In the square-law equations you use (VDS - VDS,sat) while other books use
only VDS. Would you comment on the difference?
Yes, in Eq. (6.42) we derived, for saturation operation,
ID = [(KP/2)•(W/L)•(VGS - VTHN)2]•(1 + l•(VDS - VDS,sat))
and for the triode region, Eq. (6.36)
ID = (KP)•(W/L)•[(VGS - VTHN)•VDS - V 2DS/2]
It's important to note that at the border between the triode and saturation
regions, where VDS = VDS,sat = VGS - VTHN, these equations are equal and
ID = ID,sat = [(KP/2)•(W/L)•(VGS - VTHN)2]
Okay, now as you have indicated others remove the VDS,sat and use simply
ID = [(KP/2)•(W/L)•(VGS - VTHN)2]•(1 + l•VDS)
which is the implementation also used in the Level 1 SPICE model.
So here's the rub, if you don't use VDS,sat then there is a discontinuity
at the border of the triode and saturation regions when VDS = VDS,sat. To
avoid this discontinuity you have to also multiply the equation for
operation in the triode region by (1 + l•VDS) or
ID = (KP•W/L)•[(VGS - VTHN)• VDS - V 2DS/2]•(1 + l•VDS)
This is often not mentioned or indicated by other authors, but, again it is what
is used in the level 1 MOSFET spice model.
See, also, the bottom of page 146 in the third edition.