Why MOSFET has nonlinearity if it operate in non-sat region?

Hi. It's first time to write question at this site so if i violated the rules of this site please tell me
anyway I have a question about MOSFET operation
many people said if we want to use MOSFET in analog integrated circuits, MOSFET has to be in saturation region
because of many things such as linearity , high gain and so on
however, there are no articles which explain why MOSFET has nonlinearity if it operates in non-sat, using equation
please help me to figure out this problem!

...
now i have some books "microelectronics, by neamen" "Design of analog CMOS, by Rajavi"
where do i have to see ?

• In saturation the MOSFET acts as a voltage controlled current source.We get around the nonlinear relation to obtain linear amplification.

• By dc Biasing we make MOSFET operate at a certain VGS and ID & then superimpose the signal to be amplified vgs on the dc bias VGS.Keeping the signal vgs small changes in iD almost proportional to vgs.
• In attached image,between Point A and Point B: MOSFET is saturated,iD increases with increasing vgs.
• After Point B: MOSFET is triode,Curves are very crowded,further decrease of vO slow (VTC between B and C).
• The signal vi to be amplified is superimposed on the dc voltage VIQ.Keeping vi small restrict operation to an almost linear segment of the VTC,Output signal vo is proportional to vi.
• But this isn't the way in triode region.

-------------- By the way,this post is in wrong thread.Maybe moderators can move this post to the right thread -----------------------

thanks for your helps. However, from the picture I can get only two things (between A and B, 1. the voltage gain of the amp is maximum 2. the amp is linear).
I couldn't figure out what i want to know about Nonlinearity of the MOSFET amp in NONSaturation region.
{{
while studying BJT, I read below equation
iB=Is*exp(VBE/VT)/B*exp(vbe/VT)
from the equation. I've gotten that this equation is not linear
then we assume vbe is sufficiently small so as to consider this term linear
in a Taylor series iB=Is*exp(VBE/VT)/B *(1+vbe/VT)=IBQ+ib
finally I can think that the equation has linearity and use superimpose rule
}} << like this I want to get some ideas or equation in order to see why MOSFET has nonlinearity if it operate in non-sat region through the some equations

EVERYTHING is non-linear