SwePub - search: WFRF:(Bonmann Marlene 1988)

Enumeration	Reference	Cover	Find
1.	Bonmann, Marlene, 1988, et al. (author) An Integrated 200-GHz Graphene FET Based Receiver 2018 In: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; 2018-September Conference paper (peer-reviewed)abstract A receiver composed by a graphene FET 200-GHz mixer and a 1-GHz intermediate frequency amplifier integrated on a silicon substrate was modelled, fabricated and characterized. This is the first demonstration of a millimeter wave integrated receiver based on graphene FETs. The receiver conversion loss is measured to be 25 dB across the 185-205-GHz band with 16 dBm of local oscillator pump power, which is in good agreement with the circuit simulations. The simulations show that the receiver conversion loss can be significantly reduced to 16 dB by reducing the contact resistance and by realizing a higher charge carrier mobility in the mixer transistor.
2.	Asad, Muhammad, 1986, et al. (author) Correlation between material quality and high frequency performance of graphene field-effect transistors 2019 Conference paper (other academic/artistic)abstract 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.
3.	Asad, Muhammad, 1986, et al. (author) Graphene field-effect transistors for high frequency applications 2018 In: ; November 2018 Conference paper (peer-reviewed)abstract Realization of competitive high frequency graphene field-effect transistors (GFETs) is hindered, in particular, by extrinsic scattering of charge carriers and relatively high contact resistance of the graphene-metal contacts, which are both defined by the quality of the corresponding graphene top interfaces [1]. In this work, we report on improved performance of GFETs fabricated using high quality chemical vapour deposition (CVD) graphene and modified technology steps. The modified processing flow starts with formation of the gate dielectric, which allows for preserving the high velocity of charge carriers, and, simultaneously, providing very low contact resistance. The transfer line method (TLM) analysis and fitting the GFET transfer characteristics (Fig. 1) both reveal very low specific width contact resistivity of the top contacts, down to 95 Ω⋅μm. Fitting shows also that the field-effect mobility in the GFETs can be up to 5000 cm2/(V⋅s). The measured (extrinsic) transit frequency (fT) and the maximum frequency of oscillation (fmax) are up to 35 GHz and 40 GHz, respectively, for GFETs with gate length Lg=0.5 μm (Fig. 2), which are highest among those reported so far for the GFETs with similar gate length and comparable with those of Si MOSFETs [2,3]. The dependencies of the fT and fmax on the gate length indicate that these GFETs are very promising for the scaling down and in particular for the development of power amplifiers operating in the mm-wave frequency range.
4.	Asad, Muhammad, 1986, et al. (author) The dependence of the high-frequency performance of graphene field-effect transistors on channel transport properties 2020 In: IEEE Journal of the Electron Devices Society. - 2168-6734. ; 8, s. 457-464 Journal article (peer-reviewed)abstract This paper addresses the high-frequency performance limitations of graphene field-effect transistors (GFETs) caused by material imperfections. To understand these limitations, we performed a comprehensive study of the relationship between the quality of graphene and surrounding materials and the high-frequency performance of GFETs fabricated on a silicon chip. We measured the transit frequency (fT) and the maximum frequency of oscillation (fmax) for a set of GFETs across the chip, and as a measure of the material quality, we chose low-field carrier mobility. The low-field mobility varied across the chip from 600 cm2/Vs to 2000 cm2/Vs, while the fT and fmax frequencies varied from 20 GHz to 37 GHz. The relationship between these frequencies and the low-field mobility was observed experimentally and explained using a methodology based on a small-signal equivalent circuit model with parameters extracted from the drain resistance model and the charge-carrier velocity saturation model. Sensitivity analysis clarified the effects of equivalent-circuit parameters on the fT and fmax frequencies. To improve the GFET high-frequency performance, the transconductance was the most critical parameter, which could be improved by increasing the charge-carrier saturation velocity by selecting adjacent dielectric materials with optical phonon energies higher than that of SiO2.
