INTRINSIC THRESHOLD VOLTAGE SHIFT DEPENDENCE ON THE OXIDE FIELD-INDUCED DURING OPTICALLY ASSISTED ELECTRON INJECTION

It has been found that the magnitude of the electric field across the gate insulator in an insulated gate field effect transistor (IGFET) gate oxide during optically assisted electron injection into the gate insulator of an unirradiated IGFET can have a significant impact on the observed threshold voltage shift (DELTAV(T)) associated with filling or annihilation of insulator defects, and therefore on the calculated defect density The shift due to neutral electron traps (sigma almost-equal-to 10(-16) cm2) vs. oxide field was found (i) to be constant below the field necessary to turn the device on, (ii) to decrease to a minimum as the field was increased up to an oxide field of 3.5 MV/cm, and (iii) to increase again without saturation as the field was increased. The shift due to fixed positive charge is constant below the threshold voltage field and decreases to zero as the field increases. The results indicate that at oxide fields between 1.5 and 3.5 MV/cm, the number of trapped electrons decreases with field due to a change in tra ping cross section. At fields greater than 3.5 MV/cm, the increase in threshold voltage shifts is believed to be due to the formation of new neutral electron traps by high energy electrons. These effects were evaluated using standard two-level injections (for fixed positive charge and neutral electron traps) at various fields and also by injecting electrons in a nearly continuous fashion to examine how the threshold voltage shift vs. the number of injected electrons varies with oxide field. Furthermore, by injecting devices over a range of fields (0.7 to 8 MV/cm) and then performing an additional injection at a standard low field of 0.7 MV/cm, the conclusions regarding changes in cross section and the formation of new traps were reinforced.