Gate Voltage Shift Research Articles

E-Mode AlGaN/GaN HEMTs Using p-NiO Gates

Sputtered p-NiO films were used to suppress gate leakage and produce a positive shift in gate voltage of AlGaN/GaN high electron mobility transistors for e-mode operation. A direct comparison with Schottky gate devices fabricated on the same wafer shows the utility of the NiO in increasing the on-off ratio and shifting the threshold voltage from -0.95 V (Schottky gated) to +0.9 V (NiO gated). The breakdown voltage was 780 V for a 40 µm drain-source separation. The sub-threshold swing decreased from 181 mV/dec for Schottky gate HEMTs to 128 mV/dec on NiO-gated devices. The simple fabrication process without any annealing or passivation steps shows the promise of NiO-gates for e-mode AlGaN/GaN HEMT operation. Figure 1

E-Mode AlGaN/GaN HEMTs Using p-NiO Gates

Sputtered p-NiO films were used to suppress gate leakage and produce a positive shift in gate voltage of AlGaN/GaN high electron mobility transistors for e-mode operation. A direct comparison with Schottky gate devices fabricated on the same wafer shows the utility of the NiO in increasing the on-off ratio and shifting the threshold voltage from -0.95 V (Schottky gated) to +0.9 V (NiO gated). The breakdown voltage was 780 V for a 40 µm drain-source separation. The sub-threshold swing decreased from 181 mV/dec for Schottky gate HEMTs to 128 mV/dec on NiO-gated devices. The simple fabrication process without any annealing or passivation steps shows the promise of NiO-gates for e-mode AlGaN/GaN HEMT operation.

Modeling of Ionizing Radiation Effects for Negative Capacitance Field-Effect Transistors

A theoretical model for simulating ionizing radiation effects on negative capacitance field-effect transistors (NCFETs) with a metal–ferroelectric–insulator–semiconductor (MFIS) structure was established. Based on the model, the effects of total ionizing dose (TID) and dose rate on the surface potential, ferroelectric capacitance, voltage amplification factor, and transfer characteristics of NCFETs were investigated. The simulation results demonstrated that, with the increase in total dose, the curves of surface potential versus gate voltage and driving current versus gate voltage shift left significantly, resulting in the point of voltage amplification shifting left. Meanwhile, with the increase in dose rate, the amplitude of both the surface potential and driving current decreases slightly. Meanwhile, the derived result indicated that relatively thin ferroelectric thickness can effectively reduce the effect of TID. It is expected that this model can be helpful for analyzing the radiation effects of NCFETs.

Supported Pt Nanoclusters on Single-Layer MoS2 for the Detection of Cortisol: From Atomistic Scale to Device Modeling

The development of nonenzymatic sensors is a challenge which requires, on the one hand, careful design of the sensing materials with respect to the chosen analyte, and on the other hand, suitable device architectures. In this work, we propose single-layer molybdenum disulfide (MoS2) decorated with subnanometer Pt clusters as the sensing platform for the detection of cortisol. The aim is to assess the suitability of such a sensing platform for the development of wearable and portable cortisol sensors. For this study, we performed multiscale computer simulations at the materials level up to device scale. First, ab initio simulations within the framework of density functional theory (DFT) allowed us to gain insights into the interaction, at the atomic level, between the analyte (cortisol) and the sensing platform (MoS2/Pt). Then, by carrying out technology computer-aided design (TCAD) simulations, we were able to consider a device architecture and investigate its performance as cortisol sensor. Following our multiscale simulation strategy, we were able to assess the proposed field-effect transistor (FET) sensor, whose channel is made of Pt-decorated MoS2. The sensing mechanism relies on the chemiresistive response of the device to the adsorption of cortisol on the channel, which leads to a sizable charge transfer from the analyte to the substrate and, consequently, to the measurable shift in the gate voltage threshold of the FET. Our findings suggest that both the choice of the sensing materials and the proposed FET architecture are suitable for detecting cortisol by non-enzymatic means. In the best case scenario, we predict theoretical gate voltage shifts between 76 and 780 mV with respect to the cluster concentration and between 27 and 780 mV when varying the cluster occupancy by cortisol. We may expect our results to provide the necessary basis to develop highly sensitive nonenzymatic cortisol sensors based on 2D materials decorated with Pt nanoclusters.

