The threshold voltage varies somewhat with the bias between Source and Bulk, VSB.
VT = VT0 + GAMMA [ (2 PHIF + VSB)1/2 - (2 PHIF)1/2 ]
where VT0 is the zero-bias threshold voltage, GAMMA is the body-bias factor
GAMMA = (2 q EPSILONSi Na)1/2 / Cox.
PHIF is the bulk Fermi potential
PHIF = (kT/q) ln(Na/ni).
Na is the acceptor, boron, doping density, usually on the order of 1015 cm-3.