Tunneling-injection-induced turnaround behavior of threshold voltage in thermally nitrided oxide n-channel metal-oxide-semiconductor field-effect transistors

Abstract

The threshold voltage (VT) degradation metal-oxide-semiconductor field-effect transistors (MOSFETs) with thermally nitrided oxide or pure oxide as gate dielectric was determined under Fowler-Nordheim (FN) stressing. A typical VT turnaround behavior was observed for both kinds of devices. The VT for nitrided oxide MOSFETs shifts more negatively than that for pure oxide MOSFETs during the initial period of FN stressing whereas the opposite is true for the positive shift after the critical time at turnaround point. The discovery that the shift of substrate current peak exhibits similar turnaround behavior reinforces the above results. In the meantime, the field-effect electron mobility and the maximum transconductance in the channel for nitrided oxide MOSFETs are only slightly degraded by stressing as compared to that for pure oxide MOSFETs. The VT turnaround behavior can be explained as follows: Net trapped charges in the oxide are initially positive (due to hole traps in the oxide) and result in the negative shift of VT. With increasing injection time, trapped electrons in the oxide as well as acceptortype interface states increase. This results in the positive shift in VT. It is revealed that VT degradation in MOSFETs is dominated by the generation of acceptortype interface states rather than electron trapping in the oxide after the critical time.