Detection of carbohydrate antigen 19-9 by a diamond solution-gate field-effect transistor sensor

Detection of carbohydrate antigen 19-9 (CA19-9) via hydrogen-terminated diamond (H-diamond) solution-gate field-effect transistor (SGFET) sensor had been carried out for the first time. Linker molecule 1-pyrenebutyric acid–N-hydroxy succinimide ester (Pyr-NHS) was fixed on the surface of H-diamond by π-π bonding. Then, anti-CA19-9 antibody was immobilized with Pyr-NHS. After specific binding of CA19-9 and antibody, transfer characteristics of H-diamond SGFET exhibited a gate voltage shift in negative direction at the same output current. The sensitivity of sensor was obtained as −50.98 mV/Log10[antigen concentration]. The minimum detection concentration was 0.001 U/mL. This article illustrated a potential application of H-diamond SGFET sensor in CA19-9 detection.

Electrostatic Fermi level tuning in large-scale self-assembled monolayers of oligo(phenylene-ethynylene) derivatives.

Understanding and controlling the orbital alignment of molecules placed between electrodes is essential in the design of practically-applicable molecular and nanoscale electronic devices. The orbital alignment is highly determined by the molecule-electrode interface. Dependence of orbital alignment on the molecular anchor group for single molecular junctions has been intensively studied; however, when scaling-up single molecules to large parallel molecular arrays (like self-assembled monolayers (SAMs)), two challenges need to be addressed: 1. Most desired anchor groups do not form high quality SAMs. 2. It is much harder to tune the frontier molecular orbitals via a gate voltage in SAM junctions than in single molecular junctions. In this work, we studied the effect of the molecule-electrode interface in SAMs with a micro-pore device, using a recently developed tetrapodal anchor to overcome challenge 1, and the combination of a single layered graphene top electrode with an ionic liquid gate to solve challenge 2. The zero-bias orbital alignment of different molecules was signalled by a shift in conductance minimum vs. gate voltage for molecules with different anchoring groups. Molecules with the same backbone, but a different molecule-electrode interface, were shown experimentally to have conductances that differ by a factor of 5 near zero bias. Theoretical calculations using density functional theory support the trends observed in the experimental data. This work sheds light on how to control electron transport within the HOMO-LUMO energy gap in molecular junctions and will be applicable in scaling up molecular electronic systems for future device applications.

Charge-independent protein adsorption characteristics of epitaxial graphene field-effect transistor on SiC substrate

Charge-independent biomolecule detection using field-effect transistors (FETs) with single-crystal and large-area epitaxial graphene films fabricated on SiC substrates is demonstrated. To obtain clean graphene channel surfaces, FETs were fabricated using stencil mask lithography, which is a resist-free fabrication process. Proteins with various isoelectric points (pI: 5.6–9.9) were used as targets. Transfer characteristics [drain current (ID) vs solution-gate voltage (VG) characteristics] were measured by changing the pH of the buffer solution. The ID–VG characteristics exhibited a clear negative gate voltage shift for both positively and negatively charged proteins, indicating that the epitaxial graphene FETs could not detect the charge type of the protein and electrons were doped by the adsorption of both positively and negatively charged proteins. These results cannot be explained by conventional electrostatic effects. Therefore, it can be concluded that the detection of biomolecules by the epitaxial graphene FETs occurred through charge transfer from the proteins. Moreover, the dissociation constants between the proteins and epitaxial graphene films were as small as 100 pM, indicating the high sensitivity of the graphene FETs.

