The Drain Induced Barrier Lowering (DIBL) is an electrostatic effect where the threshold voltage descreases with increasing drain bias. You can simulate the DIBL effect by adjusting the Drain Control Parameter (alphaD). The governing equation is:
DIBL = S * alphaD/(2.3kT/q)
where S is Subthreshold Swing and kT/q is the usual thermal voltage scaling factor.