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Design and analytical performance of subthreshold characteristics of CSDG MOSFET.

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The downscaling of the Metal-Oxide-Semiconductor Field Effect Transistors (MOSFET) devices have been the driving force for Nanotechnology and Very Large-Scale Integration (VLSI) systems. This is affirmed by Moore’s law which states that “The number of transistors placed in an Integrated Circuit (IC) or chip doubles approximately every two years”. The main objectives for the transistor scaling are: to increase functionality, switching speed, packing density and lower the operating power of the ICs. However, the downscaling of the MOSFET device is posed with various challenges such as the threshold roll-off, Drain Induced Barrier Lowing (DIBL), surface scattering, and velocity saturation known as Short Channel Effects (SCEs). To overcome these challenges, a cylindrically structured MOSFET is employed because it increases the switching speed, current flow, packing density, and provides better immunity to SCEs. This thesis proposes a Cylindrical Surrounding Double-Gate (CSDG) MOSFET which is an extended version of Double-Gate (DG) MOSFET and Cylindrical Surrounding-Gate (CSG) MOSFET in terms of form factor and current drive respectively. Furthermore, employing the Evanescent-Mode analysis (EMA) of a two-dimensional (2D) Poisson solution, the performance analysis of the novel CSDG MOSFET is presented. The channel length, radii Silicon film difference, and the oxide thickness are investigated for the CSDG MOSFET at the subthreshold regime. Using the minimum channel potential expression obtained by EMA, the threshold voltage and the subthreshold swing model of the proposed CSDG MOSFET are evaluated and discussed. The device performance is verified with various values of radii Silicon film difference and gate oxide thickness Finally, the low operating power and switching characteristics of the proposed CSDG MOSFET has been employed to design a simple CSDG bridge rectifier circuit for micropower electricity (energy harvester). Similar to the traditional MOSFETs, the switching process of CSDG MOSFET is in two operating modes: switch-ON (conduction of current between the drain and source) or switched-OFF (no conduction of current). However, unlike the traditional diode bridge rectifier which utilizes four diodes for its operation, the CSDG bridge rectifier circuits employs only two CSDGs (n-channel and p- channel) for its operation. This optimizes cost and improves efficiency. Finally, the results from the analyses demonstrate that the proposed CSDG MOSFET is a promising device for nanotechnology and self-micro powered device system application.


Masters Degree. University of KwaZulu-Natal, Durban.