High-field and current-induced positive charge in thermal SiO₂ layers

The generation of a bulk positive charge in SiO₂ layers 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-SiO₂ interfacial positive charge is also generated in these samples.