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In your plot, you have taken two points and have the slope between them. From the following equation
Id = 0.5 Kn (W/L) (Vgs - Vth)2 (1 + λ Vds)
For two different points,
Id2 - Id1 = 0.5 Kn (W/L) (Vgs - Vth)2 (λ (Vds2 - Vds1))
Therefore the slope that you have is (Id2 - Id1) / (Vds2 - Vds1) = 0.5 Kn (W/L) (Vgs - Vth)2 * λ.
Use the above to calculate λ.
Or you can just use the equation Id = 0.5 Kn (W/L) (Vgs - Vth)2 (1 + λ Vds)
Do the simulation for Different Id, Vgs, Vds, You have two unknowns Kn and λ which you can then calculate.
These values will change with the operating conditions. λ will vary with Vds and Id and a lot of other parameters.
You might have to do a bunch of simulations and get a bunch of values. You can take a average of them or keep all the values and use each when when required.
Ro or Rds or Gds of a transistor is provided in the dc operating point solution.
Or you can use the equation Ro = 1/(λ*Id)
Dude, don't worry, your value of λ looks OK. The variation will be small, probably in the 3rd or 4th significant digit. For a higher length, (L > 0.5um), the transistor is quite close to the ideal square law and hence the variation is minimum. For lower lengths the variation is more.
λ is inversely proportional to Length though. That is why Higher lengths give higher Ro.