5.	Bonmann, Marlene, 1988, et al. (author) Characterization of Graphene FET based 200 GHz Mixer and 1 GHz Amplifier Integrated on a Si Substrate 2018 Conference paper (other academic/artistic)abstract arises for new materials and technologies which can be used in the millimeter wave and terahertz wave regime. In this context, a receiver is an important component to be developed. It converts the received signals into useful information. A typical heterodyne receiver consists of an antenna, RF and IF filters, RF and IF amplifiers, and a mixer. The amplifiers and the mixer can be based on field effect transistors (FETs). To obtain high speed transistors the charge carrier mobility and velocity in the transistor channel should be high. Therefore, the 2D material graphene is an interesting material since it has a high room temperature charge carrier mobility and a high saturation velocity [1]. In previous works a 10 dB small-signal amplifier designed for 1 GHz [2] and a 185-215 GHz subharmonic resistive mixer [3] (designed for a center frequency at 200 GHz) based on graphene FETs (GFETs) have been demonstrated. The amplifier was further developed in [4] and the lumped inductor for matching used in [2] was replaced by an planar inductor. The measured and modeled gain for the two inductor types are shown in Fig. 1. The gain is reduced from 10 dBm to 5 dBm when using the planar inductor compared to the lumped inductor. The model shows that the gain can be increased to the designed gain of 10 dBm if the inductor resistance is reduced to Rs =5 by increasing the thickness of the gold conductor to 2 m. Additionally, the mixer design in [3] has been improved compared to the mixer design in [5] by decreasing the loss in the coplanar waveguid (CPW) circuit using air bridges. In this work, both, the amplifier and mixer are integrated together on a single silicon substrate and the characterization results are presented.
6.	Bonmann, Marlene, 1988, et al. (author) Charge carrier velocity in graphene field-effect transistors 2017 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 111:23, s. 233505- Journal article (peer-reviewed)abstract 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. 1.4×10^7 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.
7.	Bonmann, Marlene, 1988, et al. (author) Delay analysis for evaluation of carrier velocity in graphene field-effect transistors 2017 In: Graphene Week 2017, Athens, Greece, 25-29 September, 2017. Conference paper (peer-reviewed)abstract One of the main challenges in the development of graphene field-effect transistors (GFETs) forapplications in high frequency electronics is achieving high maximum frequency of oscillation (fmax),which is the power gain parameter. A promising way to achieve higher fmax is drain current saturationvia saturation of the charge carrier velocity at high electric fields [1]. Therefore, accurate evaluation ofthe charge carrier velocity in GFETs, and its field dependence, are of importance. In this work, a methodis presented that allows for the evaluation and analysis of the carrier velocity in GFETs via delay timeanalysis using measured cut-off frequencies. The measured cut-off frequency is inversely proportionalto the total delay time, which, in GFETs on Si substrates, can be expressed as the sum of intrinsic andextrinsic delay times [2, 3, 4]. The intrinsic delay is defined by the transit time, i.e. the time taken by thecharge carriers to travel across the channel, which is related to the carrier velocity. The extrinsic delaysare charging delays, i.e. RC time constants required to charge and discharge the parasitic parts of theGFETs, associated with contact resistance and gate pad capacitance. In order to evaluate the extrinsicdelays the contact resistance and gate pad capacitance are found. The contact resistance is found byapplying a drain resistance fitting model on the measured GFET transfer characteristics. The gate padcapacitance is calculated using the corresponding delay time, which is found as difference between thetotal delay and the delay in the GFETs with virtual infinite gate width W (i.e. at 1/W=0), as shown inFig. 1 [4]. The intrinsic delay time is found by subtracting the extrinsic delay from the total delay and,subsequently, used to calculate the charge carrier velocity (Fig. 2). The advantage of this method, incomparison with the previously used methods based on analysis of the GFET current-voltagecharacteristics, is that the carrier velocity is calculated directly, using measured cut-off frequency,independently from the carrier concentration, and, thereby, avoiding uncertainties associated with thecarrier generation from traps at high fields. This allows for the accurate evaluation of the charge carriervelocity and its field dependence.
8.	Bonmann, Marlene, 1988, et al. (author) Drain current saturation in graphene field-effect transistors at high fields 2018 Conference paper (other academic/artistic)abstract Development of competitive high frequency graphene field-effect transistors (GFETs) is hindered, first of all, by a zero-bandgap phenomenon in monolayer graphene, which prevents the drain current saturation and limits significantly the GFET power gain. An approach has been proposed to realise the drain current saturation in GFETs without a bandgap formation, but via velocity saturation of the charge carriers at high fields [1]. In this work, we report on the performance of GFETs fabricated using high quality CVD monolayer graphene and modified technology, which reduce the concentration of traps generating the charge carriers at high fields [2]. Fig. 1 shows typical output characteristics of GFETs with gate length of 0.5 μm. The drain current clearly reveals the saturation trends at high fields, which we associate with the saturation of the carrier velocity, see inset to Fig. 2 [2]. Fig. 2 shows typical measured (extrinsic) transit frequency (fT) and the maximum frequency of oscillation (fmax), which are characteristics of the current and power gain, respectively. Since fT and fmax are proportional to the carrier velocity, they reveal similar saturation behaviour. We analyse the saturation effects by applying the Fermi-Dirac carrier statistics. The fT and fmax are up to 34 GHz and 37 GHz, respectively, which are highest among those reported so far for the GFETs with similar gate length and comparable with those reported for Si MOSFETs [3].
