Effect of OFF-state stress on reliability of nMOSFET in SWD circuits of DRAM.

In this paper, we investigate the effect of OFF-State stress occurring at nMOSFET in a sub word-line driver (SWD) circuit under the DRAM operating conditions. Through the experimental study, we found that the increase in the number of OFF-carriers by the OFF-State stress caused the significant decrease of ON-current (Ion). During the initial period, the OFF-State stress generated the positive oxide charges (Qox) and the interface traps (Nit), which caused the increase of OFF-current (Ioff) and the decrease of Ion. After the degradations were saturated, both the Ion and Ioff decreased due to the increase in the number of OFF-carriers. Also the OFF-State degradation increased with decreasing source voltage, channel doping concentration, and channel length. Furthermore, the sensitivity of the OFF-State degradation to body bias (Vb) increased with scaling the oxide thickness (Tox). These new observations suggest that the OFF-State degradation for core transistors can impose the significant limitation on the SWD circuit design in DRAM such as scaling of dimension and determination of word-line voltage.