Calculation Of Surface Potential Research Articles

Gate All Around Metal Oxide Field Transistor: Surface Potential Calculation Method including Doping and Interface Trap Charge and the Effect of Interface Trap Charge on Subthreshold Slope

An explicit surface potential calculation method of gate-all-around MOSFET (GAAMOSFET) devices which takes into account both interface trap charge and varying doping levels is presented. The results of the method are extensively verified by numerical simulation. Results from the model are used to find qualitative and quantitative effect of interface trap charge on subthreshold slope (SS) of GAAMOSFET devices. Further, design constraints of GAAMOSFET devices with emphasis on the effect of interface trap charge on device SS performance are investigated.

A defect-based compact modeling approach for the reliability of CMOS devices and integrated circuits

Reliability simulations are critical for lifetime prediction and verification of long-term performance of integrated circuits designed in advanced CMOS technologies. The existing techniques for reliability simulation model aging effects using threshold voltage (Vth) shifts that do not reflect the bias-dependence of stress-induced defects. In this work we present a defect-based modeling approach that captures the dynamic effects of both oxide-trapped charge and interface traps through calculations of surface potential. Such defects are attributed to aging effects and to ionizing–radiation damage in advanced CMOS technologies. The approach provides a connection between physics-based reliability models and integrated circuit simulation. The model is implemented as a Verilog-A sub-circuit module and is compatible with standard EDA tools and MOSFET compact models. The model formulation is verified using two-dimensional TCAD simulations. Demonstrations with digital integrated circuit simulations in SPICE and comparisons with calculations using Vth-based models are also presented.

Interface Trap Density of Gate-All-Around Silicon Nanowire Field-Effect Transistors With TiN Gate: Extraction and Compact Model

Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface trap charge distribution of cylindrical cross-sectioned gate-all-around silicon nanowire field-effect transistor is extracted by using three-dimensional simulation. While the interface chemistry of conventional gatestack ( Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> polysilicon) in conventional planar devices is well documented, not much work is available on interface trap distribution Dit of alternate gatestacks (gatestacks employing alternate gate materials) in silicon nanowire MOSFET devices. Furthermore, a compact drain current model with interface trap charge parameter is presented. The model is based on gradual channel approximation and uses self-consistent calculation of interface trap charge and surface potential to reproduce experimental current-voltage characteristics.

Surface Potential Calculation for Dynamic-Depletion Polysilicon TFTs With Both Gaussian and Exponential DOS Distribution

A closed-form scheme for the front- and back-side surface potential calculation in the dynamic-depletion mode of polysilicon thin-film transistors is developed by including both the Gaussian deep states and the exponential tail states. The explicit relationship between the surface potentials of both sides is extracted, and the Poisson equation is also solved. The proposed scheme provides both accuracy and computational efficiency for circuit analysis in simulators. The universal solution is physical-based and is verified by both numerical simulation and experiments.

Understanding the pH-dependent immobilization efficacy of feruloyl esterase-C on mesoporous silica and its structure–activity changes

The purpose of the present investigation was to study the pH dependence of both the immobilization process and the enzyme activity of a feruloyl esterase (FoFaeC from Fusarium oxysporum) immobilized in mesoporous silica. This was done by interpreting experimental results with theoretical molecular modeling of the enzyme structure. Modeling of the 3D structure of the enzyme together with calculations of the electrostatic surface potential showed that changes in the electrostatic potential of the protein surface were correlated with the pH dependence of the immobilization process. High immobilization yields were associated with an increase in pH. The transesterification activity of both immobilized and free enzyme was studied at different values of pH and the optimal pH of the immobilized enzyme was found to be one unit lower than that for the free enzyme. The surface charge distribution around the binding pocket was identified as being a crucial factor for the accessibility of the active site of the immobilized enzyme, indicating that the orientation of the enzyme inside the pores is pH dependent. Interestingly, it was observed that the immobilization pH affects the specific activity, irrespective of the changes in reaction pH. This was identified as a pH memory effect for the immobilized enzyme. On the other hand, a change in product selectivity of the immobilized enzyme was also observed when the transesterification reaction was run in MOPS buffer instead of citrate phosphate buffer. Molecular docking studies revealed that the MOPS buffer molecule can bind to the enzyme binding pocket, and can therefore be assumed to modulate the product selectivity of the immobilized enzyme toward transesterification.

