Pinch-off Region Research Articles

Improved Temperature-Scalable DC model for SiC power MOSFET including Quasi-Saturation effect

In this paper, accurate temperature-dependent static model for Silicon-Carbide (SiC) power MOSFET is presented. The proposed model is formed by two equations relating to linear and saturation operating regions. In this model, new formalism of the saturation drain current is introduced to consider the peculiar features observed in the I-V static characteristics of the SiC power MOSFET: a) moderate inversion region, or region of low gate voltages and b) quasi-saturation region, region of high gate voltages at which the drain current becomes less sensitive to the increase of gate voltage. In addition, the model captures with high-precision the transition region between linear and saturation region, pinch-off region, noticed in the output characteristics of the SiC power MOSFETs. It will be shown that the model equations ensure continuity and smooth transition between all operating regions. Temperature scaling of the model is carried out by its temperature scaling parameters. The proposed compact model is simple and efficient using reduced number of technology independent parameters. Simple parameter extraction procedure is described that uses an optimizer algorithm based on good experimental initial guess. Excellent agreement is obtained by comparing model to TCAD simulation and device measurement.

Effective gate length determination of AlGaN/GaN HEMTs from direct measurements of thermal signatures

Determining junction temperature and two-dimensional temperature profile is critical for high-power GaN-based high electron mobility transistors to optimize performance, improve device reliability, and better thermal management. Here, we have demonstrated that resistance temperature detectors of the same material as the gate contact delineated between gate-to-source and gate-to-drain regions can accurately profile the temperature along the channel. The temperature profile is asymmetric and skewed toward the drain side, and the degree of asymmetry is used to determine the effective gate length experimentally. A two-dimensional thermodynamic model along with drift-diffusion transport matches well with the experimental data, validating the temperature profile and effective channel length extraction under bias. The vertical depth profiling of the temperature is also determined by identifying the isothermal profile through the resistance temperature detectors. The isothermal lines are largely circular in the GaN region from isotropic two-dimensional heat diffusion, with the pinch-off region acting as a heating filament. The isothermal circular profile turns elliptical in the SiC substrate due to its higher thermal conductivity.

Characterization of larger-single-Lorentzian noise deviated from 1/f characteristics detected by a power-spectral-density-integration method

Larger-single-Lorentzian noise that deviates significantly from a 1/f characteristic generated by MOSFET is quantitatively analyzed using a unique evaluation method, which integrates power spectral density over a predetermined frequency band. As a result, the appearance of larger-single-Lorentzian noise is stochastically present and for low gate voltage. According to drain voltage dependence, it is suggested that the origin of larger-single-Lorentzian noise is in a pinch-off region and results from border traps in a gate oxide film by which hot carriers are trapped/de-trapped.

Nonconducting RF and DC Hot Carrier Stresses in 14/16-nm FinFETs for RF Power Amplifiers

Hot carrier reliability under nonconducting (NC) RF and dc stresses is measured and modeled on 14/16-nm FinFETs used for RF PAs. The impact of stress on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textbf{I}-\textbf{V}$</tex-math> </inline-formula> and RF parameters is examined. RF parameter stress response suggests that degradations are located near the drain end of the channel within the pinch-off region. For classic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{V}_{\text{gs}}=\text{0}$</tex-math> </inline-formula> V OFF-state RF stress, quasi-static-approximation (QSA) significantly underestimates degradation, necessitating measurement-based lifetime modeling. At near-threshold <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{V}_{\text{gs}}$</tex-math> </inline-formula> , a condition of interest for our PAs, the degradation shows significant die-to-die variations dominated by variations of the subthreshold channel current that initiates the hot carriers. Modeling accounting for the subthreshold channel current variations shows that the near-threshold RF stress is approximately quasi-static. The results show that these FinFETs provide enough margins against NC RF stress for the intended PA applications.

