Carbon Nanotube SRAM Design With Metallic CNT or Removed Metallic CNT Tolerant Approaches

A study of an eight-transistor static random access memory (SRAM) cell and its implementation in carbon nanotube FET (CNTFET) technology are presented. Simulations of the CNTFET SRAM cell design, using a CNT SPICE model, have shown advantages over the CMOS cell in terms of static power, dynamic power, and noise margin. However, current CNT synthesis processes grow metallic CNTs alongside semiconductor CNTs. This in turn greatly degrades the performance and functionality of SRAM cells. In this paper, we present and compare two approaches to overcome the presence of metallic CNTs. The first approach tolerates metallic CNTs and uses a series of uncorrelated CNTs to form a transistor; this provides tolerance to metallic CNTs. The second approach uses an M × N array of uncorrelated CNTs to form a CNTFET and requires technologies capable of removing metallic CNTs. Both approaches have similar static noise margin. The second approach (removed metallic CNTs) consumes 1.45× more static power; on the other hand, its CNT count and write delay are reduced to 35.6% and 10.9% of the metallic tolerant approach, respectively. The realization of large memory modules in the presence of faulty SRAM cells can be achieved by having memory modules with as few as two spare columns.