A Process Compensated Gain Cell Embedded-DRAM for Ultra-Low-Power Variation-Aware Design

Gain cell embedded DRAM (GC-eDRAM) is a high-density alternative to SRAM for ultra-low-power systems. However, due to its dynamic nature, GC-eDRAM requires power-hungry refresh cycles to ensure data retention. Traditional design approaches dictate configuration of the refresh rate according to the worst bitcell, when biased at low-probability, worst-case conditions. However, due to the process variations and local mismatch that can significantly deteriorate the data retention time of a GC-eDRAM bitcell, this design approach often leads to a large power overhead. In this paper, we present a novel GC-eDRAM architecture, incorporating several techniques for variation-aware operation. The primary feature of this architecture is an improved replica scheme for process compensated access tracking that enables calibration for process variations and adaptive refresh according to the array access statistics. The array is shown to ensure data integrity, providing as much as a 7x reduction in retention power over worst-case refresh-rate design for 20% write activity.