Analytical modeling of surface accumulation behavior of fully depleted SOI four gate transistors (G4-FETs)

A charge sheet model is proposed to analyze the transistor characteristics of fully depleted SOI four gate field effect transistors (G4-FETs). The model is derived assuming a parabolic potential variation between the junction-gates and by solving 2-D Poisson’s equation. The proposed model facilitates the calculation of surface potential and charge densities as a function of all gate biases. Modifying this charge sheet model for non-equilibrium condition, current–voltage and capacitance–voltage characteristics are also analyzed. Different back surface charge conditions are considered for each analysis. The models are compared with 3-D Silvaco/Atlas simulation results which show good agreement.

Micellization and Catalytic Properties of Cationic Surfactants with Head Groups Functionalized with a Hydroxyalkyl Fragment

The catalytic activity of two homological series of cationic surfactants bearing a hydroxyalkyl fragment in the head groups R(CH3)2N+(CH2CH2OH)Br– and R(CH3)2N+(CH2CH2CH2OH)Br– toward the cleavage of the p-nitrophenyl esters of carbonic acids of different hydrophobicity (acetate, caprilate, caprinate, laurate, myristate) is shown to exceed that of typical cationic surfactants with the trimethylammonium (TMA) headgroup. The catalytic effect increases with the alkyl chain length of surfactants and nonmonotonous changes in the series: acetate < caprilate < myristate < laurate < caprinate, reaching 750 times. NMR and IR spectroscopy studies and the surface potential calculations revealed that the higher catalytic effect of hydroxyalkylated surfactants is not due to their higher surface potential and binding capacity toward substrates. This is in line with finding that binding constants for TMA series are higher than for their hydroxyalkylated analogues, which was demonstrated by the fitting of kinetic data in...

BSIM-IMG: A Compact Model for Ultrathin-Body SOI MOSFETs With Back-Gate Control

In this paper, we present an accurate and computationally efficient model for circuit simulation of ultrathin-body silicon-on-insulator MOSFETs with strong back-gate control. This work advances previous works in terms of numerical accuracy, computational efficiency, and behavior of the higher order derivatives of the drain current. We propose a consistent analytical solution for the calculation of front- and back-gate surface potentials and inversion charge. The accuracy of our surface potential calculation is on the order of nanovolts. The drain current model includes velocity saturation, channel-length modulation, mobility degradation, quantum confinement effect, drain-induced barrier lowering, and self-heating effect. The model has correct behavior for derivatives of the drain current and shows an excellent agreement with experimental data for long- and short-channel devices with 8-nm-thin silicon body and 10-nm-thin BOX.

Maturation in Action: CryoEM Study of a Viral Capsid Caught during Expansion

Bacteriophage HK97 maturation involves discrete intermediate particle forms, comparable to transitional states in protein folding, before reaching its mature form. The process starts by formation of a metastable prohead, poised for exothermic expansion triggered by DNA packaging. During maturation, the capsid subunit transitions from a strained to a canonical tertiary conformation and this has been postulated to be the driving mechanism for initiating expansion via switching hexameric capsomer architecture from skewed to 6-fold symmetric. We report the subnanometer electron-cryomicroscopy reconstruction of the HK97 first expansion intermediate before any crosslink formation. This form displays 6-fold symmetric hexamers, but capsid subunit tertiary structures exhibit distortions comparable to the prohead forms. We propose that coat subunit strain release acts in synergy with the first crosslinks to drive forward maturation. Finally, we speculate that the energetic features of this transition may result from increased stability of intermediates during maturation via enhanced inter-subunit interactions.

