On Noise for MOSFET work in linear region

[abonic]

Hi guys,
for noise calculation, we are quite familiar with MOSFET work in saturation region, the channel noise current is widely recoginized as \[\bar{I_{n}^2 }=4KT\gamma gm\]. What if it works in Linear region, or subthreshold region.
I just feel confusion that, why resistor\[r_{ds}\] should not be included in saturation region, while it must be taken into account in linear region .

 

[csolis_GT]

The resistance rds in saturation mode models a behavior of the channel, it is not a physical resistance. The channel's noise is already modeled by In, so adding noise for rds would be like double counting. In the other hand, in the linear mode, the channel is an actual resistance and the noise is modeled as such.