| 1. |
- Bonmann, Marlene, 1988, et al.
(author)
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An Integrated 200-GHz Graphene FET Based Receiver
- 2018
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In: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; 2018-September
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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.
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| 2. |
- Asad, Muhammad, 1986, et al.
(author)
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Correlation between material quality and high frequency performance of graphene field-effect transistors
- 2019
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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.
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| 3. |
- Asad, Muhammad, 1986, et al.
(author)
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Graphene field-effect transistors for high frequency applications
- 2018
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In: ; November 2018
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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.
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| 4. |
- Asad, Muhammad, 1986, et al.
(author)
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The dependence of the high-frequency performance of graphene field-effect transistors on channel transport properties
- 2020
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In: IEEE Journal of the Electron Devices Society. - 2168-6734. ; 8, s. 457-464
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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.
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| 5. |
- Bonmann, Marlene, 1988, et al.
(author)
-
Drain current saturation in graphene field-effect transistors at high fields
- 2018
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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].
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| 6. |
- Bonmann, Marlene, 1988, et al.
(author)
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Effects of self-heating on fT and fmax performance of graphene field-effect transistors
- 2020
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In: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 67:3, s. 1277-1284
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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.
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| 7. |
- Bonmann, Marlene, 1988, et al.
(author)
-
Effects of self-heating on high-frequency performance of graphene field-effect transistors
- 2019
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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.
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| 8. |
- Bonmann, Marlene, 1988, et al.
(author)
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Graphene field-effect transistors with high extrinsic fT and fmax
- 2019
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In: IEEE Electron Device Letters. - 0741-3106 .- 1558-0563. ; 40:1, s. 131-134
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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.
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| 9. |
- Bonmann, Marlene, 1988, et al.
(author)
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Studies of hysteresis in capacitance and current characteristics of flexible graphene field-effect transistors
- 2017
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In: Graphene Week 2017, Athens, Greece, 25-29 September, 2017.
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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].
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| 10. |
- Feijoo, Pedro C., et al.
(author)
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Does carrier velocity saturation help to enhance fmax in graphene field-effect transistors?
- 2020
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In: Nanoscale Advances. - : Royal Society of Chemistry (RSC). - 2516-0230. ; 2:9, s. 4179-4186
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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.
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