A critical study on the interactions of hesperitin with human hemoglobin: Fluorescence spectroscopic and molecular modeling approach

Hesperitin, a ubiquitous bioactive flavonoid abundant in citrus fruits is known to possess antioxidant, anti-carcinogenic, hypolipidemic, vasoprotective and other important therapeutic properties. Here we have explored the interactions of hesperitin with normal human hemoglobin (HbA), using steady state and time resolved fluorescence spectroscopy, far UV circular dicroism (CD) spectroscopy, combined with molecular modeling computations. Specific interaction of the flavonoid with HbA is confirmed from flavonoid-induced static quenching which is evident from steady state fluorescence as well as lifetime data. Both temperature dependent fluorescence measurements and molecular docking studies reveal that apart from hydrogen bonding and van der Waals interactions, electrostatic interactions also play crucial role in hesperitin-HbA interactions. Furthermore, electrostatic surface potential calculations indicate that the hesperitin binding site in HbA is intensely positive due to the presence of several lysine and histidine residues.

Surface-Potential-Based Drain Current Model of Polysilicon TFTs With Gaussian and Exponential DOS Distribution

An explicit approximation for surface potential as a function of terminal voltages is developed for partially depleted polysilicon thin-film transistors. The resulting model has taken both Gaussian deep states and exponential tail states into account, which provides a complete modeling for surface potential. Furthermore, a drain current model based on terms of surface potential is proposed. Surface potential and drain current calculations are successfully verified by comparisons with numerical and experimental ones.

PR3 Interacts Directly to Lipid Bilayers: Evidence from MD Simulations and SPR Experiments

Proteinase 3 (PR3) is a serine protease of the neutrophils involved in inflammation processes. Its membrane expression is a risk factor for chronic inflammatory diseases. Experimental data led to divergent hypotheses on the binding mode of PR3 to the plasma membrane of neutrophils. While several studies performed on cell lines have identified a number of partner proteins, no direct interaction between hPR3 and any of these potential partners has been demonstrated (reviewed in Hajjar et al, FEBS J., 2010). Differential scanning calorimetry and spectrophotometric measurements, on the other hand, show a direct interaction of PR3 with DMPC vesicles (Goldman et al., Eur J Biochem, 1999).Using molecular dynamics simulations, we have characterized the membrane-binding site of PR3. Electrostatic surface potential calculations and simulations with an implicit membrane model (IMM1) have showed that PR3 possesses basic amino acids that provide the driving force to orient the protein at the membrane surface, so that a hydrophobic patch can anchor into the hydrophobic region of the membrane. In vitro mutagenesis experiments have confirmed the role of both types of amino acids. All-atom MD simulations and MM/PBSA energy decomposition identify three types of interactions contributing to the anchoring of PR3: hydrogen bonding and charge-pairing with lipid phosphate groups (R177, R186A, K187 and R222), hydrophobic anchoring into the lipid bilayer core (F165, F166 and L223) and cation-π interactions with the choline groups of DMPC (W218).Surface Plasmon Resonance experiments show a strong binding of Pr3 to DMPC and POPC bilayers.Altogether our results demonstrate that Pr3 has the ability to directly bind to lipid liposomes and most probably also to the plasma membrane of neutrophils.

A novel surface potential-based short channel MOSFET model for circuit simulation

In this paper a novel analytical approximation method for surface potential (ψs) calculation in compact MOSFET model is presented. It achieves excellent accuracy and good calculation speed over all regions from accumulation to strong inversion. With this approximation method, a surface potential-based compact model for short channel MOSFET is developed. Comparison with measured data is also presented to validate the new model.

