u0 is a surface mobility, which is different than the well
characterized and fairly consistent bulk mobility. It
depends a lot on surface (underside of gate ox) quality
and this is very process-variable.
For Tox, you may see reported an optical and/or an
electrical Tox in foundry data; electrical Tox may
include things like poly depletion making it generally
larger, but more accurate for device modeling, than
the optical Tox that inline controls tend to observe.