Post-breakdown conduction in sub-5 nm gate oxides in MOS devices

We investigate the electron transport through broken down ultrathin (<5 nm) SiO/sub 2/ films in metal-oxide-semiconductor structures. We present experimental and theoretical results which demonstrate that the conduction properties of such systems are essentially governed by the cross-sectional area of the constriction connecting gate and substrate. It is shown that the hard or catastrophic breakdown mode exhibits conductance plateaus of the order of the quantum unit G/sub 0/=2e/sup 2//h (/spl sime/12.9 k/spl Omega//sup -1/), as found in quantum point contacts. On the other hand, the soft breakdown mode depends exponentially on the applied bias and can be understood in terms of tunneling through the potential barrier associated with the lower electron transversal state at the narrowmost part of the constriction.