DC sputtered ZrO2/Zn(1−x)Sn(x)O thin-film transistors and their property evaluation

A bottom gate staggered 30 nm Zn(1−x)Sn(x)O (x = 0.14) (TZO)-based thin-film transistors (TFTs) were fabricated using DC magnetron reactive sputtering method. Highly transparent 120 nm dc sputtered ZrO2 was used as a gate dielectric. The oxygen flow rate was varied from 20 to 24% during channel layer (TZO) coating and its effect on structural, morphological, optical, chemical, and electrical parameters were systematically studied. A nano scale roughness was noticed by atomic force microscopy (AFM), and ultra-smooth nature in root mean square roughness (RMS) was observed with an increment in the oxygen flow ratio. The increase in the oxygen-related defects with increase in the oxygen flow ratio in channel layer was evident from X-ray photoelectron spectroscopy (XPS). The electrical characterization of gate dielectric was carried out for Al–ZrO2–Al structure. The high capacitance density ~ 121.9 nF/cm2 for 120 nm ZrO2 was obtained from the capacitance–voltage (C–V) measurement. The fabricated TFTs operated in n-channel depletion mode and indicated pinch-off region at lower source–drain voltages. In addition, the transfer characteristics of TFTs confirmed Ion/Ioff ratio of 105, with a field effect mobility of 23 cm2/V.s. This low temperature processed TFT unlocks the possibility of use in the next generation foldable display technology.

An Analytical Model for Weak Avalanche Induced Kink Effect of PDSOI nMOSFETs

Silicon-on-insulator (SOI) devices and circuits are widely used in semiconductor industry for its superior natures on integration density, isolations degree. Inefficient body contact of partially-depleted SOI (PDSOI) devices cannot evacuate excess carriers which transported from the weak avalanche multiplication of pinch-off region at high drain voltage bias. These accumulated carriers elevate the internal body potential and trigger electrical effects namely the body bias effect of threshold voltage and the turn-on of parasitic bipolar junction transistor (BJT). A comprehensive analytical model which include all these mechanisms are proposed to describe this kink effect. This model is constructed by an equivalent circuit and five strong nonlinearly coupled equations to describe the electrical behaviors. To solve these equations, a C <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$++$</tex-math> </inline-formula> Qt-based graphical interface software is developed for the parameter extraction of the model. The fixed point iteration algorithm are adopted for the solution of equations. The effectiveness and accuracy of this model are verified by the T-gate shaped PDSOI nMOSFETs of 130 nm manufacturing process node. It can predict the electrical behaviors of kink effect. This is meaningful for the circuit simulation of PDSOI platform.

An analytical model for the body contact current of PDSOI nMOSFETs

The impact ionization of pinch-off region and the parasitic BJT structure are considerable issues for PDSOI nMOSFETs at high drain voltage bias. The measured body contact current can reflect both two mechanisms. An analytical model with impact ionization and parasitic BJT considered are proposed in this work for the body contact current modeling. This model is presented as an equivalent circuit and 5 joint nonlinear equations. The fixed-point method is applied to solve these joint equations and a graphical C++ Qt based software are developed for the model parameters extraction. The validity and accuracy of model are verified by the PDSOI nMOSFET of 130 nm process node. This model is meaningful for the electrical behavior prediction of PDSOI nMOSFETs with large external bias range.

Deformation and necking of liquid droplets in a magnetic field

Pendant droplets of water and paramagnetic solutions are studied in the presence of uniform and nonuniform magnetic fields produced by small permanent magnet arrays, both in static conditions and during dynamic pinch-off. Static measurements of the droplet shape are analyzed in terms of an apparent surface tension γapp or an effective density ρeff. The change of surface tension of deionized water in a uniform field of 450 mT is insignificant, 0.19 ± 0.21 mNm−1. Measurements on droplets of compensated zero-susceptibility solutions of Cu2+, Mn2+, and Dy3+, where the shape is unaffected by any magnetic body force, show changes of surface tension of about −1% in 500 mT. Magnetic field gradients of up to 100 T2 m−1 deform the droplets and lead to changes of ρeff that are negative for diamagnetic solutions (buoyancy effect) and positive for paramagnetic solutions. The droplet profile of strongly paramagnetic 0.1 molar DyCl3 solution is analyzed, treating the nonuniform vertical field gradient as a spatial variation of gravity. The influence of Maxwell stress on the droplet shape is discussed. In dynamic measurements, the droplet shape at pinch-off is recorded by high-speed photography and analyzed in terms of a relative change of dynamic surface tension in the presence of a magnetic field. The surface-tension-dependent prefactor of the scaling law that governs the pinch-off dynamics shows no difference for pure water or 0.11 M DyCl3 solutions in the field. The nonuniform field has no influence in the pinch-off region because the filament diameter is much less than the capillary length.

