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) ga te oxide during optically assisted electron injection into the gate in sulator of an unirradiated IGFET can have a significant impact on the observed threshold voltage shift (DELTAV(T)) associated with filling o r annihilation of insulator defects, and therefore on the calculated d efect density The shift due to neutral electron traps (sigma almost-eq ual-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 mini mum 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 threshol d voltage field and decreases to zero as the field increases. The resu lts indicate that at oxide fields between 1.5 and 3.5 MV/cm, the numbe r of trapped electrons decreases with field due to a change in tra pin g cross section. At fields greater than 3.5 MV/cm, the increase in thr eshold voltage shifts is believed to be due to the formation of new ne utral electron traps by high energy electrons. These effects were eval uated using standard two-level injections (for fixed positive charge a nd neutral electron traps) at various fields and also by injecting ele ctrons in a nearly continuous fashion to examine how the threshold vol tage shift vs. the number of injected electrons varies with oxide fiel d. 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 f ield of 0.7 MV/cm, the conclusions regarding changes in cross section and the formation of new traps were reinforced.