Re: the differences between subthreshold and saturation are
Hey guys, most of you should be sent to the starting block!!!!
Saturation means that the drain current saturates to a certain value, independent of Vds. This can be reached in weak (subthreshold), moderate and strong inversion!!!!
Why don't you re-read Tsividis book!!!!
The approximation saturation ~ Vds > Vgs-Vth is only valid for stron inversion. As the transistor approches weak inversion, Vds_sat has a theoretical limit to 2-4 times Vt (Vt=kT/q).
As somebody else mentioned it, in weak inversion (subthreshold; get used to weak inversion) drift phenomena dominates the drain current, but the transistor can still be saturated!!!! Actually, in very low power designs, you must place nearly all transistors in WI, and get a reasonable gain from your transistors because they are "saturated"!!!
A very good approximation, valid in all operations regions (WI, MI and SI) of the saturation Vds is
Vds_sat = 2/[n*(gm/Id)]
Give you anyway some margin.
Guys, in this topic you deceived me;-)