In the past, classical device simulators have been modified to incorpo
rate quantum effects using a quantum mechanical (QM) threshold-shift c
orrection. In this way, it is hoped to retain accuracy without greatly
complicating the simulation by incorporation of a coupled Schrodinger
equation solver. In this work, the accuracy of this approach is check
ed for some specific examples. The drain current of heavily doped MOSF
ET's is found using a one-dimensional (1-D) Schrodinger-Poisson solver
combined with a gradual channel model. Numerical results are compared
to classical calculations augmented by the commonly proposed channel-
current invariant QM threshold correction. Comparison of the two root
I-d(sat) versus V-GS curves shows the same threshold shifts, but diffe
rent slopes, The slope discrepancies are independent of substrate dopi
ng, and are largest for thin oxides. These differences are shown to be
due to QM effects upon the surface potential gradient, a variation ne
glected in previous studies. To simplify device simulations, two simpl
e quantum-effect corrections are proposed that show great improvement
in accuracy when compared to the earlier QM correction based on a chan
nel-current invariant V-G-shift.