Energy-Efficient 4T SRAM Bitcell with 2T Read-Port for Ultra-Low-Voltage Operations in 28 nm 3D Monolithic CoolCube™ Technology

Abstract

This paper presents a 4T-based SRAM bitcell optimized both for write and read operations at ultra-low voltage (ULV). The proposed bitcell is designed to respond to the requirements of energy constrained systems, as in the case of most IoT-oriented circuits and applications. The use of 3D CoolCube™ technology enables the design of a stable 4T SRAM bitcell by using data-dependent back biasing. The proposed bitcell architecture provides a major reduction of the write operation energy consumption compared to a conventional 6T bitcell. A dedicated read port coupled to a virtual GND (VGND) ensures a full functionality at ULV of read operations. Simulation results show reliable operations down to 0.35 V close to six sigma (6 σ) without any assist techniques (e.g. negative bitlines), achieving in worst case corner 300 ns and 125 ns in write and read access time, respectively. A 6x energy consumption reduction compared to a ULV ultra-low-leakage (ULL) 6T bitcell is demonstrated.