Radio-Frequency Linearity of Carbon-Based Nanoscale Field-Effect Transistors

  • Author / Creator
    Alam, Ahsan U.
  • This Ph.D. thesis focuses on the radio-frequency (RF) linearity of carbon-nanotube field-effect transistors (CNFETs) and graphene field-effect transistors (GFETs). The thesis can be categorized into three stages. In the first stage, the RF linearity potential of CNFETs has been investigated by considering an array-based device structure under the first approximation of ballistic transport. A nonlinear equivalent circuit for ballistic field-effect transistors is used to compare the linearity of CNFETs to conventional MOSFETs. It is shown that nanotube devices working at high frequencies are not inherently linear, as recently suggested in the literature, and that CNFETs exhibit overall linearity that is comparable to their MOSFET counterparts. The nonlinear quantum capacitance is identified to be a major source of high-frequency nonlinearity in CNFETs. The impact of device parameters such as oxide capacitance, channel width, and tube pitch are also investigated. In the second stage, a modified top-of-the-barrier model (MTBM) capable of simulating ballistic transport in GFETs is developed. The model captures band-to-band (Klein-Zener) tunneling, which is important in zero-bandgap materials, and it accounts for variations in the densities of states between the channel and the source and drain regions. The model is benchmarked against a sophisticated solver (based on the non-equilibrium Green’s function approach) and is shown to have very good quantitative agreement. The utility of the modified TBM is demonstrated by investigating and comparing the RF linearity of GFETs to that of CNFETs and conventional MOSFETs. In the third stage, the RF linearity potential of short-channel GFETs is assessed by using the modified top-of-the-barrier approach developed in stage 2, again under the first approximation of ballistic transport. An intrinsic GFET is examined to reveal the key features of GFET linearity, and extrinsic parasitics are then included to assess the overall RF linearity. It is shown that short-channel GFETs can be expected to have a signature behavior versus gate bias that includes a constant-linearity region at low gate bias, sweet spots of high linearity before and after the gate bias for peak unity-current-gain frequency, and poor linearity at the gate bias corresponding to the peak unity-current-gain frequency. It is otherwise found that a GFET offers overall linearity that is comparable to a conventional MOSFET and a CNFET, with the exception that the amount of intermodulation distortion in a GFET is dominated by the drain-injected carriers, a unique outcome of graphene’s lack of a bandgap.

  • Subjects / Keywords
  • Graduation date
    Spring 2015
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.