Radiation-Induced Variable Retention Time in Dynamic Random Access Memories

The effect of gamma-ray and neutron radiations on the variable retention time (VRT) phenomenon occurring in dynamic random access memory (DRAM) is studied. It is shown that both ionizing radiation and nonionizing radiation induce VRT behaviors in DRAM cells. It demonstrates that both Si/SiO2 interface states and silicon bulk defects can be a source of VRT. It is also highlighted that radiation-induced VRT in DRAMs is very similar to the radiation-induced dark current random telegraph signal in image sensors. Both phenomena probably share the same origin, but high-magnitude electric fields seem to play an important role in VRT only. Defect structural fluctuations (without change of charge state) seem to be the root cause of the observed VRT whereas processes involving trapping and emission of charge carriers are unlikely to be a source of VRT. VRT also appears to be the most probable cause of intermittent stuck bits in irradiated DRAMs.