| 1. |
- 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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| 2. |
- 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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| 3. |
- Bonmann, Marlene, 1988, et al.
(författare)
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Studies of hysteresis in capacitance and current characteristics of flexible 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
- 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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| 4. |
- Bonmann, Marlene, 1988, et al.
(författare)
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Effect of oxide traps on channel transport characteristics in graphene field effect transistors
- 2017
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Ingår i: Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures. - : American Vacuum Society. - 2166-2754 .- 2166-2746. ; 35:1, s. 01A115-
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Tidskriftsartikel (refereegranskat)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.
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| 5. |
- Bonmann, Marlene, 1988
(författare)
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Effects of impurities on charge transport in graphene field-effect transistors
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)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.
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