The 2.3-Angstrom Structure of Porcine Circovirus 2

Porcine circovirus 2 (PCV2) is a T=1 nonenveloped icosahedral virus that has had severe impact on the swine industry. Here we report the crystal structure of an N-terminally truncated PCV2 virus-like particle at 2.3-Å resolution, and the cryo-electron microscopy (cryo-EM) image reconstruction of a full-length PCV2 virus-like particle at 9.6-Å resolution. This is the first atomic structure of a circovirus. The crystal structure revealed that the capsid protein fold is a canonical viral jelly roll. The loops connecting the strands of the jelly roll define the limited features of the surface. Sulfate ions interacting with the surface and electrostatic potential calculations strongly suggest a heparan sulfate binding site that allows PCV2 to gain entry into the cell. The crystal structure also allowed previously determined epitopes of the capsid to be visualized. The cryo-EM image reconstruction showed that the location of the N terminus, absent in the crystal structure, is inside the capsid. As the N terminus was previously shown to be antigenic, it may externalize through viral "breathing."

De Novo Polymerase Activity and Oligomerization of Hepatitis C Virus RNA-Dependent RNA-Polymerases from Genotypes 1 to 5

Hepatitis C virus (HCV) shows a great geographical diversity reflected in the high number of circulating genotypes and subtypes. The response to HCV treatment is genotype specific, with the predominant genotype 1 showing the lowest rate of sustained virological response. Virally encoded enzymes are candidate targets for intervention. In particular, promising antiviral molecules are being developed to target the viral NS3/4A protease and NS5B polymerase. Most of the studies with the NS5B polymerase have been done with genotypes 1b and 2a, whilst information about other genotypes is scarce. Here, we have characterized the de novo activity of NS5B from genotypes 1 to 5, with emphasis on conditions for optimum activity and kinetic constants. Polymerase cooperativity was determined by calculating the Hill coefficient and oligomerization through a new FRET-based method. The Vmax/Km ratios were statistically different between genotype 1 and the other genotypes (p<0.001), mainly due to differences in Vmax values, but differences in the Hill coefficient and NS5B oligomerization were noted. Analysis of sequence changes among the studied polymerases and crystal structures show the αF helix as a structural component probably involved in NS5B-NS5B interactions. The viability of the interaction of αF and αT helixes was confirmed by docking studies and calculation of electrostatic surface potentials for genotype 1 and point mutants corresponding to mutations from different genotypes. Results presented in this study reveal the existence of genotypic differences in NS5B de novo activity and oligomerization. Furthermore, these results allow us to define two regions, one consisting of residues Glu128, Asp129, and Glu248, and the other consisting of residues of αT helix possibly involved in NS5B-NS5B interactions.

Probing the interactions of hemoglobin with antioxidant flavonoids via fluorescence spectroscopy and molecular modeling studies

Steady state and time resolved fluorescence spectroscopy, combined with molecular modeling computations, have been used to explore the interactions of two therapeutically important flavonoids, fisetin (3,7,3′,4′-OH-flavone) and 3-hydroxyflavone (3-HF), with normal human hemoglobin (HbA). Distinctive ‘two color’ fluorescence signatures and fairly high fluorescence anisotropy ( r = 0.12–0.28) of fisetin and 3-HF reveal their specific interactions with HbA. Binding constants estimated from the fluorescence studies were ≈ 4.00 × 10 4 M − 1 and 9.83 × 10 3 M − 1 for fisetin and 3-HF respectively. Specific interactions with HbA were further confirmed from flavonoid-induced static quenching of the protein tryptophan fluorescence as indicated by: (a) bimolecular quenching constant K q ≫ diffusion controlled limit (b) closely matched values of Stern–Volmer quenching constant and binding constant (c) τ o/τ ≈ 1 (where τ o and τ are the unquenched and quenched tryptophan fluorescence lifetimes respectively). Molecular docking and electrostatic surface potential calculations reveal contrasting binding modes of fisetin and 3-HF with HbA.