9.	Bonmann, Marlene, 1988, et al. (author) Effects of self-heating on fT and fmax performance of graphene field-effect transistors 2020 In: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 67:3, s. 1277-1284 Journal article (peer-reviewed)abstract It has been shown that there can be a significant temperature increase in graphene field-effect transistors (GFETs) operating under high drain bias, which is required for power gain. However, the possible effects of self-heating on the high-frequency performance of GFETs have been weakly addressed so far. In this article, we report on an experimental and theoretical study of the effects of self-heating on dc and high-frequency performance of GFETs by introducing a method that allows accurate evaluation of the effective channel temperature of GFETs with a submicrometer gate length. In the method, theoretical expressions for the transit frequency (fT) and the maximum frequency of oscillation (fmax) based on the small-signal equivalent circuit parameters are used in combination with the models of the field- and temperature-dependent charge carrier concentration, velocity, and saturation velocity of GFETs. The thermal resistances found by our method are in good agreement with those obtained by the solution of the Laplace equation and by the method of thermo-sensitive electrical parameters. Our experiments and modeling indicate that the self-heating can significantly degrade the fT and fmax of GFETs at power densities above 1 mW/μm², from approximately 25 to 20 GHz. This article provides valuable insights for further development of GFETs, taking into account the self-heating effects on the high-frequency performance.
10.	Bonmann, Marlene, 1988, et al. (author) Effects of self-heating on high-frequency performance of graphene field-effect transistors 2019 Conference paper (other academic/artistic)abstract In this work, we study the effects of self-heating (Joule heating) on the performance of graphene field-effect transistors (GFETs) with high extrinsic transit frequency (ft) and maximum frequency of oscillation (fmax) [1]. It has been shown, that self-heating in the GFETs might be significant and lead to degradation of the output characteristics with potential effects on the ft and fmax [2,3,4]. Due to relatively short gate length of 0.5 μm in the GFETs, used in this work, the local channel temperature cannot be accurately estimated by means of the infrared microscopy. Therefore, we applied the method of thermosensitive electrical parameters [5]. In particular, we analysed the gate and drain currents in response to variations of the external heater temperature and dc power (Fig. 1). The analysis allows for estimation of the thermal resistance, which is, for GFETs on SiO2/Si substrates, approx. 2e4 K/W, and in good agreement with that calculated by the model based on the solution of Laplace’s equation [6]. In turn, the known thermal resistance allows for evaluation of the GFET channel self-heating temperature. Fig. 2 shows the fmax versus dc power (Pdiss) at different external heater temperatures. The self-heating temperature at Pdiss =10 mW is approx. 130 °C. The drop in the fmax at higher Pdiss can be fully explained by self-heating. Apparently, one can expect reduced self-heating effects in the GFETs on higher thermal conductive substrates as hBN or SiC.
11.	Bonmann, Marlene, 1988, et al. (author) Graphene field-effect transistors with high extrinsic fT and fmax 2019 In: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 40:1, s. 131-134 Journal article (peer-reviewed)abstract In this work, we report on the performance of graphene field-effect transistors (GFETs) in which the extrinsic transit frequency (fT) and maximum frequency of oscillation (fmax) showed improved scaling behavior with respect to the gate length (Lg). This improvement was achieved by the use of high-quality graphene in combination with successful optimization of the GFET technology, where extreme low source/drain contact resistances were obtained together with reduced parasitic pad capacitances. GFETs with gate lengths ranging from 0.5 μm to 2 μm have been characterized, and extrinsic fT and fmax frequencies of up to 34 GHz and 37 GHz, respectively, were obtained for GFETs with the shortest gate lengths. Simulations based on a small-signal equivalent circuit model are in good agreement with the measured data. Extrapolation predicts extrinsic fT and fmax values of approximately 100 GHz at Lg=50 nm. Further optimization of the GFET technology enables fmax values above 100 GHz, which is suitable for many millimeter wave applications.
