Study on off-state hot carrier degradation and recovery of NMOSFET in SWD circuits of DRAM

We investigated threshold voltage degradation and recovery of short channel NMOS transistors in the sub wordline driver (SWD), where the source of NMOS transistors was biased with negative voltage during off-state. We found that the threshold voltage degradation occurred by the off-state hot carrier stress. The gate of NMOS transistor in sub wordline driver (SWD) in the off-state is under the subthreshold region (0GS
T
) due to the negative bias voltage applied to the source terminal. At this situation if a high voltage is applied to the drain, the subthreshold drain current increases due to the DIBL effect. Furthermore, hot carriers with higher energies generate interface traps near the dielectric/Si interface of the MOSFET. These hot carrier degradation caused the positive shift in the threshold voltage. Aforementioned degradation could be recovered by baking with bias. Furthermore, the time of such a recovery was shortened when the gate terminal was floating.