Quote:i found some transistor's vdsat is not equal to its vod even in saturation region
What is vod? Do you mean vgs-vth? Since the popular BSIM3 model is more complicated than level-1 MOS model, vdsat is not equal to vgs-vth strictly. Please refer to BSIM3 manual which is available at Berkeley website bsim.eecs.berkeley.edu.
In weak inversion region, the MOS operates like a bipolar transistor much more. When the vds is greater than vdsat, the Ids is very flat. In other word, the MOS is in saturation region.
In [Grey and Meyer], it states that the drain current is almost constant when Vds>3VT (VT=kT/q). So the minimum vds required to force the transistor to operate as a current source in weak inversion is independent of the overdrive. At 300K, 3VT ≈75 mV. That is a relative coarse but reliable value. It is not uncommon to get vds = 37mV as that in your SPICE log file.
Best regards,
Yawei