الفهرس 
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Abstract
New emergent types of nanoscale electronic devices are recently being actively investigated to satisfy the requirements of the International Technology Roadmap for Semiconductors (ITRS). At the nanoscale and in low dimensional structures such as carbon nanotubes, quantization effects and carrier quantum transport phenomena such as tunneling can be dominant, and therefore the application of quantum level device simulators is necessary to correctly predict the behaviour of these nanoscale devices.
In this thesis, we investigate nanoscale carbon nanotube-based TFETs for better performance and design. Conventional MOSFETs are also considered for comparison. The study is carried out using a two-dimensional quantum transport simulator that takes into account quantization effects as well as carrier quantum transport. Carbon Nanotubes (CNTs) are quasi 1-D nanostructures that have the potential to be suitable to replace bulk silicon in nanoelectronics applications. CNTs are characterized by their ballistic conduction, high carrier mobility, tunable bandgap and fast switching speed. These unique features make CNTs promising candidates for scaling down logic gates and for low power applications. On the other hand, TFETs are considered good candidates to replace MOSFETs due to their ability to overcome the theoretical limit of 60 mV/decade subthreshold swing of MOSFETs. Moreover, TFETs can be employed in energy efficient and ultra-low voltage circuits.
In spite of its advantages, the TFET suffers from low ON current and high ambipolar current. However, with suitable device design such deficiencies can be overcome. One possible remedy that is thoroughly investigated in this thesis is to use highly doped source and drain pockets. Effect of fabrication process tolerance resulting in non-abrupt source channel junction in these devices is also studied. Finally, application of TFETs as biosensors is presented.
The thesis is organized in six chapters as follows:
Chapter 1: This chapter gives a brief introduction to the basic properties and structures of nanoscale transistors needed for their design and performance study done in this thesis. This includes the base material which is carbon nanotubes, along with the structure and operation of TFETs and MOSFETs. Also, we review the impact of device parameters on the transfer characteristics of these TFETs and MOSFETs needed for the device optimization.
Chapter 2: This chapter reviews the main structure of the two-dimensional quantum transport simulator used in this study. The simulator is based on the non-equilibrium Green’s function (NEGF) formalism which is commonly used to model quantum transport in nanostructures. Application of the method to carbon nanotubes is also presented.
Chapter 3: In this chapter the implementation of highly doped pockets for enhancing the performance of the TFET is presented and thoroughly investigated. The parameters of the pockets such as the length and the position of the pocket are engineered for optimized design to increase the ON-current and supress the ambipolarity. Also, the frequency response of the device with the introduced pockets is investigated.
Chapter 4: In this chapter we study the effect of the gradual source doping on the performance of the device and the tolerance to the fabrication process. The performance is considered in terms of the drain ON-current, subthreshold swing, and the cut-off frequency. Further, some design parameters are examined to make the devices under study more immune to fabrication deficiencies. In addition, the effect of the gate misalignment on the device performance is investigated.
Chapter 5: In this chapter we investigate the possibility of using a nanogap embedded CNT-based TFETs for label-free bio-sensing applications and their optimization for better sensing. A nanogap is introduced under the gate. Biomolecules can be fed into the cavity through the capillary action. The ratio of the drain current in the presence of biomolecules to that without biomolecules is taken as a figure of merit for sensitivity.
Chapter 6: This chapter gives the conclusion of the thesis and introduces suggestions and recommendations for future work.
Key words: Carbon nanotube FETs, TFETs, Ambipolarity, non-equilibrium Green’s function, quantum transport, gradual source doping, gate misalignment, biosensors.