| 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. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- Generalov, Andrey, 1987, et al.
(författare)
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A 400-GHz Graphene FET Detector
- 2017
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Ingår i: IEEE Transactions on Terahertz Science and Technology. - 2156-342X .- 2156-3446. ; 7:5, s. 614-616
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Tidskriftsartikel (refereegranskat)abstract
- This letter presents a graphene field effect transistor (GFET) detector at 400 GHz, with a maximum measured optical responsivity of 74 V/W, and a minimum noise-equivalent power of 130 pW/Hz1/2. This letter shows how the detector performance degrades as a function of the residual carrier concentration in the graphene channel, which is an important material parameter that depends on the quality of the graphene sheet and contaminants introduced during the fabrication process. In this work, the exposure of the graphene channel to liquid processes is minimized resulting in a low residual carrier concentration. This is in part, an important contributing factor to achieve the record high GFET detector performance. Thus, our results show the importance to use graphene with high quality and the importance to minimize contamination during the fabrication process.
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| 6. |
- Generalov, Andrey, 1987, et al.
(författare)
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A heterodyne graphene FET detector at 400 GHz
- 2017
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Ingår i: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; , s. Artno:8067234-
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Konferensbidrag (refereegranskat)abstract
- We present a THz heterodyne detector based on a single layer graphene field effect transistor (GFET) integrated with a bowtie antenna at 400 GHz. The heterodyne detection is achieved by coupling RF and LO signals quasi-optically to the same GFET. The down converted IF signal is extracted via a coplanar stripline connected to the GFET source and drain terminals. The measured IF bandwidth is 5 GHz.
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| 7. |
- Generalov, Andrey, 1987, et al.
(författare)
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Optimization of THz graphene FET detector integrated with a bowtie antenna
- 2016
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Ingår i: European Conference on Antennas and Propagation. - 2164-3342. - 9788890701863
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Konferensbidrag (refereegranskat)abstract
- This paper discusses the integration of the split bowtie antenna with a graphene FET THz detector to maximize the detector efficiency at 1 THz. The detector utilizes the principle of distributed resistive self-mixing in GFET, and the split bowtie antenna provides an asymmetric feed to the GFET. The antenna is placed on a substrate lens to improve the directivity and can be used to create an imaging array. The dimensions of the split bowtie antenna are optimized for the best impedance matching with the GFET and to improve the pixel density of the array. The off-axis pixel performance is improved by modifying the edge-pixel antennas. The improvement in directivity of corresponding pixels is up to 1.3 dB.
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| 8. |
- Lavén, Rasmus, 1994, et al.
(författare)
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Diffusional Dynamics of Hydride Ions in the Layered Oxyhydride SrVO 2 H
- 2021
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 33:8, s. 2967-2975
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Tidskriftsartikel (refereegranskat)abstract
- Perovskite-type oxyhydrides are hydride-ion-conducting materials of promise for several types of technological applications; however, the conductivity is often too low for practical use and, on a fundamental level, the mechanism of hydride-ion diffusion remains unclear. Here, we, with the use of neutron scattering techniques, investigate the diffusional dynamics of hydride ions in the layered perovskite-type oxyhydride SrVO2H. By monitoring the intensity of the elastically scattered neutrons upon heating the sample from 100 to 430 K, we establish an onset temperature for diffusional hydride-ion dynamics at about 250 K. Above this temperature, the hydride ions are shown to exhibit two-dimensional diffusion restricted to the hydride-ion sublattice of SrVO2H and that occurs as a series of jumps of a hydride ion to a neighboring hydride-ion vacancy, with an enhanced rate for backward jumps due to correlation effects. Analysis of the temperature dependence of the neutron scattering data shows that the localized jumps of hydride ions are featured by a mean residence time of the order of 10 ps with an activation energy of 0.1 eV. The long-range diffusion of hydride ions occurs on the timescale of 1 ns and with an activation energy of 0.2 eV. The hydride-ion diffusion coefficient is found to be of the order of 1 × 10-6 cm2 s-1 in the temperature range of 300-430 K, which is similar to other oxyhydrides but higher than for proton-conducting perovskite analogues. Tuning of the hydride-ion vacancy concentration in SrVO2H thus represents a promising gateway to improve the ionic conductivity of this already highly hydride-ion-conducting material.
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| 9. |
- Tanzid, Mehbuba, 1988, et al.
(författare)
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Microwave noise characterization of graphene field effect transistors
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:1, s. 013502-
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Tidskriftsartikel (refereegranskat)abstract
- The microwave noise parameters of graphene field effect transistors (GFETs) fabricated using chemical vapor deposition graphene with 1 μm gate length in the 2 to 8 GHz range are reported. The obtained minimum noise temperature (Tmin) is 210 to 610 K for the extrinsic device and 100 to 500 K for the intrinsic GFET after de-embedding the parasitic noise contribution. The GFET noise properties are discussed in relation to FET noise models and the channel carrier transport. Comparison shows that GFETs can reach similar noise levels as contemporary Si CMOS technology provided a successful gate length scaling is performed.
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| 10. |
- Yang, Xinxin, 1988, et al.
(författare)
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A flexible graphene terahertz detector
- 2017
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 111:2
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Tidskriftsartikel (refereegranskat)abstract
- We present a flexible terahertz (THz) detector based on a graphene field-effect transistor fabricated on a plastic substrate. At room temperature, this detector reveals voltage responsivity above 2 V/W and estimated noise equivalent power (NEP) below 3 nW/Hz1/2 at 487 GHz. We have investigated the effects of bending strain on DC characteristics, voltage responsivity, and NEP of the detector, and the results reveal its robust performance. Our findings have shown that graphene is a promising material for the development of THz flexible technology.
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