Analytical model of the drain current in amorphous silicon junction field effect transistors

This paper presents an analytical model of a hydrogenated amorphous silicon (a-Si:H) junction field effect transistor (JFET) based on a p-type/intrinsic/n-type stacked structure. The p-doped layer is connected to the transistor gate electrode, while the n-layer acts as the device channel. The analysis shows the effect of the geometrical and physical parameters of the intrinsic and n-doped layers on the transistor characteristics. In particular, the intrinsic layer thickness plays a central role in determining the depletion region of the n-channel and, as a consequence, the device threshold voltage. The drain current behavior achieved with a modeled parametric analysis is in very good agreement with the experimental drain current measured on fabricated JFET, both in triode and pinch-off regions. This demonstrates the model feasibility as an effective tool to design thin film electronic circuit as a sensor signal amplifier based on a-Si:H p-i-n junction.

Leaky-dielectric phase field model for the axisymmetric breakup of an electrified jet

We explore the axisymmetric breakup of electrified jets under wide ranges of the Taylor number (\ensuremath{\Gamma}), the Ohnesorge number (\ensuremath{\mathit{O}}h) and the wavenumber (\ensuremath{\mathit{k}}) which are relevant to electrosprays operating in the cone-jet mode. We extend prior numerical and experimental results by developing a phase field EHD model to account for the behavior of the jet after the pinch-off. For \ensuremath{\mathit{O}}h &gt; 1, a much broader distribution of droplet size is observed which originates from the pinch-off region of the jet. For \ensuremath{\Gamma} \ensuremath{\ge} 1.5, the primary droplets are always unstable irrespective of the viscosity level.

Negative-to-Positive Differential Resistance Transition in Ferroelectric FET: Physical Insight and Utilization in Analog Circuits.

In this article, for the first time, we explained a detailed physical insight for negative differential resistance (NDR) to positive differential resistance (PDR) transition in a ferroelectric (FE)-based negative capacitance (NC) FET and also its dependence on the device terminal voltages. Using extensive well-calibrated TCAD simulations, we have investigated this phenomenon on fully depleted silicon on insulator (FDSOI)-NCFET. The NDR-to-PDR transition occurs due to FE layer capacitance changes from a negative to positive state during channel pinchoff. This, in turn, results in a valley point in the output characteristic ( IDS - VDS ) at which the output resistance is infinite. We also found that we could alter the valley point location by modulating the vertical electric field through the FE layer in the channel pinchoff region using body bias ( VBB ). The interface oxide charges also impacted the NDR to PDR transition, and a positive interface charge results in faster NDR to PDR transition. Furthermore, we have utilized the modulation in the NDR-to-PDR transition due to VBB for designing a current mirror. Results show that the output current ( IOUT ) variation due to VDS reduces from ~8% to ~2% with VBB . We have also designed a single-stage common source (CS) amplifier and provided design guidelines to achieve a higher gain in the NDR region. The results obtained using a small-signal model of the FDSOI-NCFET demonstrate that ~25% higher gain can be achieved with the discussed design guidelines in the NDR region compared to the transition region of IDS - VDS . We have also explored the device scaling effect on the amplifier gain and found that ~ 2.23× gain can be increased with smaller channel length and higher device width.

Enhancing Photovoltage of Metal-Insulator-Semiconductor (MIS) Photoanode By Coordination Polymer Via Facile Chemical Deposition