Observations regarding deep-level states causing p-type doping in AlTiO gate and positive threshold voltage shift in AlGaN/GaN high electron mobility transistors

Application of ternary oxides has emerged as a potential enabler to achieve enhancement mode (normally-OFF) operation in AlGaN/GaN high electron mobility transistors (HEMTs). However, it is not well understood what leads to the 2-Dimensional Electron Gas depletion or positive threshold voltage shift by the integration of these oxides in the gate stack. In this work, an electro-optical experiment-based method is used to probe the underlying mechanism. For experiments, AlGaN/GaN metal–oxide–semiconductor high electron mobility transistors with gate stack consisting of AlTiO (ternary p-type oxide), Al2O3, TiO2 (undoped binary oxides), and NiO (conventional p-type oxide) are used. Optical exposure with wavelengths falling in the UV regime (365 nm) was found to induce a negative threshold voltage shift in AlTiO based devices. Experimentation on different GaN buffer stacks, different gate oxides, and selective UV exposure only to the gate region established the phenomenon to be governed by properties of the ternary oxide. This was further confirmed by the fact that the negative threshold voltage shift with UV exposure was proportional to the positive shift achieved as a function of Al% in AlTiO. The negative shift was found to be due to de-ionization of deep-level negative states in AlTiO, which resulted from the presence of Al at Ti sites ([Al]Ti′). These negatively ionized deep-level states at room temperature result in p-type doping of the oxide, leading to the positive threshold voltage shift in AlTiO gate based HEMTs when compared with HEMTs with Al2O3 or TiO2 as gate oxide.

Universal Bacterial Detection Utilizing PEDOT:PSS-Based Organic Electrochemical Transistors

Bacteria are one of the most common sources of illness, responsible for a wide range of infections. Bacterial infections can be caused from a variety of sources, with thousands of deaths attributed to waterborne outbreaks alone [1]. Detection of both infectious and non-infectious bacteria is important to ensure commercial goods and water are safe for consumers. Current detection of bacterial contamination typically uses cell culture plates to measure the number of colony forming bacteria (CFU/ml) in a liquid media. This technique is widely used due to its high sensitivity and easy visual readout. Unfortunately, culture counting technique is time consuming (1-3 days), requires skilled lab technicians, can be contaminated during preparation, and cannot be integrated into the desired testing media directly.To overcome these limitations, many bacterial sensors and detection techniques have been developed over the last few decades [2]. A recent sensor for bacteria detection is utilizing the organic electrochemical transistor (OECT). OECTs are polymer-based transistors that utilize ionic solutions to de-dope and re-dope a conducting channel. These devices are known for the high transconductance, bio-compatibility, low operating voltage, low cost, small size, ease of integration into measurement systems, and ability to function in aqueous environments [3]. Detection of a single bacteria is typically desired and achieved through the capture of bacteria cells onto the OECT channel area [4]. This approach allows for rapid detection of a specific pathogen with high sensitivity. However, for detecting contamination from a variety of bacteria a different approach must be used.We have investigated the OECT response to the presence of non-captured bacteria present within the OECT operating media. In this approach cells are detected both in solution and on the surface of the source-drain channel and on the gate electrode. Presence of bacteria between the gate and channel region will cause a shift in effective gate voltage, leading to a change in source-drain current. Detection and characterization of (non-bacteria) whole cells has previously been reported utilizing this approach [5, 6]. In our work we have focused on the bacteria pseudomonas fluorescens (p. fluorescens) due to its size (~1µm) and potential as a water borne pathogen. P. fluorescens was cultured in Luria-Bertani (LB) and Dey-Engley (D/E) media, diluted to different concentrations in broth, and used as the operating media for OECT. Concentrations of 1.9e5 to 6.8e8 CFU/ml were found to cause a shift in effective gate voltage between 36.8 to 97.5mV. Craun, G.F., Statistics of waterborne outbreaks in the US (1920–1980). Waterborne diseases in the United States, 2018: p. 73-159.Ahmed, A., et al., Biosensors for whole-cell bacterial detection. Clinical microbiology reviews, 2014. 27(3): p. 631-646.Strakosas, X., M. Bongo, and R.M. Owens, The organic electrochemical transistor for biological applications. Journal of Applied Polymer Science, 2015. 132(15).Demuru, S., et al. Flexible Organic Electrochemical Transistor with Functionalized Inkjet-Printed Gold Gate for Bacteria Sensing. 20th International Conference on Solid-State Sensors, Actuators and Microsystems & Eurosensors, 2019.Liao, J., et al., Organic electrochemical transistor based biosensor for detecting marine diatoms in seawater medium. Sensors and Actuators B: Chemical, 2014. 203: p. 677-682.Ramuz, M., et al., Monitoring of cell layer coverage and differentiation with the organic electrochemical transistor. Journal of Materials Chemistry B, 2015. 3(29): p. 5971-5977. Figure 1

