DESIGN AND DEVELOPMENT OF RADHARD SRAM CELL

Abstract

One of the major causes of the single event upset in the electronic circuit/device operating in geostationary orbit is heavy ion radiation. Due to continuous shrinking of deep submicron CMOS technologies single event effect due to ionizing particles has become a severe problem. Due to shrinking the critical charge which is required for the upset is reduced in newer technologies trends, upset is even caused by lightly ionizing particles low energy protons and muons. Soft errors are dominated in the VLSI circuit that affects the memory the most. These physical non-destructive mechanisms are described in chapter 2. Upset mechanisms in dynamic random access memories and static random access memories are described in detail in chap 3. Two unique modified SRAM cells have been proposed in the work presented. The first circuit propose a new way which uses ex-or gate to recover the initial state of memory cell. It has a versatile block which can be connected to any SRAM cell to make it immune to single event caused by particles. The recovery circuit is implemented on both 6T and 9T SRAM cell. The second proposed circuit uses the dynamic body biasing to change the threshold of the restoring transistor in real time. The body of the two restoring transistor (each for the two storage node) is varied depending on the states and the cycle of the memory cell. That changes the threshold of the transistor according to the potential at each storage node. It forbids the change in the state of the memory cell unless there is read or write cycle by decreasing the restoring time. Thus the first circuit which uses the ex-or gate rectifies the state of the memory cell if there is any change due to single event while the second proposed circuit doesn’t allow any change by decreasing the restoring time. Simulation also played an important role in understanding the SEU mechanisms thus reducing the number of fab and test cycle for the development of radhard circuit to zero. Visual TCAD Monte Carlo code software is used to simulate the radiation environment.