Photoelectrochemical (PEC) water splitting is a promising route to convert intermittent sunlight into chemical fuels, which can be stored and used on-demand to produce electricity. One of the remaining challenges for the development of efficient PEC water splitting systems is related to the photoanode, which requires a high-performance light absorber with an excellent catalyst to drive the OER that needs high potential [1]. The metal-insulator-semiconductor (MIS) architecture employing Si as the photoanode is an attractive system due to the wide light absorption range of Si (Eg 1.1 eV) and high OER catalytic activity of metal [2-3]. For the latter, metal nanoparticles including Co and Ni can be dispersed on the semiconductor surface via electrodeposition in high-performance Si MIS photoanodes [2]. Furthermore, surrounding the metal nanoparticles and the uncoated Si with a higher-work-function material leads to significant improvement of photovoltage due to the pinch-off effect [4]. Up to this point, the surrounding layer is generally the oxidized phase of the base metal, such as its oxide or oxyhydroxide, and the fabrication techniques used to introduce the surrounding layer are limited to a few choices, i.e., electrodeposition, photodeposition, or annealing methods [4]. Copper(I) thiocyanate (CuSCN) is a coordination polymer which can be prepared by a simple chemical deposition through conversion of Cu [6]. Because Cu and CuSCN have work functions of 4.5-4.9 [7] and 5.4 eV [8], respectively, the Cu/CuSCN junction on n-Si is expected to develop the pinch-off region and increase the performance of Si/SiOx/Cu photoanode. In this presentation, we aim to highlight two key developments of Cu-based MIS Si photoanodes prepared by facile methods. Firstly, we employed electrodeposition to coat 30-nm Cu nanoparticles on n-Si, resulting in a photoanode with a low OER onset potential of 1.37 V vs reversible hydrogen electrode (RHE). Secondly, we added CuSCN on the n-Si/SiOx/Cu photoanode via a simple chemical deposition in an aqueous bath containing ammonium thiocyanate (NH4SCN) and ethylenediaminetetraacetic (EDTA). The presence of CuSCN on n-Si/SiOx/Cu was confirmed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Raman spectroscopy. The optimal n-Si/SiOx/Cu/CuSCN with a short CuSCN deposition time showed a 100-mV improvement in the onset potential relative to the unmodified n-Si/SiOx/Cu. Based on open-circuit-potential (OCP) measurements, the CuSCN-modified photoanode exhibited a high photovoltage of 460 mV. We believe that the findings can provide a new strategy for further surface modification of PEC devices to achieve efficient and low-cost solar harvesting technology.[1] Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110 (11), 6446–6473.[2] Loget, G. Water Oxidation with Inhomogeneous Metal-Silicon Interfaces. Curr. Opin. Colloid Interface Sci. 2019, 39 (l), 40–50.[3] Aroonratsameruang, P.; Pattanasattayavong, P.; Dorcet, V.; Mériadec, C.; Ababou-Girard, S.; Fryars, S.; Loget, G. Structure–Property Relationships in Redox-Derivatized Metal–Insulator–Semiconductor (MIS) Photoanodes. J. Phys. Chem. C 2020, 124 (47), 25907–25916.[4] Laskowski, F. A. L.; Oener, S. Z.; Nellist, M. R.; Gordon, A. M.; Bain, D. C.; Fehrs, J. L.; Boettcher, S. W. Nanoscale Semiconductor/Catalyst Interfaces in Photoelectrochemistry. Nat. Mater. 2020, 19 (1), 69–76.[5] Lee, S. A.; Choi, S.; Kim, C.; Yang, J. W.; Kim, S. Y.; Jang, H. W. Si-Based Water Oxidation Photoanodes Conjugated with Earth-Abundant Transition Metal-Based Catalysts. ACS Mater. Lett. 2020, 2 (1), 107–126.[6] Xu, J.; Xue, D. Fabrication of Upended Taper-Shaped Cuprous Thiocyanate Arrays on a Copper Surface at Room Temperature. J. Phys. Chem. B 2006, 110 (23), 11232–11236.[7] Gartland, P. O.; Berge, S.; Slagsvold, B. J. Photoelectric Work Function of a Copper Single Crystal for the (100), (110), (111), and (112) Faces. Phys. Rev. Lett. 1972, 28 (12), 738–739.[8] Worakajit, P.; Hamada, F.; Sahu, D.; Kidkhunthod, P.; Sudyoadsuk, T.; Promarak, V.; Harding, D. J.; Packwood, D. M.; Saeki, A.; Pattanasattayavong, P. Elucidating the Coordination of Diethyl Sulfide Molecules in Copper(I) Thiocyanate (CuSCN) Thin Films and Improving Hole Transport by Antisolvent Treatment. Adv. Funct. Mater. 2020, 30 (36), 2002355.

