ENHANCED HOT-CARRIER-DEGRADATION IN LDD MOSFETS UNDER PULSED STRESS

Hot-carrier degradation of lightly doped drain (LDD) MOSFET's under ac stress is investigated. Enhanced ac degradation occurs in LDD MOSFET' s as well as in single drain MOSFET's. However, there is a peculiar de gradation mechanism in LDD MOSFET's. For single-drain MOSFET's, enhanc ed ac degradation appears in both threshold voltage and transconductan ce at stress drain voltages larger than a critical value. On the other hand, for LDD MOSFET's, although the enhanced degradation in threshol d voltage and transconductance appears at stress drain voltages larger than a critical value, the enhanced degradation in transconductance a ppears even under stress drain voltages lower than the critical value. The difference in the ac-enhanced degradation between LDD and single- drain structures can be explained by a hot hole generated neutral-elec tron-trap model and the change in hot-hole-injected oxide region accor ding to stress bias conditions.