ACWWong wrote on Jun 7th, 2006, 5:21am:sorry to butt-into this conversation... i agree with mikki... slow speed for CMOS circuits is usually at high temperature... inspite of vth reduction...
My understanding is that mobility is increasing (as temp goes up) at low temperatures because the ions will have less effect on a hotter carrier. Then mobility decreases because the lattice vibrates and there are collisions. So the curve of mobility vs. temp is an inverted 'V'.
The Vth change is -0.6 mV/K but mobility goes down with T^(-1.5), so yes, mobility is going to really affect your circuit speed more. Plus Ids is a linear function of Vgs-Vth with short-channel devices, so the temp has less of an effect on current via Vth than for square-law devices.
Cheers,
Marc
P.S. I am using the handle 'Croaker' now. It comes from the 'Black Company' books.