Leakage current characterization and projection in carbon nanotube transistors
Keywords: Engineering.
Electrical Engineering.

Carbon nanotubes (CNTs) are promising candidates as channel materials for extremely-scaled technology nodes due to their naturally 1-nm thin body and high charge carrier mobility. Moreover, the low-temperature fabrication (i.e., > 400 °C) of CNT field effect transistors (CNFETs) enables monolithic three-dimensional (3D) ultra-dense integration of logic and memory, leading to energy and throughput benefits at the application level. However, CNFETs suffer from large off-state leakage due to small effective mass and band gap. While much progress has been made in improving the on-state current of CNFETs, there is a lack of benchmarking their off-state current. The off-state leakage is often under-estimated in simulation models, which ignore additional tunneling contribution in CNFETs. While strategies for suppressing leakage exist, the control of off-state current has yet to be demonstrated. This thesis aims to address the leakage challenge in CNFETs through a comprehensive leakage study encompassing: (1) characterization of leakage current; (2) calibration of simulation models; and (3) projection of low-leakage design space. In this dissertation, I will present a systematic study of the following leakage mechanisms in carbon nanotube MOSFETs: Gate Leakage -- To mitigate the gate leakage, a gate oxide bilayer for CNT is employed consisting of a 0.35 nm interfacial dielectric (k = 7.8) and 2.5 nm high-k dielectric (k = 24). Gate leakage was reduced to below 1 pA/CNT at 10 nm gate length and 0.7 V supply voltage, surpassing the technology target requirements. Superior electrostatic control of 65 mV/dec subthreshold slope and 20 mV/V drain-induced barrier lowering (DIBL) were achieved in top-gated CNT MOSFETs at 15 nm gate length. Band-to-Band Tunneling (BTBT) -- The BTBT leakage in CNT MOSFETs is influenced strongly by the CNT band gap, supply voltage, and extension doping level. However, existing studies typically estimate the CNT band gap indirectly from the CNT diameter, resulting in limited accuracy due to various band gap-diameter approximations. To address this, a novel direct CNT band gap extraction method is developed. The lower limit of off-state current was measured in electrostatically-doped CNT MOSFETs across a range of band gaps, supply voltages, and extension doping levels. A non-equilibrium Green's function (NEGF) model confirms the dependence of BTBT leakage on CNT band gap, supply voltage, and extension doping level. Based on the calibrated NEGF model, a leakage current design space is projected for long-channel CNT MOSFETs, enabling identification of appropriate device design choices across CNT band gap, supply voltage, and extension doping. Source-Drain Tunneling (SDT) -- Short-channel CNT MOSFETs with gate lengths ranging rom 6.5 nm to 14.0 nm were fabricated, as simulations predict significant SDT leakage below 12 nm gate length. Temperature-dependent electrical measurements from 6.5 K to 300 K were used to distinguish between short-channel effects and source-drain tunneling. Three short-channel MOSFETs were examined as examples, demonstrating different temperature dependencies and the extent of SDT