Compute-in-Memory Array Design Using Stacked Hybrid IGZO/Si eDRAM cells

To effectively accelerate neural networks in compute-in-memory (CIM) based systems, higher memory cell density is essential to handle the increasing computational workload and number of parameters. While CMOS embedded dynamic random access memory (eDRAM) is being explored as an alternative, improving the short retention time $(t_{ret})$ (<1 ms) remains a challenge that must be addressed for system applications. Recent studies show that InGaZnO (IGZO)-based eDRAM demonstrates an exceptionally long retention time $t_{ret}$ (> 100 s), but additional improvements are needed due to its substantial cell variability and slower operating speed compared to CMOS-based cells. This paper proposes a cell and array design for CIM using 3T-based stacked hybrid IGZO/Si eDRAM (Hybrid-3T) and performs a system-level deep neural network (DNN) evaluation. The Hybrid-3T cell, designed based on 7-nm FinFET technology, achieves $t_{ret}$ that is 100 s longer compared to IGZO-based 3T eDRAM (IGZO-3T). The proposed Hybrid-3T offers a 3.4 × higher bit cell density compared to 8T SRAM bit cells and a 2 × higher density compared to CMOS-based 3T eDRAM (CMOS-3T), while demonstrating similar throughput and variability levels to CMOS eDRAM and SRAM-based systems. Furthermore, we evaluate DNN inference accuracy for vision and natural language processing (NLP) tasks using the proposed CIM design, examining the impact of improved cell variability and retention time on system-level characteristics. The retention time for ensuring CIM operation accuracy $(t_{ret,CIM})$ is 10⁸ times longer in Hybrid-3T than CMOS-3T, and the $t_{ret,CIM}$ considering variability $(t_{ret,CIM_{v}})$ is more than 3 × longer than IGZO-3T eDRAM. As a result, the proposed Hybrid-3T eDRAM CIM leverages the advantages of both CMOS-3T and IGZO-3T CIM designs, enabling the development of high-performance, reliable systems.