A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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Subject: search.filters.subject.carbon nanotube

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    HIGH FREQUENCY GENERATION BASED ON CARBON NANOTUBE FIELD-EFFECT TRANSISTORS
    (2014) Song, Da; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Carbon nanotubes (CNTs) are promising materials in radio frequency (RF) applications due to their high mobility, high current density and low capacitance. Over the past several years, extensive experimental and theoretical works have been focused on increasing the cut-off frequency of carbon nanotube field effect transistors (CNTFETs). However, there is limited study aiming for understanding the linearity of CNTFETs, which is an important aspect when radio frequency transistors are working in multiple frequency environments. In this dissertation, CNTFETs are fabricated based on horizontally aligned carbon nanotubes grown on quartz substrate. DC characterization shows three conduction regions in the transfer curve of the device, p-type and n-type linear regions, and ambipolar nonlinear region. The single tone excitation measurement shows extra harmonic generations as a result of the nonlinearity of the device. Same measurement is conducted with control devices without carbon nanotubes in the channel and confirms the nonlinearity is from the carbon nanotubes in the channel. Comparison between the 1st order harmonic amplitude and the 2nd order derivative of current with respect to gate voltage indicates that nonlinear transconductance is the cause of nonlinearity in the device. In order to understand the nonlinearity thoroughly, an elementary model based on 1D electronic transport and Drude model is built. The model can accurately predict the DC performance and nonlinearity of the device. Taking advantage of the transitions between linear and nonlinear transfer regions, we build our CNTFETs into gate controlled radio frequency mixers. Two-tone mixing measurement shows clearly that intermodulation terms in the output spectrum are strong in ampibolar regions and suppressed to noise floor in the linear regions. We further perform passive mixing (no source/drain voltage applied) in higher frequency regime and demonstrate the generation of harmonic and intermodulation signals in the output frequency range between 75 and110GHz, which is the among the highest output frequency observed from CNTFETs to date.
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    Ultrafast nano-oscillators based on interlayerbridged carbon nanoscrolls
    (2011-07-25) Zhang, Zhao; Li, Teng
    We demonstrate a viable approach to fabricating ultrafast axial nano-oscillators based on carbon nanoscrolls (CNSs) using molecular dynamics simulations. Initiated by a single-walled carbon nanotube (CNT), a monolayer graphene can continuously scroll into a CNS with the CNT housed inside. The CNT inside the CNS can oscillate along axial direction at a natural frequency of tens of gigahertz. We demonstrate an effective strategy to reduce the dissipation of the CNS-based nano-oscillator by covalently bridging the carbon layers in the CNS. We further demonstrate that such a CNS-based nano-oscillator can be excited and driven by an external AC electric field, and oscillate at more than 100 GHz. The CNS-based nano-oscillators not only offer a feasible pathway toward ultrafast nano-devices but also hold promise to enable nanoscale energy transduction, harnessing, and storage (e.g., from electric to mechanical).
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    Development of Carbon Nanotube Field-Effect Transistor Arrays for Detection of HER2 Overexpression in Breast Cancer
    (2011) Aschenbach, Konrad Hsu; Gomez, Romel D; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    We developed a carbon nanotube biosensor platform that was deployed at the National Cancer Institute and successfully detected the HER2 oncogene in real cancer cells at clinically relevant levels. HER2 is a receptor protein that resides on the surface of certain cancer cells and is associated with higher aggressiveness in breast cancers. Overabundance of HER2 at the chromosomal, cell surface, and intermediate gene expression levels can all indicate a dangerous HER2 status. At the present, testing for HER2 status requires labor-intensive laboratory procedures using expensive reagents. Cost remains the major barrier to widespread screening. We propose an integrated electronic testing platform based on direct label-free gene detection. The system would integrate the various labor-intensive processes that are usually performed by skilled laboratory technicians. The heart of the system is an array of carbon nanotube field-effect transistors that can detect unlabelled nucleic acids via their intrinsic electric charges. We developed a scalable fabrication technique for carbon nanotube biosensor arrays, hardware and software for data acquisition and analysis, theoretical models for detection mechanism, and protocols for immobilization of peptide nucleic acid probes and hybridization of nucleic acids extracted from cells. We demonstrated detection of HER2 from real cell lines which express cancer genes, thereby lowering the technological barrier towards commercialization of a low-cost gene expression biosensor. The system is suitable for lab-on-a-chip integration, which could bring rapid, low-cost cancer diagnoses into the clinical setting.
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    Thermal and performance modeling of nanoscale mosfets, carbon nanotube devices and integrated circuits
    (2006-05-31) Akturk, Akin; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    We offer new paradigms for electronic devices and digital integrated circuits (ICs) in an effort to overcome important performance threatening problems such as self heating. To investigate chip heating, we report novel methods for predicting the thermal profiles of complex ICs at the resolution of a single device. We resolve device and IC temperatures self-consistently, with individual device performances, while accounting for IC layout and software application details. At the device level, we calculate performance and generated heat details. We then extend these performance figures to the overall chip using a stochastic or Monte Carlo type methodology. Next, at the IC level, we solve for the device temperatures using the chip's layout and application software details. Here, we apply our mixed-mode algorithm to two-dimensional (planar) and three-dimensional ICs. To relieve thermal stresses and performance degradation in specific areas of extreme heating or hot spots, we offer design strategies using thermal contacts or different IC layouts. Moreover, we also show chips that we had designed and fabricated through IC fabrication clearing house MOSIS for experimental investigations. We also investigate carbon nanotubes (CNTs) and CNT embedded MOSFETs as new device paradigms for future electronic circuits. To examine the effects of CNTs on device performance, we develop a CNT Monte Carlo simulator, and determine scattering rates and CNT electron transport. Here, we report position-dependent velocity oscillations and length effects in semiconducting single-walled zig-zag carbon nanotubes. Our calculated results indicate velocity oscillations in the Terahertz range, which approaches phonon frequencies. This may facilitate new high frequency RF device and circuit designs, opening new paradigms in communication networks. Furthermore, to obtain device performance figures for MOSFETs that embed CNTs in their channels, our device solver determines interactions between the CNT and silicon (Si) by obtaining quantization and transport effects on the tube and the Si, and at the CNT-Si barrier. We predict that the CNT-MOSFET yields a better performance than the traditional MOSFET. Especially, CNT-MOSFETs employing lower diameter tubes exhibit improved performance capabilities. We also perform similar analyses for CNT embedded SOI-MOSFETs.