MOSFET Replacement Devices for Energy-Efficient Digital Integrated Circuits
Technical Report Identifier: EECS-2009-182
December 17, 2009
Abstract: Increasing power density is a daunting challenge for continued MOSFET scaling due to non-scalability of the thermal voltage kBT/q. To circumvent this CMOS power crisis and to allow for aggressive supply voltage reduction, alternative switching device designs have been proposed and demonstrated to achieve steeper than 60mV/dec subthreshold swing (S). This dissertation begins with a general overview of the physics and operation of these MOSFET-replacement devices. It then applies circuit-level metrics to establish evaluation guidelines for assessing the promise of these alternative transistor designs. This dissertation then investigates the abrupt "pull-in" effect of an electrostatically actuated beam to achieve abrupt switching behavior in the nano-electro-mechanical field effect transistor (NEMFET). To facilitate low-voltage NEMFET design, the Euler-Bernoulli beam equation is solved simultaneously with the Poisson equation in order to accurately model the switching behavior of a NEMFET. The impact of various transistor design parameters on the gate pull-in voltage and release voltage are examined. A unified pull-in/release voltage model is developed. Finally, this dissertation proposes the use of micro-relays for zero-standby-power digital logic applications. To mitigate the contact reliability issue, it is demonstrated that since relatively high on-state resistance can be tolerated while extremely high endurance is a necessity, hard contacting electrode materials and operation with low contact force are preferred for reliable circuit operation. Using this contact design approach, a reliable relay technology that employs titanium dioxide (TiO2) coated tungsten (W) electrodes is developed for digital logic applications. Relay miniaturization will lead to improvements in density (for lower cost per function), switching delay (for higher performance), and power consumption. A scaled relay technology is projected to provide >10x energy savings for digital circuits operating at up to ~100MHz.