The conduction mechanism under quasibreakdown of ultrathin gate oxide has been studied systematically in both n and p metal-oxide-semiconductor field effect transistors (MOSFETs) with a 3.7 nm gate oxide. The carrier separation experiment is conducted to investigate the evolutions of gate, source/drain, and substrate currents before and after quasibreakdown. It is shown that after quasibreakdown, the substrate current and the source-drain current versus the gate voltage curves are surprisingly analogous to those curves observed in fresh MOSFET with a gate oxide of direct tunneling thickness. This strongly supports the quasibreakdown model based on the local physically damaged region by which the effective oxide thickness is reduced. When direct tunnelings of conduction band electrons, valence band electrons and holes through the effectively thinned gate oxide are taken into account, the major experimental observations in the quasibreakdown can be explained in a unified way. (C) 1999 American Institute of Physics. [S0003-6951(99)03542-1].