Graphene field-effect transistors and devices for advanced high-frequency applications

New device technologies and materials are continuously investigated, in order to increase the bandwidth of high-speed electronics, thereby extending data rate and range of applications. The 2D-material graphene, with its intrinsically extremely high charge carrier velocity, is considered as a promising new channel material for advanced high frequency field-effect transistors. However, most fabrication processes introduce impurities and defects at the interface between graphene and adjacent materials, which degrade the device performance. In addition, at high drain fields, required for high transistor gain, the close proximity of the adjacent materials limits the saturation velocity, and there is a significant increase in the channel temperature caused by self-heating.In this thesis, the influence of impurities and defects on charge transport, the limitations of the saturation velocity, and the effect of velocity saturation and self-heating on the transit frequency (fT) and the maximum frequency of oscillation (fmax) of graphene field effect transistor (GFETs) are analysed.In addition, GFETs with state-of-the-art extrinsic fT =34 GHz and fmax =37 GHz, and an integrated 200-GHz GFET based receiver are presented. Also, through the development of a fabrication process of GFETs with a buried gate configuration, this work contributed to the direct nanoscopic observation of plasma waves in the GFET channel during terahertz illumination.The study was conducted by (i) setting up a model describing the influence of impurities and defects on capacitance and transfer characteristics at low electric fields, (ii) by developing a method for studying the limiting mechanisms of the charge carrier velocity in the graphene channel at high electric fields and answering the question whether velocity saturation improves fmax, (iii) by developing a method to study the channel temperature and its effect on fT and fmax.It was found that scattering by remote optical phonons limits the saturation velocity and charge carriers emitted from interface states at high fields are preventing the current to saturate and, hence, limiting fT and fmax. Additionally, the study shows that the channel temperature in GFETs can increase significantly causing degradation of the high frequency performance due to the decrease of charge carrier mobility and velocity. In summary, this work shows that it is necessary to develop a GFET design and fabrication process providing clean and defect-free interfaces, to minimise parasitic effects, and to use materials with higher optical phonon energies and higher thermal conductivities than those used today. This will allow for realisation of GFETs with extrinsic fT and fmax in the sub-terahertz range

340 GHz FMCW pulse-Doppler radar to characterize the dynamics of particle clouds

This is a document that will describe a 340 GHz pulse Doppler radar. It will describe the system and its performanc

Effect of oxide traps on channel transport characteristics in graphene field effect transistors

A semiempirical model describing the influence of interface states on characteristics of gate capacitance and drain resistance versus gate voltage of top gated graphene field effect transistors is presented. By fitting our model to measurements of capacitance–voltage characteristics and relating the applied gate voltage to the Fermi level position, the interface state density is found. Knowing the interface state density allows us to fit our model to measured drain resistance–gate voltage characteristics. The extracted values of mobility and residual charge carrier concentration are compared with corresponding results from a commonly accepted model which neglects the effect of interface states. The authors show that mobility and residual charge carrier concentration differ significantly, if interface states are neglected. Furthermore, our approach allows us to investigate in detail how uncertainties in material parameters like the Fermi velocity and contact resistance influence the extracted values of interface state density, mobility, and residual charge carrier concentration

Characterization of Al2O3 gate dielectric for graphene electronics on flexible substrates

In this work, we have fabricated parallel-plate capacitor test structures consisting of 35 nm thick Al2O3 dielectric film and graphene as bottom electrode on polyethylene terephthalate (PET) to characterize the electrical properties of the dielectric film for graphene electronics on flexible substrates.It was found out that leakage current density in the Al2O3 film is less than 0.1 mA/cm2 at 5 V, which allows for applying it as a gate dielectric in graphene-based field effect transistors (GFETs) on flexible substrates. Dielectric constant of the Al2O3 film is approx. 7.6, which is close to the bulk value and confirms good quality of the Al2O3 film. Analysis indicates that the measured loss tangent, which is up to 0.2, is governed mainly by the dielectric loss in the Al2O3 and can be associated with defects in Al2O3 and Al2O3/graphene interface. Our results will be used in further development of GFETs on flexible substrates

Correlation between material quality and high frequency performance of graphene field-effect transistors

Correlations between material quality, equivalent circuit and high frequency parameters of the graphene field-effect transistors, such as mobility, contact resistivity, carrier velocity, drain conductivity, transit frequency and maximum frequency of oscillation, have been established via applying drain resistance, velocity and saturation velocity models. The correlations allow for understanding dominant limitations of the high frequency performance of transistors, which clarifies the ways of their further development. In particular, the relatively high drain conductivity is currently main limiting factor, which, however, can be counterbalanced by increasing the carrier velocity via operating transistors at higher fields, in the velocity saturation mode

