Impact ionization and positive charge formation in silicon dioxide films on silicon

Positive charge formation and its possible relationship to impact ionization in silicon dioxide have been controversial issues for many years. In this study, band-gap ionization due to the development of a high-energy tail in the hot-electron energy distribution is shown to occur in films thicker than 20.0 nm at fields higher than 7 MV/cm. This process is demonstrated to "directly" account for hole currents in the substrate circuit of n-channel field-effect transistors and for the observation of positively trapped charges accumulating at the substrate-silicon/silicon-dioxide interface at low injected-carrier fluences (less than 0.001 C/cm2) before the onset of trap creation.