Pinch-off process of Burke–Schumann flame under acoustic excitation

The pinch-off phenomenon in a flame refers to flame separation after reaching the critical frequency and amplitude under acoustic excitation. The pinch-off flame consists of the main flame attached to the nozzle and the detached pocket flame, and it is reported that more pollutants are discharged as the residence time of the pocket gas (i.e., the hot products) increases. This study focuses on the mechanism of the pinch-off phenomenon. The flame structure and flow field were analyzed by the simultaneous measurement of OH planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV). Due to a lack of previous research on the necessary conditions for a pinch-off flame, this study mapped the conditions for pinch-off in terms of the forcing frequency and velocity perturbation intensity (u′/u¯) under acoustic excitation. The flame height in the pinch-off region was defined in two ways: the main flame height from the nozzle tip to the tip of the attached flame, and the total flame height including the detached pocket. Strouhal number was calculated based on these two definitions of flame height. In addition, the vortex flow and reverse flow under external forces were measured through analyzing the non-reacting and reacting flow fields. Flame deformation due to the entrainment of air in the vortex flow and the reverse flow was confirmed, and it was found to correlate with the pinch-off phenomenon. To analyze this observation quantitatively, time-dependent strain rate analysis was performed, and a high strain was confirmed to be present upon flame extinction. Therefore, these results demonstrate that pinch-off is correlated with the vortex flow and the reverse flow, and caused by a high strain rate acting on the flame surface.

Physics &amp; Modeling of Ambipolar Snapback Behavior in Gate Grounded NMOS

This paper models first snapback ambipolar action in NMOS, when subjected to high current stress across the drain terminal. We analyze 2 − D ambipolar current in Gate Grounded NMOS (GGNMOS) devices close to pinch-off region, which is related with S shaped I-V characteristics under highly injected avalanche generated carriers. Furthermore, the first snap-back phenomenon in GGNMOS owing to surface bipolarity is investigated. Due to ambipolar current flow and barrier lowering at source-body junction a novel model of surface bipolar activity has been presented. An analytical model for the surface electrostatics is provided and further compared with 2 − D TCAD device simulation results. Moreover, the electron and hole coupling is reported through a simplified model for a complex and distributed 2 − D parasitic bipolar transistor.

A nonlinear empirical I/V model for GaAs and GaN FETs suitable to design power amplifiers

This article presents an improved nonlinear empirical I/V model suitable for GaAs and GaN FETs. The new drain-to-source current formulation accurately represents the symmetric and the asymmetric bell-shaped transconductance (gm) for all VDS values. Besides modeling with high accuracy the I/V characteristic, the proposed model can fit the first and second derivatives of the transconductance, from the ohmic to the saturation region, including the pinch-off region. The high correlation between experimental and simulated data of the I/V curves of GaAs and GaN FETs, ACPR, load-pull, and a class-J power amplifier designed in S-band corroborates the usefulness of the proposed model, considering only the static I/V model as the main nonlinear element of the electrical equivalent circuit model of the transistor.

Impacts of Stress Voltage and Channel Length on Hot-Carrier Characteristics of Tunnel Field-Effect Thin-Film Transistor

Hot-carrier stress (HCS) characteristics of polycrystalline-silicon (poly-Si) tunnel field-effect thin-film transistors are investigated and compared with the conventional poly-Si thin-film transistors (TFTs). After HCS with ${V}_{{\text {DS}}} = {2}{V}_{{\text {GS}}} = {20}$ V for 1000 s, poly-Si TFT exhibits significant ON-state current ( ${I}_{{\text {on}}}{)}$ degradation ~85% and two-step subthreshold swing (SS) behavior due to the serious lattice damage between the pinch-off point and the drain. For the poly-Si tunnel field-effect transistor (TFET), less ${I}_{{\text {on}}}$ degradation ~70% is observed. In addition, the stress drain voltage ( ${V}_{{\text {DS}}}$ ) effect of the HCS is also studied. When the stress ${V}_{{\text {DS}}}$ is increased from 10 to 15 V, the poly-Si TFETs exhibit more obvious stressed ${I}_{{\text {on}}}$ degradation (~24.4%) than that of poly-Si TFTs (~6.5%). When the stress ${V}_{{\text {DS}}}$ is further increased from 15 to 20 V, the poly-Si TFETs exhibit less stressed ${I}_{{\text {on}}}$ degradation (~62.4%) than that of poly-Si TFTs (~85%). Because the increase in the stress drain voltage is dropped in the pinch-off region of the poly-Si TFETs after the pinch-off phenomenon occurs, resulting in less impact on the stress field at the tunnel junction. When the channel length is decreased, the results show that the HCS effect of poly-Si TFETs is insensitive to the channel length, and the poly-Si TFTs show more serious electrical degradation and breakdown behavior with the scaling down of the channel length. The stronger HCS immunity of poly-Si TFETs would be an important advantage to replacing poly-Si TFTs for applications of 3-D integrated circuits.