One-dimensional continuous analytic potential solution to generic oxide–silicon–oxide system

A one-dimensional continuous analytic potential solution to a generic oxide–silicon–oxide system is developed. With the analytic solution, the potential distribution in the silicon film is predicted. A physics-based relation between surface potentials is also derived and then applied to the generic oxide–silicon–oxide metal–oxide–semiconductor field-effect transistors (MOSFETs) for the calculation of surface potentials

Accelerating electrostatic surface potential calculation with multi-scale approximation on graphics processing units.

Tools that compute and visualize biomolecular electrostatic surface potential have been used extensively for studying biomolecular function. However, determining the surface potential for large biomolecules on a typical desktop computer can take days or longer using currently available tools and methods. Two commonly used techniques to speed-up these types of electrostatic computations are approximations based on multi-scale coarse-graining and parallelization across multiple processors. This paper demonstrates that for the computation of electrostatic surface potential, these two techniques can be combined to deliver significantly greater speed-up than either one separately, something that is in general not always possible. Specifically, the electrostatic potential computation, using an analytical linearized Poisson-Boltzmann (ALPB) method, is approximated using the hierarchical charge partitioning (HCP) multi-scale method, and parallelized on an ATI Radeon 4870 graphical processing unit (GPU). The implementation delivers a combined 934-fold speed-up for a 476,040 atom viral capsid, compared to an equivalent non-parallel implementation on an Intel E6550 CPU without the approximation. This speed-up is significantly greater than the 42-fold speed-up for the HCP approximation alone or the 182-fold speed-up for the GPU alone.

Redox properties and catalytic activity of surface-bound human sulfite oxidase studied by a combined surface enhanced resonance Raman spectroscopic and electrochemical approach

Human sulfite oxidase (hSO) was immobilised on SAM-coated silver electrodes under preservation of the native heme pocket structure of the cytochrome b5 (Cyt b5) domain and the functionality of the enzyme. The redox properties and catalytic activity of the entire enzyme were studied by surface enhanced resonance Raman (SERR) spectroscopy and cyclic voltammetry (CV) and compared to the isolated heme domain when possible. It is shown that heterogeneous electron transfer and catalytic activity of hSO sensitively depend on the local environment of the enzyme. Increasing the ionic strength of the buffer solution leads to an increase of the heterogeneous electron transfer rate from 17 s(-1) to 440 s(-1) for hSO as determined by SERR spectroscopy. CV measurements demonstrate an increase of the apparent turnover rate for the immobilised hSO from 0.85 s(-1) in 100 mM buffer to 5.26 s(-1) in 750 mM buffer. We suggest that both effects originate from the increased mobility of the surface-bound enzyme with increasing ionic strength. In agreement with surface potential calculations we propose that at high ionic strength the enzyme is immobilised via the dimerisation domain to the SAM surface. The flexible loop region connecting the Moco and the Cyt b5 domain allows alternating contact with the Moco interaction site and the SAM surface, thereby promoting the sequential intramolecular and heterogeneous electron transfer from Moco via Cyt b5 to the electrode. At lower ionic strength, the contact time of the Cyt b5 domain with the SAM surface is longer, corresponding to a slower overall electron transfer process.

Experimental measurement of work function in doped silicon surfaces

Theory and experimental measurement of work function for electrons in doped silicon surfaces are presented in the article. Definitions of work function and of the local work function are given in the introduction. It was shown, that the value of work function defines the position of Fermi energy level. Numerical calculations of surface potential and electric field for theoretical semiconductor surface models show the difference between finite and infinite surfaces. Photovoltage dependence on charge carriers injection ratio for doped silicon surface was numerically calculated as well. Experimentally determined Fermi energies of doped silicon samples show good agreement with theory for low and moderately doped samples only. Appropriate explanations of disagreement between theory and experiment are given in the conclusion. Suggestions for the system improvement are presented in the Appendix.