that you use in your circuit starts to open around 1V (as you correctly pointed out) but unlike what you claim it is supposed to enter the saturation region around 7V (not 2V). Check it here: http://www.irf.com/product-info/datasheets/data/jantx2n6764.pdf
See Fig.