Sub-unity body factor: The next CMOS and beyond CMOS technology booster for enhanced energy efficiency?

CMOS is the most successful technology for integrated circuits; for more than 50 years the efforts on scaling the MOSFET transistor dimensions towards nanometer scale, together with performance improvement and cost reduction have resulted in a revolution in computing and communication technologies that penetrated all domains of human life. Today we experience the mobile computing and the onset of the Internet of Things era, where the demands on enhanced integrated functionalities and energy efficiency are as important as the system miniaturization. Despite the fact that new paradigms in computing consider possible future roles of neuromorphic and quantum computing, it appears that, at least for the near future CMOS will continue to play a dominant role. One of the remaining challenges for transistors with nanometer dimensions is the voltage scaling, today saturated at value near 0.75V. Until very recently this supply voltage have been scaled at an average rate of around 50mV/year by applying an extraordinary set of technology boosters that transformed CMOS in a multi-material three dimensional technology, exploiting advanced quantum effects in semiconductors. However, the present voltage supply saturation is explained by the impossibility to further scale the threshold voltage without exploding the OFF current of the transistor and is due to the incompressible thermionic subthreshold slope of the MOSFET, having as limit 60mV/decade at room temperature. This limitation generated tremendous research efforts on devising new types of so called subthermionic steep slope switches, where the principle of the MOSFET switch is changed with physical mechanisms that can provide such steeper transition between OFF and ON state. The goal is to enable further supply voltage scaling down to 0.2-0.3V, where logic state distinguishability is ensured with an improvement of energy efficiency of more than 10x. This is also expected to help the limitations of dark silicon, essentially related to the power density and to the impossibility to turn on more than a limited fraction of the total number of transistors of a microprocessor.