978-1-4673-1036-9/12/$31.00 ©2012 IEEE 275 13th Int'l Symposium on Quality Electronic Design A Body-Voltage-Sensing-Based Short Pulse Reading Circuit for Spin-Torque Transfer RAMs (STT-RAMs)

With scaling of CMOS and Magnetic Tunnel Junction (MTJ) devices, conventional low-current reading techniques for STT-RAMs face challenges in achieving reliability and performance improvements that are expected from scaled devices. The challenges arise from the increasing variability of the CMOS sensing current and the reduction in MTJ switching current. This paper proposes a short-pulse reading circuit, based on a body-voltage sensing scheme to mitigate the scaling issues. Compared to existing sensing techniques, our technique shows substantially higher read margin (RM) despite a much shorter sensing time. A narrow current pulse applied to an MTJ significantly reduces the probability of read disturbance. The RM analysis is validated by Monte-Carlo simulations in a 65-nm CMOS technology with both CMOS and MTJ variations considered. Simulation results show that our technique is able to provide over 300 mV RM at a GHz frequency across process-voltage-temperature (PVT) variations, while the reference designs require 4.3 ns and 2.3 ns sensing time for a 200 mV RM, respectively. The effective read energy per bit required by the proposed sensing circuit is around 195 fJ in the nominal case