Carrier tunneling in the gate dielectric, especially in the direct-tunneling regime where large current densities flow through the gate oxide, are known to result in substantial changes in the drain-current characteristics of MOSFETs. In this paper, we present simulation results of the drain-current characteristics of MOSFETs with ultrathin oxide using Tunnel-PISCES, a MOSFET tunneling simulator that models electron tunneling through the gate dielectric in a self-consistent manner with carrier transport by drift and diffusion in the substrate. We are able to predict the experimental trends reported for the dependence of the drain current of ultrathin-oxide MOSFETs on gate-oxide thickness. This tunneling simulation capability provides a means for generating MOSFET sizing guidelines to avoid tunneling-induced drain-current degradation.