Based on uniform hot carrier injection (optically assisted electron injection) across the $Si-SiO_2$ interface into the gate insulator of n-channel IGFETs, the threshold voltage shifts associated with electron injection of $1.25{ imes}l0^{16}{;}e/ extrm{cm}^2 between 0.5 and 7 MV/cm were found to decrease from positive to negative values, indicating both a decrease in trap cross section ($E_{ox}{geq}1.5 MV/cm$) and the generation of FPC $E_{ox}{geq}5{;}MV/cm$). It was also found that FNC and large cross section NETs were generated for $E_{ox}{geq}5{;}MV/cm$. Continuous, uniform low-field (1MV/cm) electron injection up to $l0^{19}{;}e/ extrm{cm}^2 is accompanied by a monatomic increase in threshold voltage. It was found that the data could be modeled more effectively by assuming that most of the threshold voltage shift could be ascribed to generated bulk defects which are generated and filled, or more likely, generated in a charged state. The injection method and conditions used in terms of injection fluence, injection density, and temperature, can have a dramatic impact on what is measured, and may have important implications on accelerated lifetime measurements.