| 1. |
- Asad, Muhammad, 1986, et al.
(författare)
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Correlation between material quality and high frequency performance of graphene field-effect transistors
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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.
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| 2. |
- Bonmann, Marlene, 1988, et al.
(författare)
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Effects of self-heating on high-frequency performance of graphene field-effect transistors
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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.
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| 3. |
- Bonmann, Marlene, 1988, et al.
(författare)
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Graphene field-effect transistors with high extrinsic fT and fmax
- 2019
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Ingår i: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 40:1, s. 131-134
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Tidskriftsartikel (refereegranskat)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.
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| 4. |
- Li, Junjie, 1995, et al.
(författare)
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High frequency noise characterisation of graphene field-effect transistors at different temperatures
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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.
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| 5. |
- Nylander, Andreas, 1988, et al.
(författare)
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RF properties of carbon nanotube / Copper composite through silicon via based CPW structure for 3D integrated circuits
- 2019
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Ingår i: 2019 IEEE 14th Nanotechnology Materials and Devices Conference, NMDC 2019.
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Konferensbidrag (refereegranskat)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.
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| 6. |
- Vorobiev, Andrei, 1963, et al.
(författare)
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Graphene Field-Effect Transistors for Millimeter Wave Amplifiers
- 2019
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Ingår i: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; 2019-September
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Konferensbidrag (refereegranskat)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.
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| 7. |
- Bonmann, Marlene, 1988
(författare)
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Graphene field-effect transistors and devices for advanced high-frequency applications
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
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| 8. |
- Soltani, Amin, et al.
(författare)
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Unveiling the plasma wave in the channel of graphene field-effect transistor
- 2019
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Ingår i: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; 2019-September
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Konferensbidrag (refereegranskat)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.
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