Gate-oxide thickness dependence of hot-carrier-induced degradation was investigated for sub-10-nm gate-oxide nMOSFET’s. It was confirmed that a thinner gate oxide nMOSFET shows smaller degradation. Mechanisms for the smaller degradation were analyzed using a simple degraded MOSFET model. It was found that the number of the generated interface states is defined uniquely by the amount of peak substrate current, independently from the gate-oxide thickness. It was found that the major cause of the smaller degradation in the thinner gate-oxide device is smaller mobility degradation due to the generated interface states. The degraded mobility was measured and formulated. The smaller mobility degradation was explained by the difference between vertical electric field dependence of the Coulomb scattering term and that of the phonon term under the inversion condition. The effect of a larger channel conductance, due to the larger inversion charges for the thinner gate-oxide device, was found to be the secondary cause for the smaller degradation.