Deriving equations from the MOSFET Characteristics curve

[khatus]

Hello Guys,
I can't understand some parts of the MOSFET static characteristics curve. Here the equation given for the Triode region for the characteristics curve of the MOSFET is VP≤ VGS ≤ 0 and
VDS ≤ VGS-VP. My question is How I can find these two equations by looking at the graph? Or it would be better if someone could explain how to determine these two equations by analyzing the graph shown in the picture.

 

[dick_freebird]

Not that graph.

Most of what you want for DC params
comes from ID-Vgs pulls. Fit VT with a ruler
on a sqrt(ID) plot and get gm / subthreshold slope off log(ID) vs Vgs.

Only thing you can get off the plot you show
is lambda and you want a L series to fit the
delta-L params.

 

[FvM]

VP≤ VGS ≤ 0 and VDS ≤ VGS-VP aren't equations, these are unequations defining triode and saturation region.

Figure 7.12 array of curves is defined by exactly 2 parameters, Vp and Idss. With this two parameters, you can recreate the curves, e.g. in Excel, using equations 7.1 and 7.3.

It's however a simplified MOSFET characteristic, abstracting e.g. from channel length modulation parameter lambda, as mentioned by dick_freebird.

 

[FvM]

I forget to mention that post #1 and #3 curves are for a depletetion mode transistor with negative threshold voltage. The original curve describes a JFET, but there are also a few depletion mode MOSFET available. Most MOSFET are however enhancement mode type with positive threshold (in case of NMOSFET). The discussed equations are valid for both types.