The generation of a bulk positive charge in SiO2layers of silicon gate metal‐oxide‐silicon (MOS) devices, under the conditions of high‐field and charge injection is studied. The time dependence of the positive charge and its spatial distribution as a function of the oxide thickness and electric field are all consistent with an impact ionization‐recombination model which takes into account both the spatial and the field dependence of the ionization probability. The nature of the ionization, either band‐to‐band, or traps ionization, is still unknown. Bulk positive charge of the same nature is also formed in Al gate oxides. Nevertheless, it was not always observed in previous works since a much larger Si‐SiO2interfacial positive charge is also generated in these samples.