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New kinds of hot-carrier injection mechanisms, which are different from channel hot-electron and substrate hot-electron injection mechanisms already reported by Ning, et al., are presented. These are first drain avalanche hot-carrier (DAHC) injection and secondly substrate current induced hot-electron (SCHE) injection. DAHC injection is due to the emission of electrons and holes heated in the drain avalanche plasma. SCHE injection is caused by the emission of excess electrons originating from impact ionisation of substrate current. The authors have succeeded in directly observing these two injection phenomena by measuring gate current as low as of the order of 10Â¿15 A for small size MOS devices having effective channel length below 1.0 Â¿m and gate oxide thinner than 10nm. The measured gate currents due to both injection mechanisms are also modelled numerically using the CADDET, a 2-D analysis program. In addition, device degradation due to these new injections is clarified. Base on the experimendal results, it is shown that DAHC effect rather than channel hot-electron effect, is more responsible for hot carrier related device degradation. The SCHC is also found to cause significant threshold shift, even at the VD<3V bias condition. thus, the presented injection phenomena prove to impose more Severe constraints on the submicron VLSI device design.