Analysis on Retention Time and Adaptive Refresh in Embedded DRAMs with Aging Benefits

Embedded DRAMs (eDRAMs) are a promising solution to replace SRAMs for on-chip memories in low-power applications. Gain cells-based eDRAMs, because of their compatibility with standard CMOS process, offer a viable solution to high density storage required by modern SoCs. However, they are usually characterized by a short retention time, which increases their power consumption due to the need of frequent refresh. In this paper, we first analyze the beneficial effects of BTI aging for leakage reduction in eDRAMs and consequent retention time increase. By means of SPICE simulations, we show that, after only a month of operation, retention time increases between 7.2% and 57.9%, depending on cell structure. Retention time increase may exceeds 150% in less than 5 years of operation. Finally, we show how to capitalize on this beneficial effect by adopting an adaptive refresh rate, leading to a significant refresh power reduction over time that, for the considered eDRAM cells, ranges between 10% and 51% in 10 years of operation.