RRAM-based Compute-in-memory (CIM) has emerged as a promising computing paradigm for artificial intelligence (AI) algorithms. However, the low on/off ratio and high on-current have been the major challenges to enhance the accuracy, parallelism, and energy efficiency. In this paper, we propose a novel Hybrid RRAM-Capacitor (HRC) cell based CIM macro to address these issues. The proposed HRC cell achieves a high on-off ratio with sub-100nA on-current and eliminates direct current path during computation, which significantly enhances both parallelism and energy efficiency. The write-verify scheme for RRAM programming is optimized for HRC cell array, and is further supported by a quantization result calibration technique using a dummy column to ensure high linearity and accuracy in analog domain multiply-and-accumulate (MAC) operations. A HRC-CIM macro has been designed and demonstrated using 28nm technology node, enabling block-level parallelism across 64 rows with 4-bit input per row, and delivering an energy efficiency of up to 40.40 TOPS/W @8b-IN/8b-W.

Recently, an in-memory analog circuit based on crosspoint memristor arrays was reported, which enables solving linear regression problems in one step and can be used to train many other machine learning algorithms. To explore its potential for computing accelerator applications, it is of fundamental importance to improve the computing speed of the circuit,i.e., the circuit response towards correct outputs. In this work, we comprehensively studied the transfer function of this circuit, resulting in a quadratic eigenvalue problem that describes the distribution of poles. The minimal real part of non-zero eigenvalues defines the dominant pole, which in turn dominates the response time. Simulations for multiple linear regression solutions with different datasets evidence that, the computing time does not necessarily increase with problem size. The dominant pole is related to parameters in the circuit, including feedback conductance, and gain bandwidth products of operational amplifiers. By optimizing these parameters synergistically, the dominant pole shifts to higher frequencies and the computing speed is consequently optimized. Our results provide a guideline for design and optimization of in-memory machine learning accelerators with analog memristor arrays. Also, issues including power consumption, impact of noise and variation of sources and memristors are investigated to offer a comprehensive evaluation of the circuit performance.