Avalanche and Fowler-Nordheim tunneling electron injections have been performed at constant current on a broad variety of differently processed Al-gate metal-oxide-semiconductor capacitors. It is found that the same type of positive charge (the ''slow states'') is generated during low-field and high-field electron injection. The maximum amount of positive charge which can be generated at a given electric field depends on processing and increases linearly with the average field in the oxide. However, the rate at which the positive charge is generated is controlled uniquely by the anode field, for a given polarity of the gate voltage. It follows that the role of the electron traps in the bulk SiO2 - independent of their nature - is that of increasing both the rate and the total number of created defects by enhancing, respectively, the anode field, as a result of the distortion of the potential in SiO2, and the average field which must be increased to maintain a constant injected current. Processes described earlier for the generation of the interfacial defects such as interband impact-ionization and water-triggered mechanisms, do not account for the experimental results. In particular, the role of the water-related electron traps appears to be simply ''electrostatic'' - as for other electron traps, such as arsenic - rather than ''electrochemical,'' as previously proposed. Instead, it is suggested that during the hot electron injection, a positive or neutral species emitted from the anodic region propagates to the Si-SiO2 interface creating the damage.