12.	Bonmann, Marlene, 1988, et al. (author) Studies of hysteresis in capacitance and current characteristics of flexible graphene field-effect transistors 2017 In: Graphene Week 2017, Athens, Greece, 25-29 September, 2017. Conference paper (peer-reviewed)abstract Owing to the unique combination of mechanical and electrical properties of graphene, e.i., flexibility andhigh carrier velocity, it is a promising material for emerging applications in flexible high frequencyelectronics. One of the challenges in the development of reliable high performance devices is associatedwith impurities, which are normally present at the graphene/dielectric interfaces. Impurities reduce thecarrier mobility via scattering (Ref. 1) and introduce interface states. Interface states can trap and detrapcharge carriers which typically leads to hysteresis. Fig.1 and Fig.2 show hysteresis in the gatecapacitance and drain current versus gate voltage dependences measured in this work in the graphenefield-effect transistors (GFETs) on flexible PET substrates. It is important to clarify the nature and thedistribution of traps to be able to improve the GFET design, materials and fabrication process in thedevelopment of hysteresis-free flexible GFETs. In this work, we continue developing the model (Ref. 2),which describes the influence of interface states on gate capacitance-voltage and drain resistancevoltagecharacteristics and allows for reasonable good fitting of the forward sweep (Fig.1 and Fig.2,solid lines). Here, we include also the backward sweep, which, as it can be seen, requires moreadvanced modelling, taking into account trapping/ de-trapping dynamics and the analysis of interfacestate distribution. This work helps to clarify the origin of hysteresis in greater depth and allows forcombination with other models, e.g., include hysteresis effects in the model of the responsivity of flexibleGFET THz power detectors [3].
13.	Feijoo, Pedro C., et al. (author) Does carrier velocity saturation help to enhance fmax in graphene field-effect transistors? 2020 In: Nanoscale Advances. - : Royal Society of Chemistry (RSC). - 2516-0230. ; 2:9, s. 4179-4186 Journal article (peer-reviewed)abstract It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get optimal maximum oscillation frequency (f(max)). This paper investigates whether velocity saturation can help to get better current saturation and if that correlates with enhancedf(max). We have fabricated 500 nm GFETs with high extrinsicf(max)(37 GHz), and later simulated with a drift-diffusion model augmented with the relevant factors that influence carrier velocity, namely: short-channel electrostatics, saturation velocity effect, graphene/dielectric interface traps, and self-heating effects. Crucially, the model provides microscopic details of channel parameters such as carrier concentration, drift and saturation velocities, allowing us to correlate the observed macroscopic behavior with the local magnitudes. When biasing the GFET so all carriers in the channel are of the same sign resulting in highly concentrated unipolar channel, we find that the larger the drain bias is, both closer the carrier velocity to its saturation value and the higher thef(max)are. However, the highestf(max)can be achieved at biases where there exists a depletion of carriers near source or drain. In such a situation, the highestf(max)is not found in the velocity saturation regime, but where carrier velocity is far below its saturated value and the contribution of the diffusion mechanism to the current is comparable to the drift mechanism. The position and magnitude of the highestf(max)depend on the carrier concentration and total velocity, which are interdependent and are also affected by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the highestf(max)from similar to 60 to similar to 40 GHz.
14.	Li, Junjie, 1995, et al. (author) High frequency noise characterisation of graphene field-effect transistors at different temperatures 2019 Conference paper (other academic/artistic)abstract Graphene is a promising material for high frequency electronics applications thanks to its intrinsically high carrier mobility and velocity, and graphene transistors are continuously pushed toward higher operating frequencies [1]. For high frequency low noise amplifiers, it is important to evaluate the noise parameters of the graphene field-effect transistors (GFETs). In this work, we present the noise performance of the GFETs made of chemical vapour deposition (CVD) in the frequency and temperature ranges of 2-18 GHz and -60-25 C. The noise figure with 50 Ohm impedance termination (F50) was measured using the cold-source method and then the minimum noise figure (Fmin) was estimated using the Pospieszalski’s noise model [2, 3]. In Fig. 1 and Fig. 2, the Fmin of a GFET with a gate length of 0.5 μm as a function of the frequency (f) and drain voltage (Vd) at different temperatures are shown. This GFET revealed maximum frequency of oscillation (fmax) of 18 and 21 GHz at 25 and -60 °C, respectively. It can be seen from Fig. 1, that the Fmin at 8 GHz is approx. 2 dB lower than that of the previously published CVD GFETs and comparable with that of the best published SiC GFETs [4, 5]. The Fmin decreases significantly with temperature down to 0.3 dB at 8 GHz, competing with Si CMOS [6]. It can be seen from Fig. 2, that Fmin decreases with the Vd and saturates above approx. 1 V, where GFETs operate in the velocity saturation mode [1]. Analysis of the dependences allows for further development of the GFETs for advanced low noise amplifiers.
