Delay Analysis of Graphene Field-Effect Transistors

In this letter, we analyze the carrier transit delay in graphene field-effect transistors (GFETs). GFETs are fabricated at the wafer-scale on sapphire substrate. For a device with a gate length of 210 nm, a current gain cut-off frequency fT of 18 GHz and 22 GHz is obtained before and after de-embedding. The extraction of the internal (Cgs,i, Cgd,i) and external capacitances (Cgs,ex and Cgd,ex) from the scaling behavior of the gate capacitances Cgs and Cgd allows the intrinsic ({\tau}_int), extrinsic ({\tau}_ext) and parasitic delays ({\tau}_par) to be obtained. In addition, the extraction of the intrinsic delay provides a new way to directly estimate carrier velocity from the experimental data while the breakdown of the total delay into intrinsic, extrinsic, and parasitic components can offer valuable information for optimizing RF GFETs structures.Comment: 3 pages, 3 figures, accepted for publication in IEEE Electron Device Letter

Inhibitory effect of a tyrosine-fructose Maillard reaction product, 2,4-bis(p-hydroxyphenyl)-2-butenal on amyloid-β generation and inflammatory reactions via inhibition of NF-κB and STAT3 activation in cultured astrocytes and microglial BV-2 cells

<p>Abstract</p> <p>Background</p> <p>Amyloidogenesis is linked to neuroinflammation. The tyrosine-fructose Maillard reaction product, 2,4-bis(<it>p</it>-hydroxyphenyl)-2-butenal, possesses anti-inflammatory properties in cultured macrophages, and in an arthritis animal model. Because astrocytes and microglia are responsible for amyloidogenesis and inflammatory reactions in the brain, we investigated the anti-inflammatory and anti-amyloidogenic effects of 2,4-bis(<it>p</it>-hydroxyphenyl)-2-butenal in lipopolysaccharide (LPS)-stimulated astrocytes and microglial BV-2 cells.</p> <p>Methods</p> <p>Cultured astrocytes and microglial BV-2 cells were treated with LPS (1 μg/ml) for 24 h, in the presence (1, 2, 5 μM) or absence of 2,4-bis(<it>p</it>-hydroxyphenyl)-2-butenal, and harvested. We performed molecular biological analyses to determine the levels of inflammatory and amyloid-related proteins and molecules, cytokines, Aβ, and secretases activity. Nuclear factor-kappa B (NF-κB) DNA binding activity was determined using gel mobility shift assays.</p> <p>Results</p> <p>We found that 2,4-bis(<it>p</it>-hydroxyphenyl)-2-butenal (1, 2, 5 μM) suppresses the expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) as well as the production of nitric oxide (NO), reactive oxygen species (ROS), tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β) in LPS (1 μg/ml)-stimulated astrocytes and microglial BV-2 cells. Further, 2,4-bis(<it>p</it>-hydroxyphenyl)-2-butenal inhibited the transcriptional and DNA binding activity of NF-κB--a transcription factor that regulates genes involved in neuroinflammation and amyloidogenesis via inhibition of IκB degradation as well as nuclear translocation of p50 and p65. Consistent with the inhibitory effect on inflammatory reactions, 2,4-bis(<it>p</it>-hydroxyphenyl)-2-butenal inhibited LPS-elevated Aβ₄₂levels through attenuation of β- and γ-secretase activities. Moreover, studies using signal transducer and activator of transcription 3 (STAT3) siRNA and a pharmacological inhibitor showed that 2,4-bis(<it>p</it>-hydroxyphenyl)-2-butenal inhibits LPS-induced activation of STAT3.</p> <p>Conclusions</p> <p>These results indicate that 2,4-bis(<it>p</it>-hydroxyphenyl)-2-butenal inhibits neuroinflammatory reactions and amyloidogenesis through inhibition of NF-κB and STAT3 activation, and suggest that 2,4-bis(<it>p</it>-hydroxyphenyl)-2-butenal may be useful for the treatment of neuroinflammatory diseases like Alzheimer's disease.</p

Deeply-scaled GaN high electron mobility transistors for radio frequency applications

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 173-186).Due to the unique combination of large critical breakdown field and high electron velocity, GaN-based high electron mobility transistors (HEMTs) have great potential for next generation high power RF amplifiers. The performance of GaN devices has increased continuously in the last two decades. However, in spite of the improvements, there are still several critical issues limiting the high frequency operation of these devices. One of the key challenges is the lower-than-expected maximum current gain cutoff frequency (fT) of deeply-scaled GaN HEMTs. The fT of the short channel devices is well below both projections from maximum frequency in the long channel devices and theoretical expectations based on material properties. Another important issue is a roll-off of the device frequency performance under wide bias range, which limits the large-signal high speed operation in the deeply-scaled devices. This thesis focuses on these two important problems and investigates them both analytically and experimentally. First, through systematic study of the transistor delay, the critical factors limiting intrinsic and extrinsic device speed are clarified and several technologies are demonstrated to overcome these limits. This has allowed the demonstration of state-of-the-art high frequency performance GaN HEMTs. Second, in order to understand the origin of the decrease in device speed at high drain and gate bias, a new extraction method and novel transistor structure have been developed, which provide an excellent guide for future device optimization.by Dong Seup Lee.Ph. D

Citizens’ Preference and Perception of Street Trees of Main Boulevards in Busan, South Korea

Street tree species are selected mainly based on functional utility. This selection practice results in effective management. If the value of street trees as perceived by citizens is added to the existing selection criteria, satisfaction will be improved in addition to effective management. This study attempted to find a way to reflect citizens’ values in street tree selection criteria through empirical analysis. A preference survey and the visual and affective evaluation of 12 street tree species in Busan Metropolitan City were conducted. With the collected data, principal component analysis (PCA) was performed to draw common factors of evaluation. Additionally, multidimensional scaling (MDS) was used to group tree species that were considered similar. The study results revealed that citizens prefer species that provide visually positive feelings and intimacy. Some species appropriate for urban greenspaces are not preferred for street trees. Although a low level of nuisance is an important aspect of street trees, citizens do not prefer species that are excessively free from nuisance. When selecting species with a polarized preference, dealing with undesired aspects is required. It is necessary to acknowledge the different preferences of citizens and street tree authorities; reflecting citizens’ values can provide more citizen-friendly street plans

Gate-geometry dependence of electrical characteristics of p-GaN gate HEMTs

In this Letter, we experimentally investigate the impact of gate geometry on forward operation of Schottky-gate p-GaN high electron mobility transistors (HEMTs). In particular, we analyze devices with changing gate-metal/p-GaN junction area and p-GaN/AlGaN/GaN heterostructure area in the linear regime. These devices exhibit unique threshold voltage and subthreshold swing scaling dependence with gate geometry that is in contrast with classic field-effect transistors. On the other hand, peak transconductance and ON resistance are found to scale classically. We find that these results arise from the fact that with a Schottky contact to the p-GaN layer, under steady-state conditions, the p-GaN layer voltage is set by current continuity across the gate stack. Furthermore, a detailed scaling study of the gate current reveals that current flow across the p-GaN/AlGaN/GaN heterostructure is not uniform—instead, it preferentially flows through the ungated portion of the p-GaN layer. Our study concludes that in Schottky-type p-GaN gate HEMTs, the respective areas of two junctions constitute an additional design degree of freedom to fine-tune device performance. </jats:p