Optimizing the operating voltage of tunnel FET-based SRAM arrays equipped with read/write assist circuitry

Abstract

This paper deals with obtaining the minimum operating voltage of memory arrays based on TFET SRAM cells. First, we compare the I-V characteristics of two TFETs and one FDSOI using SPICE simulations. The results reveal that TFET devices exhibit high ON/OFF current ratios at different power supply voltage levels. This observation suggests a higher stability for SRAM cells based on these devices. Next, the characteristics of 6T SRAM cells implemented using minimum sized transistors based on these three device structures are compared. The comparison, which considers two TFET cell structures, i.e., inward and outward SRAMs, is performed at different supply voltages. The results for the hold static noise margin show that at low supply voltages (i.e., below 300mV), the FDSOI SRAM cell cannot hold data whereas both the inward and outward structures of TFET have acceptable noise margins at all supply voltages. Among the two TFET structures, the outward cell is selected because of higher speed especially for the write operation. TFET SRAMs suffer from long read access latency at ultra-low supply voltages (e.g., 150mV). The problem, however, may be overcome by using the negative GND read-assist technique. The results show that for a 32×32 TFET outward SRAM array, the minimum energy consumption (energy-delay product) may be achieved at the supply voltage of 200mV (300mV) with 1.32GHz (4.55GHz) as the read access frequency.