Quantum-mechanical modeling of electron tunneling current from the quantized inversion layer of ultra-thin-oxide (<40 Angstrom) nMOSFET's is presented, together with experimental verification, An accurate determination of the physical oxide thickness is achieved by fitting experimentally measured capacitance-versus-voltage curves to quantum-mechanically simulated capacitance-versus-voltage results. The lifetimes of quasibound states and the direct tunneling current are calculated using a transverse-resonant method, These results are used to project an oxide scaling limit of 20 Angstrom before the chip standby power becomes excessive due to tunneling currents.