15.	Nylander, Andreas, 1988, et al. (author) RF properties of carbon nanotube / Copper composite through silicon via based CPW structure for 3D integrated circuits 2019 In: 2019 IEEE 14th Nanotechnology Materials and Devices Conference, NMDC 2019. Conference paper (peer-reviewed)abstract The development of integrated circuits (ICs) has seen exponential growth in performance over the last couple of decades and has pushed the boundaries for how we use our electronics in our daily lives. The scaling of ICs, and therefore also the performance development, is now starting to slow down when the physical designs are reaching critical dimensions where quantum effects starts to become noticeable. One proposed route to circumvent these issues for a continued scaling is based on the implementation of 3D integration by chip stacking for an increased miniaturization potential. Miniaturisation will soon also result in interconnect dimensions that surpass the mean free path (MFP) in Cu, the commonly used material for interconnects today, with a sharp increase in resistivity as a result. By changing the through silicon via (TSV) interconnect material from Cu to a carbon nanotube (CNT)/Cu composite, continued scaling can be ensured both in terms of electrical conductivity, ampacity and signal delays. Furthermore, a reduced skin effect can be achieved ensuring lower signal losses at higher RF frequencies making the CNT/Cu composite an ideal candidate to replace tranditional Cu interconnects. In this paper, we are demonstrating a coplanar waveguide (CPW) test structure using CNT/Cu filled TSVs connected to Au transmission lines on SiO2-passivated high resistivity Si substrates. The parasitic losses of the CNT/Cu TSV based CPW test structure were measured using a Sparameters test setup. The results showed that the CNT/Cu TSVs with affiliated contacts increased the signal losses up to S21 = -5.5 dB compared to Au reference transmission lines. These results are in line with previous results using CNT based TSVs and will serve as a basis for future improvements of CNT based interconnect technology for 3D integration.
16.	Vorobiev, Andrei, 1963, et al. (author) Graphene Field-Effect Transistors for Millimeter Wave Amplifiers 2019 In: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; 2019-September Conference paper (peer-reviewed)abstract In this work, we analyze high frequency performance of graphene field-effect transistors (GFETs), applying models of drain resistance, carrier velocity and saturation velocity. This allows us to identify main limitations and propose an approach most promising for further development of the GFETs suitable for advanced mm-wave amplifiers. Analysis indicates, that the saturation velocity of charge carriers in the GFETs can be increased up to 5e7 cm/s via encapsulating graphene by hexagonal boron nitride layers, with corresponding increase of extrinsic maximum frequency of oscillation up to 180 GHz at 200 nm gate length.
17.	Yang, Xinxin, 1988, et al. (author) Characterization of Al2O3 gate dielectric for graphene electronics on flexible substrates 2016 In: 2016 Global Symposium on Millimeter Waves (GSMM) & ESA Workshop on Millimetre-Wave Technology and Applications. - 9781509013487 ; , s. 153-156 Conference paper (peer-reviewed)abstract 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.
18.	Yang, Xinxin, 1988, et al. (author) Test structures for evaluating Al2O3 dielectrics for graphene field effect transistors on flexible substrates 2018 In: IEEE International Conference on Microelectronic Test Structures. ; 31, s. 75-78 Conference paper (peer-reviewed)abstract We have developed a test structure for evaluating 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.
19.	Asad, Muhammad, 1986, et al. (author) Enhanced high-frequency performance of top-gated graphene FETs due to substrate-induced improvements in charge carrier saturation velocity 2021 In: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 68:2, s. 899-902 Journal article (peer-reviewed)abstract 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 µm 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 2·10^7 cm/s from 1.5·10^7 cm/s.
20.	Bonmann, Marlene, 1988, et al. (author) Effect of oxide traps on channel transport characteristics in graphene field effect transistors 2017 In: Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures. - : American Vacuum Society. - 2166-2754 .- 2166-2746. ; 35:1, s. 01A115- Journal article (peer-reviewed)abstract A semiempirical model describing the influence of interface states on characteristics of gatecapacitance and drain resistance versus gate voltage of top gated graphene field effect transistors ispresented. By fitting our model to measurements of capacitance–voltage characteristics and relatingthe applied gate voltage to the Fermi level position, the interface state density is found. Knowing theinterface state density allows us to fit our model to measured drain resistance–gate voltagecharacteristics. The extracted values of mobility and residual charge carrier concentration arecompared with corresponding results from a commonly accepted model which neglects the effect ofinterface states. The authors show that mobility and residual charge carrier concentration differsignificantly, if interface states are neglected. Furthermore, our approach allows us to investigate indetail how uncertainties in material parameters like the Fermi velocity and contact resistanceinfluence the extracted values of interface state density, mobility, and residual charge carrierconcentration.
