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Circuit techniques for enhancing the retention time and random cycle of logic-compatible embedded DRAMs (eDRAMs) are presented. An asymmetric 2T gain cell utilizes the gate and junction leakages of a PMOS write device to maintain a high data `1' voltage level which enables fast read access using an NMOS read device. A current-mode sense amplifier (C-S/A) featuring a cross-coupled PMOS latch and pseudo-PMOS diode pairs is proposed to overcome the innate problem of small read bit-line (RBL) voltage swing in 2T eDRAMs with improved voltage headroom and better impedance matching under process-voltage-temperature (PVT) variations. A half-swing write bit-line (WBL) scheme is adopted to improve the WBL speed by 33% and reduce its power dissipation by 25% during write-back operation with no effect on retention time. A stepped write word-line (WWL) driver reduces the current drawn from the boosted high and low supplies by 67%. A 192 kb eDRAM test chip with 512 cells-per-BL implemented in a 65 nm low-power (LP) CMOS process shows a random cycle frequency and latency of 667 MHz and 1.65 ns, respectively, at 1.1 V and 85 × °C. The measured refresh period at a 99.9% bit yield condition was 110 μs which is comparable to that of recently published 1T1C eDRAM designs.
Date of Publication: Feb. 2012