Research targets highly energy-efficient CMOS logic systems

A team of researchers from the Tokyo Institute of Technology has proposed an architecture of power-gating using non- volatile SRAM (NV-SRAM) and non-volatile FF (NV-FF) circuits, called non-volatile power-gating. According to them, these allow the size of logic circuit domains for power-gating to be optimally designed, and to make supply voltages and logic circuits becomes more efficient.

Developments in low power, high performance CMOS logic technology are vital to the future of microprocessors and SoC devices for personal computers, servers and mobile/smartphones. Much of the processing in these computing systems is carried out using a volatile hierarchical memory system in which bistable circuits such as static random access memory (SRAM) and flip-flop (FF) play an essential role for fast data-access. However, the power to these bistable circuits cannot be switched off without losing their data. This inability to turn off power is a fundamental problem for energy consumption in CMOS logic systems.

The method for saving energy in CMOS logic systems, called power-gating, uses architecture to cut the supply voltage to idle circuit domains, effectively putting them to power shut-off state to avoid leakage and thereby save static energy.

Over the past few years, Satoshi Sugahara and his team have been developing non-volatile bistable memory circuits (NV-SRAM and NV-FF) required to establish non-volatile power-gating systems with better overall performance and energy efficiency than conventional power-gating systems. In particular, the researchers have built pseudo-spin metal-oxide-semiconductor field-effect transistors (PS-MOSFETs) for use in the non-volatile bistable memory circuits.

The PS-MOSFET can be configured with an ordinary MOSFET coupled with a spin-transfer torque magnetic tunnel junction (STT-MTJ), and it can reproduce the functions of spin-transistors in which different electrons spin states or magnetisation configurations of the ferromagnetic electrodes are used to control transistor output. Spin transistors can also store non-volatile information. In a typical bistable memory circuit, an inverter loop consisting of cross-coupling two CMOS gates is used to store each memory bit. In the new non-volatile bistable circuits, PS-MOSFETs are added to the inverter loop.

Previous attempts to build non-volatile bistable circuits with STT-MTJs have resulted in performance degradation, because the STT-MTJs interfere with their fundamental circuits of the inverter loops. To overcome this problem, the team designed NV-SRAM and NV-FF circuits using PS-MOSFETs. In these circuits, the STT-MTJs can be electrically separated from the inverter loops by the PS-MOSFETs and thus have no degradation effects on the bistable circuit performance.

The NV-SRAM and NV-FF circuits built by Sugahara's team have performed well under tests so far, compared to conventional SRAM/FF circuits. They also developed architectures for minimising break-even time (that is an important performance index of power-gating) of the NV-SRAM and NV-FF circuits, including a 'store-free' shutdown, wherein existing data is not rewritten, thereby dramatically saving energy.

The new transistor and circuit designs could be pivotal in the development of faster, more energy-efficient processing in future CMOS logic systems, said the researchers.