p-channel Si-gate metal-oxide-semiconductor transistors of very thin oxides are used for the study of quantum yield of electron impact ionization in silicon. Electrons are injected into silicon from the polysilicon gate by tunneling to give an approximate δ-function energy distribution. This energy distribution is preserved when electrons pass through the oxide by direct tunneling. Using the carrier-separation properties of the induced junction, we are able to experimentally measure the number of generated electron-hole pairs as a function of the incident electron energy, up to 5 eV. Our results are found to be in excellent agreement with recent theoretical calculations of quantum yield. Beyond 5 eV, the interpretations on the experimental data are difficult due to the broadening of the incident electron energy distribution. This broadening effect is caused by strong scattering in the oxide when electrons tunnel by the Fowler-Nordheim (F-N) process. It is observed that the average energy of those electrons tunneled by the Fowler-Nordheim process becomes a function of oxide field, relatively independent of the oxide thickness.