Size Independent Sensitivity to Biomolecular Surface Density Using Nanoscale CMOS Technology Transistors

We investigate the impact of field effect transistor (FET) transducer size on its sensitivity to biomolecular surface charge density, as would be encountered in a bulk biomolecule concentration measurement. We characterize electrolytically gated transistor sensors fabricated completely using CMOS FinFET technology and analyse a wide range of channel lengths (3000 nm to 450 nm) and widths (1000 nm to 60 nm). We measure saturated density of biomolecules on the transducer surface in order to eliminate variabilities related to association kinetics or affinity effects of the analyte-receptor biomolecules. We use transistor gate voltage shift to quantify the saturated PNA/DNA hybridization signal. We observe no significant dependence of saturated DNA hybridization signal (29.5 ± 3.7 mV) on transistor dimensions for 15 base pair long DNA with an areal density of about 8.3 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> DNA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> measured in 1.5 mM buffer. We also confirm these experimental findings by 3D TCAD simulations for transistor widths down to 10 nm, with less than 6% variation in voltage transduction sensitivity versus width. TCAD analysis was also performed for scaling of the FET height from 250 nm down to 5 nm, resulting in a similar small drop in voltage transduction sensitivity (~ 5.5% for a 250 nm wide FET and ~ 14.6% for a 10 nm wide FET). These results show that the FET sensor downscaling does not significantly improve or degrade voltage transduction sensitivity to a charge (or molecule) density.

Electric field thermopower modulation analyses of the operation mechanism of transparent amorphous SnO2 thin-film transistor

Transparent amorphous oxide semiconductors (TAOSs) based transparent thin-film transistors (TTFTs) with high field effect mobility (μFE) are essential for developing advanced flat panel displays. Among TAOSs, amorphous (a-) SnO2 has several advantages against current a-InGaZnO4 such as higher μFE and being indium-free. Although a-SnO2 TTFT has been demonstrated several times, the operation mechanism has not been clarified thus far due to the strong gas sensing characteristics of SnO2. Here we clarify the operation mechanism of a-SnO2 TTFT by electric field thermopower modulation analyses. We prepared a bottom-gate top-contact type TTFT using 4.2-nm-thick a-SnO2 as the channel without any surface passivation. The effective thickness of the conducting channel was ∼1.7 ± 0.4 nm in air and in vacuum, but a large threshold gate voltage shift occurred in different atmospheres; this is attributed to carrier depletion near at the top surface (∼2.5 nm) of the a-SnO2 due to its interaction with the gas molecules and the resulting shift in the Fermi energy. The present results would provide a fundamental design concept to develop a-SnO2 TTFT.

Attomolar Label-Free Detection of DNA Hybridization with Electrolyte-Gated Graphene Field-Effect Transistors.

In this work, we develop a field-effect transistor with a two-dimensional channel made of a single graphene layer to achieve label-free detection of DNA hybridization down to attomolar concentration, while being able to discriminate a single nucleotide polymorphism (SNP). The SNP-level target specificity is achieved by immobilization of probe DNA on the graphene surface through a pyrene-derivative heterobifunctional linker. Biorecognition events result in a positive gate voltage shift of the graphene charge neutrality point. The graphene transistor biosensor displays a sensitivity of 24 mV/dec with a detection limit of 25 aM: the lowest target DNA concentration for which the sensor can discriminate between a perfect-match target sequence and SNP-containing one.

