Acoustic-phonon runaway and impact ionization by hot electrons in silicon dioxide

We present model calculations for high-field electron transport in silicon dioxide based on recently measured energy-dependent electron-phonon scattering rates and impact ionization rates. We find a hot-electron runaway phenomenon in ${\mathrm{SiO}}_2$, ‘‘acoustic-phonon runaway.’’ This phenomenon occurs at electric fields exceeding 7 MV/cm, when acoustic-phonon scattering can no longer stabilize the hot electrons. A fraction of the electrons are accelerated in the electric field to energies high enough to generate electron-hole pairs by impact ionization. Simulated hole currents due to high-field impact ionization in ${\mathrm{SiO}}_2$ gate oxides with thicknesses greater than 200 Å agree well with measured substrate hole currents in n-channel field-effect transistors. This suggests that these currents are due to holes generated by hot-electron impacts in the gate oxide.