21.	Bonmann, Marlene, 1988 (author) Effects of impurities on charge transport in graphene field-effect transistors 2017 Licentiate thesis (other academic/artistic)abstract In order to push the upper frequency limit of high speed electronics further, thereby extending the range of applications, new device technologies and materials are continuously investigated. The 2D material graphene, with its intrinsically extremely high room temperature charge carrier velocity, is regarded as a promising candidate to push the frequency limit even further. However, so far most fabrication processes unintentionally introduce impurities at the interface between graphene and adjacent materials, which affect the performance. Additionally, due to the lack of a band gap, the important power gain parameter, the maximum frequency of oscillation ($f_\text{max}$), is not impressively high. In this thesis, results of the studies of the effect of impurities on charge transport in a graphene field effect transistor (GFETs) are presented. This study was performed was done by, firstly, setting up a semi-empirical model describing the influence of impurities, i.e., interface states on capacitance and transfer characteristics at low electric fields and, secondly, by developing a method for studying the limiting mechanisms of the charge carrier velocity in the graphene channel at high electric fields. It was found that uncertainties in the material parameters of graphene, such as the Fermi velocity, hamper the possibility to find the correct mobility value by direct measurements on a GFET. Furthermore, it was shown that remote optical phonons limit the saturation velocity and charge carriers emitted from interface states at high fields are preventing the current to saturate and, hence, restricting $f_\text{max}$. By studying the effects and the limitations set by impurities and other parasitic effects in the GFET it is possible to clarify strategies for further development of GFETs towards reliable performance and higher $f_\text{max}$. As is shown in this work, it is necessary to develop a fabrication process which results in clean interfaces and adjacent materials with higher optical phonon energies than today.
22.	Bonmann, Marlene, 1988 (author) Graphene field-effect transistors and devices for advanced high-frequency applications 2019 Doctoral thesis (other academic/artistic)abstract 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.
23.	Bonmann, Marlene, 1988, et al. (author) Sub-millimetre wave range-Doppler radar as a diagnostic tool for gas-solids systems - solids concentration measurements 2023 In: Advanced Powder Technology. - : Elsevier BV. - 0921-8831 .- 1568-5527. ; 34:1 Journal article (peer-reviewed)abstract 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. In addition, the radar hardware offers flexibility in terms of installation. After signal processing, the output of the radar is range-velocity images of the solids flowing along the radar’s line-of-sight. The image frame rate can be close to real-time, allowing the solids flow dynamics to be observed. While the well-established Doppler principle is used to measure the solids velocity, 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 µm. 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 0.2 × 10-4 m3/m3 up to 40 × 10-4 m3/m3 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 time resolution. In future work, the radar technique will be applied to study non-controlled solids flow at a larger scale, and to understand flow conditions relevant to industrial reactor applications, e.g., fluidised bed, entrained flow, and cyclone units.
24.	Bonmann, Marlene, 1988, et al. (author) Terahertz radar observes powder dynamics for pharmaceutical manufacturing 2024 In: IEEE Sensors Journal. - 1558-1748 .- 1530-437X. ; 24:13, s. 20512-20522 Journal article (peer-reviewed)abstract The optical opaqueness of powders has precluded the observation of powder flow dynamics in processing tubes, with important implications, for example, in the pharmaceutical industry, where non-destructive monitoring during the manufacturing process is essential to ensure the quality of the final product and the effectiveness of the process. Taking advantage of the high penetration of terahertz electromagnetic waves in powders and its wavelength-to-particle size ratio, we demonstrate that a submillimeter-wave pulse-Doppler radar can overcome the present challenges and characterize powder flow dynamics in pharmaceutical manufacturing processes. Mimicking typical vessel shapes in pharmaceutical operations, we were able to characterize falling powder streams in a tube with a sample volume resolution of a few cubic centimeters and a range resolution of about 5 mm. We successfully monitored particle velocity, particle distribution within the tube, and mass flow rate in real-time. This remote sensing method, based on advanced terahertz electronics, opens up the possibility to study and monitor powder dynamics in a wide range of applications.
25.	Bryllert, Tomas, 1974, et al. (author) 340 GHz FMCW pulse-Doppler radar to characterize the dynamics of particle clouds 2022 Journal article (other academic/artistic)abstract This is a document that will describe a 340 GHz pulse Doppler radar. It will describe the system and its performance
26.	Bryllert, Tomas, 1974, et al. (author) A Submillimeter-Wave FMCW Pulse-Doppler Radar to Characterize the Dynamics of Particle Clouds 2023 In: IEEE Transactions on Terahertz Science and Technology. - 2156-342X .- 2156-3446. ; 13:4, s. 389-395 Journal article (peer-reviewed)abstract This work presents a 340-GHz frequency-modulated continuous-wave (FMCW) pulse-Doppler radar. The radar system is based on a transceiver module with about one milli-Watt output power and more than 30-GHz bandwidth. The front-end optics consists of an off-axis parabola fed by a horn antenna from the transceiver unit, resulting in a collimated radar beam. The digital radar waveform generation allows for coherent and arbitrary FMCW pulse waveforms. The performance in terms of sensitivity and resolution (range/cross-range/velocity) is demonstrated, and the system's ability to detect and map single particles (0.1–10 mm diameter), as well as clouds of particles, at a 5-m distance, is presented. A range resolution of ca 1 cm and a cross-range resolution of a few centimeters (3-dB beam-width) allow for the characterization of the dynamics of particle clouds with a measurement voxel size of a few cubic centimeters. The monitoring of particle dynamics is of interest in several industrial applications, such as in the manufacturing of pharmaceuticals and the control/analysis of fluidized bed combustion reactors.