Bipolar effects in snapback mechanism in advanced n-FET transistors under high current stress conditions

This work models high current snapback behavior in n-FET transistors with bottom body contact under high current stress at the drain for ZRAM (Zero capacitor RAM). We analyze 2D current flow in n-FET near the pinch-off region and relate the results to the S-shaped snapback characteristics under high injection avalanche generated carriers conditions. The role of surface bipolar effects on the first snapback phenomenon in the GG-NMOS (Gate-Grounded NMOS) is investigated. A novel physical insight of the bipolar activity is modeled through current flow and barrier lowering at the source-substrate junctions. Moreover, the coupling of electron and hole injection is described. Finally, a model for surface potential inside the substrate before and after snapback is derived and compared with TCAD simulation results. A very good agreement is observed between the model and the TCAD results.

Towards manufacturing high uniformity polysilicon circuits through TFT contact barrier engineering

The predicted 50 billion devices connected to the Internet of Things by 2020 has renewed interest in polysilicon technology for high performance new sensing and control circuits, in addition to traditional display usage. Yet, the polycrystalline nature of the material presents significant challenges when used in transistors with strongly scaled channel lengths due to non-uniformity in device performance. For these new applications to materialize as viable products, uniform electrical characteristics on large areas will be essential. Here, we report on the effect of deliberately engineered potential barrier at the source of polysilicon thin-film transistors, yielding highly-uniform on-current (<8% device-to-device, accounting for material, as well as substantial geometrical, variations). The contact-controlled architecture of these transistors significantly reduces kink effect and produces high intrinsic gain over a wide range of drain voltage (2–20 V). TCAD simulations associate critical grain boundary position and the two current injection mechanisms in this type of device, showing that, for the geometry considered, the most unfavorable location is ~150 nm inside the source area. At this point, grain boundary contributes to increasing the resistance of the source pinch-off region, reducing the current injection from the bulk of the source area. Nevertheless, the effect is marginal, and the probability of a grain boundary existing at this position is low. This new understanding is instrumental in the design of new signal conversion and gain circuits for flexible and low-power sensors, without the need for complex compensation methods.

Droplet pinch-off with pressure fluctuations

Dynamics of Newtonian fluid pinch-off is universal, excluding the possibility of manipulating pinch-off behavior by varying initial and boundary conditions which is desirable in applications such as inkjet printing and microfluidics. Here we show that the dynamics of two-fluid pinch-off with disturbed inlet pressure (such as the profile of the conical liquid neck, cone slope, and neck thinning rate) depends on initial perturbations. The nonuniversality arises from pressure-fluctuation-induced nonlocal flow velocity that stretches the axial length scale of the pinch-off region. We renormalize the disturbed pinch-off using the linear ratio, (1 + βε), with β being the empirical constant and ε the dimensionless pressure fluctuation. We further apply the pressure fluctuation in engineering pinch-off where the disturbed and undisturbed systems have identical pinch-off dynamics but distinct material properties. Our results could provide useful guidelines for controlling the breakup of liquid threads with extreme physical properties, such as ultrahigh viscosity and ultralow interfacial tension, in inkjet printing and microfluidics for a range of applications.

Visualization of channel formation of organic field-effect transistors by scanning photocurrent microscopy

Abstract Scanning modulated photocurrent microscopy by means of chopped red laser light for local excitation is employed in organic field-effect transistors having pentacene as active layer. The spatial modulated photoresponse presents a gradual increase from the source-to drain-side with increasing the negative gate-source voltage and undergoes enhancement under additional full device blue bias-bandgap illumination. The electric field-assisted exciton dissociation mechanism alone cannot explain the photoresponse enhancement under blue bias-light. This enhancement can be explained by considering that the electrons from the laser light-created triplet excitons have a large probability to be transferred to trapped holes accumulated at the pentacene-insulator interface creating mobile holes. In the framework of this interpretation, an increased local photoresponse reveals the channel region with upwards band bending and accumulated trapped holes from holes injected and extra holes created in case where an additional blue bias-light bandgap excitation is employed. Taking advantage of this property, the gradual increase of the spatial photoresponse along the channel, observed during turning-on the transistor with increasing the negative gate voltage, can be used to visualize the growing accumulation channel region from the source-to drain-side against the pinch-off depletion region. Moreover, the spatial photoresponse changes observed during transition from the linear to the saturation regime with increasing the negative drain voltage, can be used to monitor the suppression of the accumulation region and the increase of the electric field in the drain-side.