  Portable devices like smartphones, tablets, wearable electronic devices,
medical implants, wireless sensor nodes, and Internet-of-Things (IoT) devices
have tremendous constraints on their energy consumption. Adding more
functionalities onto the portable devices increases its energy consumption
significantly. However, the energy capacity of the battery does not increase
proportionally. Hence, to overcome the constraints on energy consumption, two
main approaches are being undertaken by the designers to integrate more
functionalities onto the energy-constrained systems. One approach involves
reducing the inherent energy consumption of circuits, and the other involves
harvesting energy from the ambient sources and utilizing it to power the
circuits. In this thesis, both the approaches mentioned above are followed in
developing energy-saving techniques for energy-constrained CMOS circuits and
systems. Switched-capacitor-assisted power gating technique is proposed to
reduce the leakage current of large digital circuits. Benefits in overall
energy saving using nano-electromechanical switches (NEMS) for power gating is
quantified on a system-on chip for a mobile platform made using a 14 nm gate
length FinFET. For reducing the quiescent current in the analog front-end
circuits, this thesis proposes a novel technique of realizing discrete-time
(D-T) signal amplification using nano-electromechanical switches (NEMS). Apart
from the three energy reduction techniques mentioned above, harvesting energy
from mechanical vibrations using an efficient technique of power extraction
from the piezoelectric transducer is proposed.
