Sven Mothes

Toward Linearity in Schottky Barrier CNTFETs

Carbon nanotube field-effect transistors are expected to be beneficial for analog high-frequency applications due to, among others, their inherent linearity and, thus, very low signal distortion. Achieving this linearity has so far been assumed to depend on meeting the following conditions: Ballistic single-subband transport, ohmic contacts, and quantum capacitance limited operation. However, these conditions are very difficult to meet in realistic devices and circuit applications. It is shown in this paper that high linearity is also possible under significantly relaxed and, in particular, more practical conditions. This paves the way toward the exploration of linearity in realistic devices suffering from carrier scattering in the channel and with Schottky-like contacts, as well as thicker and lower-κ gate oxides, which do not allow operation in the quantum capacitance limit. This study is based on results obtained with a Boltzmann transport equCarbon nanotube field-effect transistors are expected to be beneficial for analog high-frequency applications due to, among others, their inherent linearity and, thus, very low signal distortion. Achieving this linearity has so far been assumed to depend on meeting the following conditions: Ballistic single-subband transport, ohmic contacts, and quantum capacitance limited operation1. However, these conditions are very difficult to meet in realistic devices and circuit applications. It is shown in this paper that high linearity is also possible under significantly relaxed and, in particular, more practical conditions. This paves the way toward the exploration of linearity in realistic devices suffering from carrier scattering in the channel and with Schottky-like contacts, as well as thicker and lower-κ gate oxides, which do not allow operation in the quantum capacitance limit. This study is based on results obtained with a Boltzmann transport equation solver that includes tunneling through potential barriers.ation solver that includes tunneling through potential barriers.

High-Frequency Ballistic Transport Phenomena in Schottky Barrier CNTFETs

The effective-mass Schrödinger equation is solved directly for the wave functions to explore the steady state and the time-dependent quantum-ballistic transport in Schottky barrier carbon nanotube (CNT) field-effect transistors (FETs). The employed contact parameters allow for discontinuities of the effective mass at the metal-CNT interface, and carefully chosen boundary conditions minimize spurious reflections at the simulation domain boundaries. Two-port Y-parameters of a selected device structure are computed and qualitatively explained with the time dependence of coherent quantum-ballistic charge injection. The finite escape times of the charge carriers in an open quantum system are identified for determining the inertial response to high-frequency terminal signals. Since the escape times depend on the shape of the Schottky barriers, the latter contribute to the dynamic behavior of ballistic CNTFETs.

High frequency noise in manufacturable carbon nanotube transistors

HF noise parameters were measured and modeled for the first time for wafer-scale manufacturable CNTFETs. These first multi-tube multi-finger CNTFETs exhibit still relatively high values for the minimum noise figure (NFmin = 3.5 dB at 1 GHz). Based on detailed compact modeling, the origin of this noise can be explained by the existence of the parasitic network and metallic tubes.

A Semiphysical Large-Signal Compact Carbon Nanotube FET Model for Analog RF Applications

A compact large-signal model, called Compact Carbon Nanotube Model (CCAM), is presented that accurately describes the shape of DC and small-signal characteristics of fabricated carbon nano-tube FETs (CNTFETs). The new model consists of computationally efficient and smooth current and charge formulations. The model allows, for a given gate length, geometry scaling from single-finger single-tube to multifinger multitube transistors. Ambipolar transport, temperature dependence with self-heating, noise, and a simple trap model have also been included. The new model shows excellent agreement with the data from both the Boltzmann transport equation and measurements of Schottky-barrier CNTFETs and has been implemented in Verilog-A, making it widely available across circuit simulators.

Towards a multiscale modeling framework for metal-CNT interfaces

This paper gives a short overview on our recent investigations towards a multiscale modeling and simulation framework for metal-CNT interfaces. We employ three simulation approaches with well defined interfaces. For the simulation at device level we make use of a recently developed wave-function based effective-mass Schrödinger-Poisson solver which employs a hetero-junction like contact model to capture the physics in the contact region where the CNT is embedded into metal. The required model parameters are adjusted to TB and DFT simulation results. A comparison with experimental data for a short channel device shows the applicability of the proposed approach.

Impact of incomplete metal coverage on the electrical properties of metal-CNT contacts: A large-scale ab initio study

Using a dedicated combination of the non-equilibrium Green function formalism and large-scale density functional theory calculations, we investigated how incomplete metal coverage influences two of the most important electrical properties of carbon nanotube (CNT)-based transistors: contact resistance and its scaling with contact length, and maximum current. These quantities have been derived from parameter-free simulations of atomic systems that are as close as possible to experimental geometries. Physical mechanisms that govern these dependences have been identified for various metals, representing different CNT-metal interaction strengths from chemisorption to physisorption. Our results pave the way for an application-oriented design of CNT-metal contacts.

Impact of functionalization patterns on the performance of CNTFETs

Covalent functionalization of carbon nanotubes (CNTs) might be an option to optimize the behavior of CNT field effect transistors (FETs) [1] (despite all related technological problems). In principle, the atoms or molecules used for functionalizing are placed randomly along the CNT or they are high-ordered in decoration patterns. Here, only high-ordered functionalization patterns are studied (i) to convert metallic CNTs into semiconducting CNTs and (ii) to reduce or to increase the ambipolarity of a semiconducting CNT.

About the charge injection limitation in Schottky barrier CNTFETs

The injection of charges into the channel of a carbon nanotube (CNT) field effect transistor (FET), and, hence, the overall device behavior are strongly affected by the interface between the CNT and the metall contacts and the tube portion underneath the metall contacts (contact region). The properties of the interface and the contact region depend on details of the fabrication process and the materials used. Thus, a detailed understanding of the related phenomena facilitates device and process optimization. A major issue in optimizing the interface and the contact region is the limitation of charge injection due to potential barriers in the contact region and clipping. The limitation along with the transport within the channel is studied by means of the Boltzmann-transport equation (BTE) in combination with a phenomenological contact model which also considers tunneling through near-contact potential barriers. The related results are discussed with respect to CNT-based 1D-electronics for high-frequency (HF) analog applications.

Modeling of NQS effects in carbon nanotube transistors

Time-dependent quantum simulations are used to rigorously identify non-quasi-static (NQS) effects in Carbon nanotube transistors. A complete physics-based small signal equivalent circuit is derived which captures important NQS effects for circuit design and simulation. This model agrees well with high-frequency measurements. Additionally, the impact of Schottky barriers on the kinetic inductance and the charging resistances is discussed and the role of the contact resistances is investigated.

Carbon nanotube field-effect transistor performance in the scope of the 2026 ITRS requirements

Top gate, global back gate and buried gate CNTFET structures with a channel length of 5.9 nm are studied in the scope of the 2026 ITRS requirements. The studies are performed using a numerical device simulator. Figures of merit and performance parameters such as the switching speed, the switching energy, Ion/Ioff-ratio, among others, are obtained for each structure and compared with the 2026 ITRS requirements for different application scenarios. Most of the requirements are met with the buried gate CNTFET. The requirement for the Ion/Ioff-ratio is met at the cost of other performance parameters.