MCUs and Transistors of the Subthreshold Variety
Low-priced digital watches make use of FETs that function in the sub-threshold region. The sub-threshold region has been an avoided topic for a lot of reasons. It so happens that characterizing FETs in the sub-threshold region is difficult.
However, sub-threshold has made a comeback in the embedded arena with the Ambiq Apollo MCU. The Ambiq Apollo MCU has FETs functioning in the sub-threshold region. It is basically a Cortex M4F MCU that boasts of very low power consumption. In fact, it is believed to be as low as 34 uA/MHz while operating through flash. It is a number that is drastically lower than competing MCUs. Plus, it is an M4 based MCU with an in-built hardware floating point.
The numbers are almost hard to believe. 34 uA/MHz is literally one-tenth of what the MSP430F11 uses and it is supposed to be an ultra-low power MCU. The MSP430F11 operates on 16 bits and costs the same as the Ambiq.
Generally, ultra-low power is all about getting an optimum amount of work done with a fair amount of sleeping time. But, the M4F is capable of executing tasks in very little time and even during execution, the MCU requires only a fraction of the power used by other processors.