Single-Supply 3T Gain-Cell for Low-Voltage Low-Power Applications
Logic compatible gain cell (GC) embedded DRAM (eDRAM) arrays are considered an alternative to SRAM due to their small size, non-ratioed operation, low static leakage, and 2-port functionality. However, traditional GC-eDRAM implementations require boosted control signals in order to write full voltage levels to the cell to reduce the refresh rate and shorten access times. These boosted levels require either an extra power supply or on-chip charge pumps, as well as non-trivial level shifting and toleration of high voltage levels. In this paper, we present a novel, logic compatible, 3T GC-eDRAM bitcell that operates with a single supply voltage and provides superior write capability to conventional GC structures. The proposed circuit is demonstrated with a 2kb memory macro that was designed and fabricated in a mature 0.18um CMOS process, targeted at low-power, energy-efficient applications. The test array is powered with a single supply of 900mV, showing an 0.8ms worst-case retention time, a 1.3ns write-access time, and 2.4pW/bit of retention power. The proposed topology provides a bitcell area reduction of 43%, as compared to a redrawn 6T SRAM in the same technology, and an overall macro area reduction of 67% including peripherals.