High-performance interactive analysis of split aptamer and HIV-1 Tat on multiwall carbon nanotube-modified field-effect transistor

Interaction between split RNA aptamer and the clinically important target, HIV-1 Tat was investigated on a biosensing surface transduced by functionally choreographed multiwall carbon nanotubes (MWCNTs). Acid oxidation was performed to functionalize MWCNTs with carboxyl functional groups. X-ray photoelectron spectroscopy analysis had profound ~2.91% increment in overall oxygen group and ~1% increment was noticed with a specific carboxyl content owing to CO and OCO bonding. The interaction between split RNA aptamer and HIV-1 Tat protein was quantified by electrical measurements with the current signal (Ids) over a gate voltage (Vgs). Initially, 34.4 mV gate voltage shift was observed by the immobilization of aptamer on MWCNT. With aptamer and HIV-1 Tat interaction, the current flow was decreased with the concomitant gate voltage shift of 23.5 mV. The attainment of sensitivity with split aptamer and HIV-1 Tat interaction on the fabricated device was 600 pM. To ensure the genuine interaction of aptamer with HIV-1 Tat, other HIV-1 proteins, Nef and p24 were interacted with aptamer and they displayed the negligible interferences with gate voltage shift of 3.5 mV and 5.7 mV, which shows 4 and 2.5 folds lesser than HIV-1 Tat interaction, respectively.

Impact of Heavy Ion Energy on Charge Yield in Silicon Dioxide

The impact of heavy ion energy on charge yield in silicon dioxide was investigated by irradiating power VDMOSFETs with heavy ions at energies from 3 to 25 MeV/amu. The ion charge yield was determined by analyzing the gate voltage shifts of power VDMOSFETs induced by irradiation with 60Co $\gamma $ -rays and several different heavy ion species. A strong dependence of the charge yield on the ion energy is revealed. The charge yield at ion energy of 25 MeV/amu is greater by nearly an order of magnitude than that at the ion energy of 3 MeV/amu. The results are consistent with simulations of heavy ion track structures for low- and high-energy ions.

Model for Charge Accumulation in n- and p-MOS Transistors during Tunneling Electron Injection from a Gate

A quantitative model for charge accumulation in an undergate dielectric during tunneling electron injection from a gate according to the Fowler–Nordheim mechanism is developed. The model takes into account electron and hole capture at hydrogen-free and hydrogen-related traps as well as the generation of surface states during the interaction of holes with hydrogen-related centers. The experimental dependences of the threshold voltage shift and gate voltage shift of n- and p-channel MOS (metal–oxide–semiconductor) transistors on the injected charge in the constant current mode are analyzed based on the model.

Characterization of SiO2/SiC interface states and channel mobility from MOSFET characteristics including variable-range hopping at cryogenic temperature

The characteristics of SiC MOSFETs (drain current vs. gate voltage) were measured at 0.14−350 K and analyzed considering variable-range hopping conduction through interface states. The total interface state density was determined to be 5.4×1012 cm−2 from the additional shift in the threshold gate voltage with a temperature change. The wave-function size of interface states was determined from the temperature dependence of the measured hopping current and was comparable to the theoretical value. The channel mobility was approximately 100 cm2V−1s−1 and was almost independent of temperature.

Модель накопления зарядов в n- и p-МОП-транзисторах при туннельной инжекции электронов из затвора

AbstractA quantitative model for charge accumulation in an undergate dielectric during tunneling electron injection from a gate according to the Fowler–Nordheim mechanism is developed. The model takes into account electron and hole capture at hydrogen-free and hydrogen-related traps as well as the generation of surface states during the interaction of holes with hydrogen-related centers. The experimental dependences of the threshold voltage shift and gate voltage shift of n - and p -channel MOS (metal–oxide–semiconductor) transistors on the injected charge in the constant current mode are analyzed based on the model.

