Statistical analysis of dynamic-random-access-memory data-retention characteristics

Charges stored in a memory cell of a dynamic random access memory are lost by the Shockley-Read-Hall (SRH) current that is generated at carrier traps in the space-charge-region (SCR) of a junction. Magnitude of the SRH current is determined by the trap levels that are distributed not only among cells, but also within a cell. This traplevel distribution causes the temperature-dependent variation in the data retention times. The SRH current is enhanced by an SCR field, and the distribution of the field among cells also increases the variation in the retention times. Variation in the number of traps, on the other hand, contributes only slightly to the retention-time distribution. From these results we find that reduction of the electric-field distribution, as well as of the average field, is important to improve the data-retention characteristics.