Body biasing and self-assembled molecular coatings enable 50-mV operation of ultra-small mechanical relays for low-power digital computing.
For decades, the electronics industry has relied on scaling of their basic switching component — the metal–oxide–semiconductor field-effect transistor (MOSFET) — to provide continually increasing capabilities (processing data and speed) to digital integrated systems, while simultaneously decreasing key economic aspects such as area and fabrication cost. However, as scaling has continued, managing power consumption in integrated circuits (IC) has become increasingly challenging and is now one of the key bottlenecks facing IC designers1. The common complementary metal–oxide–semiconductor (CMOS) logic scheme, which uses two MOSFET transistors switching in a complementary way, should ideally consume power only when switching (dynamic power), which can be alleviated by decreasing the power supply voltage levels. But the presence of subthreshold currents (static power consumption) and the need for clearly distinguishable on–off current levels, which requires moderate power supply levels, creates a power scaling limit for CMOS technology2.
The fundamental physical limitations of MOSFET transistors, which cannot act as ideal switching devices, are the principal reason for this power scaling limit. For example, the presence of the subthreshold current detracts from the ideal zero current level when a transistor is switched to the off state, which in turn leads to an increase in the static power consumption. Additionally, a limited on–off current ratio with a relatively high activation or threshold voltage (the voltage required to start turning the transistor on) makes it difficult to reduce the power supply voltage (typically kept close to 1 V), setting a limit on the reduction of the dynamic power consumption. To quantify these characteristics, two main parameters are used: the subthreshold slope, defined as the slope of the current in log10 scale versus gate voltage (limited to approximately 60 mV per decade of current at room temperature), and the threshold voltage (typically more than 0.2 V) (Fig. 1a).
A variety of new devices are now emerging to overcome the limitations of CMOS-based digital systems as a power-hungry technology3,4,5,6,7,8. One such approach uses purely mechanical relays, fabricated using advanced micro- and nanofabrication techniques, to create very small switches with almost ideal switching characteristics9,10,11 (Fig. 1b,c). Reporting at the 2018 IEEE International Electron Devices Meeting in San Francisco, Zhixin Alice Ye and colleagues at the University of California, Berkeley now show that micromechanical relays can provide a low-power alternative to conventional CMOS switches12.
The researchers fabricated relays that were tens of micrometres in size and could exhibit ideal zero off-currents due to a lack of physical contact between the electrodes. Furthermore, due to the nature of the mechanical switching, extremely steep subthreshold slopes of around 10–20 mV per decade were achieved. By biasing the relay body, and through the use of self-assembled molecular coatings, Ye and colleagues were able to fabricate devices with threshold voltages of 50 mV, with unprecedentedly small switching hysteresis voltages. When combined, these properties allow the team to fabricate relay-based logic gates, such as AND, OR, and XOR gates, that operate at voltages as low as 100 mV — an order of magnitude lower than conventional CMOS-based logic.
This is by far the most advanced device of this type, exhibiting the properties needed to decrease the power consumption in digital integrated systems. The micromechanical relay could, in particular, be of use in the rapidly emerging market of networked smart objects — the Internet of Things — where low-power, low-voltage systems are a key requirement. Further down-scaling of the micromechanical relays, and studies on yield and durability of the coating, will be needed before the viability of these devices in practical applications is confirmed. However, the work of Ye and colleagues represents a significant advance in the fight to overcome the power scaling limit imposed by the use of MOSFET transistors. It also provides an important step towards a new approach to digital computing in which very low-power computing is achieved using purely mechanical systems or using hybrid systems that incorporate CMOS circuits and CMOS-compatible micromechanical relays13.
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Barniol, N. Mechanical relays for digital circuits. Nat Electron 1, 616–617 (2018). https://doi.org/10.1038/s41928-018-0181-2