Fabrication of Aptamer-Immobilized Multi-Target Field-Effect Transistor Biosensor for Sensing Mental Stress

The sensing of biomarkers in the body is very effective for diagnosis of disease and monitoring of human health condition. However, blood sampling of mental stress markers is stressful for those who do not feel serious health concerns when they want to check only their own stress level. We need to measure stress markers noninvasively in our daily lives. Saliva is an ideal biological sample to be taken noninvasively. α-amylase and cortisol are known as stress markers in saliva. Since individual differences in stress response are large, it is important to simultaneously detect multiple markers and evaluate the stress level. In this study, we attempted to fabricate two field effect transistor (FET) biosensors using aptamers against α-amylase and cortisol as indexes of stress. Our purpose was to investigate the characteristics of sensing two types of biomarkers, a protein and a steroid molecule, using aptamer-immobilized FET biosensors. The aptamer against α-amylase and the one against cortisol were obtained after eight rounds of selection by SELEX method from an initial DNA library. The gate insulator (SiO2) of FET was modified with 3-aminopropyltriethoxysilane, followed by addition of glutaraldehyde. Each of two aptamers, which are terminated with amino group, was immobilized on the thus-prepared surface of insulator. We detected α-amylase and cortisol in phosphate buffer using the aptamer against α-amylase and the one against cortisol respectively. An increase in the magnitude of response was observed with an increase of α-amylase concentration in the range between 1 nmol/dm3 and 100 nmol/dm3. FET sensor responded as a gate voltage shift to the change in charge on the gate insulator surface. The observed response was suggested the binding of negatively charged α-amylase (isoelectric point = 6.34) to the aptamer. Though cortisol is an electrically neutral molecule, an increase in the magnitude of response was observed with an increase of cortisol concentration in the range between 1 μmol/dm3 and 1 mmol/dm3. It was a similar behavior toward the increase of negative charge or the decrease of positive charge. The phosphate groups of DNA aptamers and their counter ions have an electrical charge on the gate insulator. This suggests that the cortisol aptamer contracted and its negative charge penetrated Debye length or that the counter ion bound to the aptamer beforehand was removed after the association of cortisol. The concentration of α-amylase in saliva is 1 - 3 μmol/dm3. The sensitivity of this α-amylase sensor is at a practical level. The concentration of cortisol in saliva is 3 - 11 nmol/dm3. This cortisol FET sensor requires optimization of aptamer and refinement of the device for practical application. The quantitative detection of α-amylase and cortisol using aptamer-immobilized FET sensor was achieved. This sensor can be sensitive to a structural change of aptamer induced by the target binding. It was demonstrated that the detection of the neutral target molecule is possible. AcknowledgementThis work is supported by Adaptable and Seamless Technology transfer Program through targetdriven R&D (A-STEP), and the Center of Innovation S&T-Based Radical Innovation & Entrepreneurship Program (COI-STREAM), both from the Japan Science and Technology Agency (JST) and the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Fabrication and Operation of Monolayer Mott FET at Room Temperature

Abstract Self-assembled monolayer FET based on a TTF derivative is described (FET = field-effect-transistor, TTF = tetrathiafulvalene). The molecule is anchored on an alumina dielectric layer through covalent bonding of a phosphonic acid linker. A p-type monolayer FET device is achieved and subsequent chemical doping of this monolayer with F4TCNQ dopants results in an ambipolar device. (F4TCNQ = 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane) Several strange behaviors including a gate voltage shift upon doping seem to be consistent with organic monolayer Mott FET. Finally, temperature dependence of the FET performance, which also fit the anticipated Mott FET behavior, is discussed.

Interacting mesoscopic capacitor out of equilibrium

We consider the full nonequilibrium response of a mesoscopic capacitor in the large transparency limit, exactly solving a model with electron-electron interactions appropriate for a cavity in the quantum Hall regime. For a cavity coupled to the electron reservoir via an ideal point contact, we show that the response to any time-dependent gate voltage $V_{\rm g}(t)$ is strictly linear in $V_{\rm g}$. We analyze the charge and current response to a sudden gate voltage shift, and find that this response is not captured by a simple circuit analogy. In particular, in the limit of strong interactions a sudden change in the gate voltage leads to the emission of a sequence of multiple charge pulses, the width and separation of which are controlled by the charge-relaxation time $\tau_{\rm c} = h C_{\rm g}/e^2$ and the time of flight $\tau_{\rm f}$. We also consider the effect of a finite reflection amplitude in the point contact, which leads to nonlinear-in-gate-voltage corrections to the charge and current response.