Enhanced high-frequency performance of top-gated graphene FETs due to substrate-induced improvements in charge carrier saturation velocity

High-frequency performance of top-gated graphene field-effect transistors (GFETs) depends to a large extent on the saturation velocity of the charge car-riers, a velocity limited by inelastic scattering by surface optical phonons from the dielectrics surrounding the chan-nel. In this work, we show that by simply changing the graphene channel surrounding dielectric with a material having higher optical phonon energy, one could improve the transit frequency and maximum frequency of oscillation of GFETs. We fabricated GFETs on conventional SiO2/Si substrates by adding a thin Al2O3 interfacial buffer layer on top of SiO2/Si substrates, a material with about 30% higher optical phonon energy than that of SiO2, and compared performance with that of GFETs fabricated without adding the interfacial layer. From S-parameter measurements, a transit frequency and a maximum frequency of oscillation of 43 GHz and 46 GHz, respectively, were obtained for GFETs on Al2O3 with 0.5 \ub5m gate length. These values are approximately 30% higher than those for state-of-the-art GFETs of the same gate length on SiO2. For relating the improvement of GFET high-frequency performance to improvements in the charge carrier saturation velocity, we used standard methods to extract the charge carrier veloc-ity from the channel transit time. A comparison between two sets of GFETs with and without the interfacial Al2O3 layer showed that the charge carrier saturation velocity had increased to 2\ub710^7 cm/s from 1.5\ub710^7 cm/s

Charge carrier velocity in graphene field-effect transistors

To extend the frequency range of transistors into the terahertz domain, new transistor technologies, materials, and device concepts must be continuously developed. The quality of the interface between the involved materials is a highly critical factor. The presence of impurities can degrade device performance and reliability. In this paper, we present a method that allows the study of the charge carrier velocity in a field-effect transistor vs impurity levels. The charge carrier velocity is found using high-frequency scattering parameter measurements followed by delay time analysis. The limiting factors of the saturation velocity and the effect of impurities are then analysed by applying analytical models of the field-dependent and phonon-limited carrier velocity. As an example, this method is applied to a top-gated graphene field-effect transistor (GFET). We find that the extracted saturation velocity is ca. 1.4 710^7 cm/s and is mainly limited by silicon oxide substrate phonons. Within the considered range of residual charge carrier concentrations, charged impurities do not limit the saturation velocity directly by the phonon mechanism. Instead, the impurities act as traps that emit charge carriers at high fields, preventing the current from saturation and thus limiting power gain of the GFETs. The method described in this work helps to better understand the influence of impurities and clarifies methods of further transistor development. High quality interfaces are required to achieve current saturation via velocity saturation in GFETs

Test structures for evaluating Al2O3 dielectrics for graphene field effect transistors on flexible substrates

We have developed a test structure for evalua\uadting the quality of Al2O3 gate dielectrics grown on graphene for graphene field effect transistors on flexible substrates. The test structure consists of a metal/dielectric/ graphene stack on a PET substrate and requires only one lithography step for the patterning of the topside metal electrodes. Results from measurements of leakage current, capacitance and loss tangent are presented

Sub-millimetre wave range-Doppler radar as a diagnostic tool for gas-solids systems -- solids concentration measurements

Current non-intrusive measurement techniques for characterising the solids flow in gas-solids suspensions are limited by the low temporal or low spatial resolution of the sample volume, or in the case of optical methods, by a short range of sight. In this work, a sub-millimetre wave range-Doppler radar is developed and validated for non-intrusive sensing of solids concentrations in a gas-solids particle system with known characteristics. The radar system combines favourable features, such as the ability to see through at optical frequencies opaque materials, to measure the local solids velocity and the reflected radar power with a spatial resolution of a few cubic centimetres over distances of a few metres. This paper introduces a method to relate the received radar signal power to the solids volumetric concentrations (cv) of different particulate materials. The experimental set-up provides a steady stream of free-falling solids that consist of glass spheres, bronze spheres or natural sand grains with known particle size distributions and with particle diameters in the range of 50-300 $\mu$m. Thus, the values of cv found using the radar measurements are validated using the values of cv retrieved from closure of the mass balance derived from the measured mass flow rate of the solids stream and the solids velocity. The results show that the radar system provides reliable measurements of cv, with a mean relative error of approximately 25% for all the tested materials, particle sizes and mass flow rates, yielding values of cv ranging from 0.2x10$^{-4}$ m$^{3}$/ m$^{3}$ up to 40x10$^{-4}$ m$^{3}$/ m$^{3}$ and solids velocities within the range of 0-4.5 m/s. This demonstrates the ability of the radar technology to diagnose the solids flow in gas-solids suspensions using a unique combination of penetration length, accuracy, and spatial and velocity resolution.Comment: submitted to Advanced Powder Technology Elsevie