27.	Generalov, Andrey, 1987, et al. (author) Distinction of the thermoelectric effect in graphene FET THz detectors 2020 In: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; 2020-November, s. 504-505 Conference paper (peer-reviewed)abstract This work presents an approach to distinguish the thermoelectric detection mechanism from the resistive mixing or plasma wave rectification in graphene FET THz detectors. Numerical full-wave simulations validate the asymmetric feeding of the existing antenna design and allow for comparison with a reference design of thermoelectric detectors. The experimental results verify quantitively the thermoelectric contribution to the overall rectification, which allows for more accurate modelling of the GFET THz detectors.
28.	Guio Perez, Diana Carolina, 1985, et al. (author) Radar-based measurements of the solids flow in a circulating fluidized bed 2023 In: Fuel. - 0016-2361. ; 345 Journal article (peer-reviewed)abstract The aim of this work is to demonstrate the value of radar technology for studying experimentally the solids flows in gas-solids fluidized beds. The work presents original results regarding the solids concentration and velocity acquired in a non-intrusive manner from a cold flow model. The tailored radar setup operates at submillimeter wave frequencies (0.34 THz) and can measure the location of solids with a spatial resolution of 1/8 mm−1 in the direction of the radar beam, and of 40–60 mm across the radar beam. The solids velocity in the direction of the beam propagation is determined through measurement of the Doppler shift caused by the reflection of the transmitted radar signal by solids moving in relation to the antenna. The measurements were performed in both the horizontal and vertical directions in the riser of a circulating fluidized bed (cross-sectional area of 0.45 m2 and height of 3.1 m) operated with glass beads (mean particle size of 106 µm, and particle density of 2,486 kg/m3) and using air at ambient temperature as the fluidization agent, with superficial velocities in the range of 0.3–1.3 m/s. The measurements are used to assess the validity of the technique and are not intended to characterize the unit fluid dynamically. The solids concentrations derived from the radar measurements follow the qualitative trends derived from pressure-drop measurements, resembling the expected changes that occur in the concentration profiles as the fluidization velocity increases. Concentrations in the range from 10-6 m3/m3 to 10-1 m3/m3 are measurable. In quantitative terms, for low concentrations of solids (<5·10-3 m3/m3, approximately) the radar measurements exhibited the ability to provide more consistent measurements of the solids concentration than those obtained from pressure transducers, for which the small pressure differences lead to unstable and even negative values for solids concentrations. The two measurement methods were in quantitative agreement for solids volume fractions higher than the threshold. Concentrations ≥ 1·10-1 m3/m3, though measurable, strongly attenuate the radar signal, thereby reducing the beam penetration to a depth of centimeters. For each position along the radar beam, the distribution of solids velocity measured from the Doppler effect was found to be within the expected ranges and allowed observations of solids back-mixing. The radar technique applied in this work is a promising technique for detailed characterization of the solids flow in fluidized beds, offering high spatial and temporal resolutions, allowing the determination of both solids velocity and concentration, and having a reasonably high penetration depth.
29.	Moradikouchi, Anis, 1990, et al. (author) Terahertz radar sensing for real-time monitoring of powder streams 2023 In: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. Conference paper (peer-reviewed)abstract In pharmaceutical manufacturing processes, the flow properties of powder streams moving in the manufacturing pipes directly impact the properties of the final drug product, and there is a need for real-time non-invasive monitoring of the powder flow properties with process analytical tools. In this study, we propose a frequency-modulated continuous wave (FMCW) Doppler radar system with a center frequency of 340 GHz to measure the flow properties of falling powder streams in a vertical transparent tube. We successfully measured the velocity profile and powder flow density variation along the height of the tube with a spatial resolution of about 5 mm. In conclusion, the terahertz FMCW Doppler radar system was shown to be highly promising for real-time sensing of flow properties of powder streams in the pharmaceutical manufacturing processes.
