| 1. |
- Bonmann, Marlene, 1988, et al.
(författare)
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An Integrated 200-GHz Graphene FET Based Receiver
- 2018
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Ingår i: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; 2018-September
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Konferensbidrag (refereegranskat)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.
(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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| 3. |
- Asad, Muhammad, 1986, et al.
(författare)
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Graphene field-effect transistors for high frequency applications
- 2018
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Ingår i: ; November 2018
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Konferensbidrag (refereegranskat)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.
(författare)
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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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Ingår i: IEEE Journal of the Electron Devices Society. - 2168-6734. ; 8, s. 457-464
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Characterization of Graphene FET based 200 GHz Mixer and 1 GHz Amplifier Integrated on a Si Substrate
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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.
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| 6. |
- Bonmann, Marlene, 1988, et al.
(författare)
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Charge carrier velocity in graphene field-effect transistors
- 2017
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 111:23, s. 233505-
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Tidskriftsartikel (refereegranskat)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.
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| 7. |
- Bonmann, Marlene, 1988, et al.
(författare)
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Delay analysis for evaluation of carrier velocity in graphene field-effect transistors
- 2017
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Ingår i: Graphene Week 2017, Athens, Greece, 25-29 September, 2017.
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Konferensbidrag (refereegranskat)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.
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| 8. |
- Bonmann, Marlene, 1988, et al.
(författare)
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Drain current saturation in graphene field-effect transistors at high fields
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)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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| 9. |
- Bonmann, Marlene, 1988, et al.
(författare)
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Effects of self-heating on fT and fmax performance of graphene field-effect transistors
- 2020
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Ingår i: IEEE Transactions on Electron Devices. - 1557-9646 .- 0018-9383. ; 67:3, s. 1277-1284
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Tidskriftsartikel (refereegranskat)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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| 10. |
- 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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