since overdrive voltage is directly proportional to current, the worst case corner for vdsat depends on the biasing current which biases that particular transistor. since VTH is maximum at slow process corner, vdsat is minimumfor a given current. see where the bias current gives min current at max or min operating temperature depending on whether bias circuit is a PTAT or NTAT or bandgap current. based on above, we can guess at which corner, vdsat will be min or max.
we have tobe careful where vdsat is max since we have tocheck whether all transistors are in saturation only at this corner.