Actually, I need a general expression that would hold both in strong an weak inversion.  So far the best I have found is the following expression from Tsividis' book "Operation and Modelling of the MOS transistor" (eq. 8.2.48 in the 2nd edition / eq. 7.2.58 in the 3rd edition):

gds = (W/L)uCox*[Vgb-VFB-PSIsL-gamma*sqrt(PSIsL)]  

(PSIsL is the surface potential at the drain) which I guess can be rewritten in terms of Vgs as:

gds = (W/L)uCox*[Vgs-Vsb-VFB-PSIsL-gamma*sqrt(PSIsL)]

However, I'm not sure if this expression is valid all the way from strong to weak inversion and in linear and saturation regions (that would be great). Can someone please confirm this?

Note that I am not interested in the modelling of the Ids dependence on Vds (channel length modulation, DIBL, etc): I just need an expression to show that, when Vgs diminishes, the drain-to-source conductance diminishes too!

Thanks, Jorge.