30.	Pälli, Samu Ville, et al. (author) Imaging experiments with a 340-GHz FMCW radar and frequency-diverse holograms 2023 In: Proceedings of SPIE - The International Society for Optical Engineering. - 0277-786X .- 1996-756X. ; 12535 Conference paper (peer-reviewed)abstract We present recent developments of a standoff imaging system based on a frequency-diverse phase hologram and deep neural networks. The single-pixel imaging system operates in a monostatic configuration consisting of a 340-GHz FMCW radar and a frequency-diverse phase hologram to interrogate the radar down range direction with spatially varying, frequency-dependent field patterns. The measured back-reflected signal contains spatial reflectivity information from the target, and the fast chirp rate of the radar enables real-time imaging performance. Together with simultaneously acquired visible-light images, a deep neural network integrated into the submillimeter-wave data readout electronics can map the received signal onto a 2D image without mechanical or active electrical beam scanning. In experiments, we have collected submillimeter-wave and visible-light data of a moving target in the region of interest with a 60-Hz frame rate. The results suggest that the system can image the moving target with a resolution comparable to the theoretical diffraction limit. The minimal hardware complexity and good imaging performance of the demonstrated computational submillimeter-wave imaging system support its potential as a cost-effective and easily deployable solution for various imaging applications.
31.	Soltani, Amin, et al. (author) Direct nanoscopic observation of plasma waves in the channel of a graphene field-effect transistor 2020 In: Light: Science and Applications. - : Springer Science and Business Media LLC. - 2047-7538 .- 2095-5545. ; 9:1 Journal article (other academic/artistic)abstract Plasma waves play an important role in many solid-state phenomena and devices. They also become significant in electronic device structures as the operation frequencies of these devices increase. A prominent example is field-effect transistors (FETs), that witness increased attention for application as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies, where they exhibit very good sensitivity even high above the cut-off frequency defined by the carrier transit time. Transport theory predicts that the coupling of radiation at THz frequencies into the channel of an antenna-coupled FET leads to the development of a gated plasma wave, collectively involving the charge carriers of both the two-dimensional electron gas and the gate electrode. In this paper, we present the first direct visualization of these waves. Employing graphene FETs containing a buried gate electrode, we utilize near-field THz nanoscopy at room temperature to directly probe the envelope function of the electric field amplitude on the exposed graphene sheet and the neighboring antenna regions. Mapping of the field distribution documents that wave injection is unidirectional from the source side since the oscillating electrical potentials on the gate and drain are equalized by capacitive shunting. The plasma waves, excited at 2 THz, are overdamped, and their decay time lies in the range of 25–70 fs. Despite this short decay time, the decay length is rather long, i.e., 0.3-0.5 μm, because of the rather large propagation speed of the plasma waves, which is found to lie in the range of 3.5–7 × 10^6 m/s, in good agreement with theory. The propagation speed depends only weakly on the gate voltage swing and is consistent with the theoretically predicted 1414 power law.
32.	Soltani, Amin, et al. (author) Unveiling the plasma wave in the channel of graphene field-effect transistor 2019 In: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; 2019-September Conference paper (peer-reviewed)abstract Coupling an electromagnetic wave at GHz to THz frequencies into the channel of a graphene field-effect transistor (GFET) provokes collective charge carrier oscillations of the two-dimensional electron gas (2DEG) known as plasma waves. Here, we report the very first experimental and direct mapping of the electric field distribution in a gated GFET at nanometer length scales using scattering-type scanning near-field microscopy (s-SNOM) at 2 THz. Based on the experimental results we deduce the plasma wave velocity for different gate bias voltages, which is in good agreement with the theoretical prediction.
33.	Wu, Wanqiang, 1992, et al. (author) Radar-based measurement of solids back-mixing in the freeboard of a circulating fluidized bed 2024 In: Chemical Engineering Journal. - 1385-8947. ; 488 Journal article (peer-reviewed)abstract This work investigates solids back-mixing in the freeboard of a circulating fluidized bed based on THz-radar measurements of the concentrations and velocity distributions of the solid particles along the height of the riser. These data allow height-resolved closure of the solids mass balance and, thereby, quantification and further insight of the two main mechanisms for the back-mixing of the solids entrained from the bottom region of the fluidized bed: (i) solids disengagement and backmixing within the core region of the riser cross-section,; and (ii) solids lateral transfer of from the core region to the wall layers. The experiments were carried out in a circulating fluidized bed riser (3.1 m in height and 0.45 m2 in cross-section), which was operated with Geldart B solids fluidized with air at room temperature and for different gas velocities. The experimentally-derived data are expressed in terms of the disengagement rate and a lateral core-to-wall layer mass transfer coefficient. From the results, it is estimated that the presence of disengagement-based solids back-mixing is significant all along the 3-m riser. The disengagement rate shows a non-linear dependency on the solids concentration, with the lateral solids transfer to the walls (which follows a linear dependency on the solids concentration) eventually becoming the dominant form of back-mixing at upper heights.

Skapa referenser, mejla, bekava och länka

Permalink

Träfflista för sökning "WFRF:(Bonmann Marlene 1988) "

Refine your search

Year