Cross-Coupled Ferroelectric FET-Based Ternary Content Addressable Memory With Energy-Efficient Match Line Scheme

Fast communication between networking devices increases the importance of the ternary content addressable memory (TCAM). The demands for low energy in networking devices have accelerated the research on nonvolatile TCAMs that store data without the power supply. Recently, ferroelectric field-effect transistors (FeFETs), nonvolatile three-terminal devices with a high on/off ratio, have been adopted for TCAMs. Although the previous FeFET TCAMs consume low write energy with a significantly compact TCAM cell area, they suffer from write problems: 1) the write scheme cannot afford the saturated polarization switching in FeFETs and 2) the states stored in the unselected TCAM cells are changed during the write operation. In addition, search yield is deteriorated by wide process variations of FeFETs. This paper proposes a novel FeFET TCAM that is free from the write problems of the previous FeFET TCAMs and tolerant to process variations. This paper also proposes a novel match line scheme to improve search energy and time by reducing match line capacitance and the amount of discharged voltage in the match evaluation phase. Industrial-compatible 28 nm technology-based simulation results with the Preisach FeFET model show that the proposed FeFET TCAM achieves the highest search yield.

I. Introduction

The recent development of the Internet of Things (IoT) has increased the exchange of a large amount of data among networking devices. In the rapid exchange of data, a content-addressable memory (CAM) plays a critical role in improving the searching speed of the networking devices for route look-up, packet classification, or packet forwarding. A ternary CAM (TCAM), which provides the ‘X (don’t care)’ as well as ‘0’ and ‘1’, enables a more flexible search operation. The most typical structure of a TCAM is based on static random access memories (SRAMs). Although the SRAM TCAM supports fast writing and searching, it consumes a high static power for maintaining the data [1]. To resolve this problem, several nonvolatile TCAMs have been developed using magnetic tunnel junction (MTJ) or flash [2], [3], [4]. These nonvolatile TCAMs can maintain the stored data without power supply during standby mode, however, several drawbacks impede the practical use of nonvolatile TCAMs. First, the low on/off ratio of the MTJ [5], [6], [7] requires many transistors for ensuring enough sensing margin to distinguish match and mismatch accurately, resulting in a large area overhead. Second, since two-terminal devices like MTJ operate only as storage, they require a serially connected transistor as a switch, which also imposes area overhead. Third, current-based write operation of MTJ [8] or a large write voltage () and a long write time () of flash [9] cause a high write energy consumption.