Quasi-static Capacitance Research Articles

Static and dynamic modeling of organic thin film transistors: effect of channel length on the Mayer–Neldel rule energy and quasistatic capacitances

In this work, pentacene based thin film transistors (TFTs) with different channel lengths (L = 2.5, 5, 10 and 20 μm) have been fabricated and characterized electrically. Exploiting the electrical characteristics, we have analyzed the channel length effect on the key parameters of fabricated TFTs. We found that the performance of pentacene-TFTs was enormously enhanced by the reduction of channel length .We have also examined the influence of contact and channel resistances (RCandRch) on the electrical proprieties of fabricated TFTs, using the transmission line method (TLM). Then, we have modeled the dependence of the total resistanceRTon the gate voltageVGusing the grain boundary trapping Meyer–Neldel rule (GBT-MNR) model and we have successfully reproduced, the output characteristic of pentacene TFTs using the overall resistance extracted from the GBT-MNR model. Finally, in order to investigate the channel length effect on the dynamic behavior of fabricated devices, we have reported a dynamic model based on the quasistatic assumptions which were used for metal-oxide-semiconductor field-effect transistor (MOSFET). Accordingly, we have presented a simple small-signal equivalent circuit to calculate theoretically the capacitances of pentacene-TFTs for different channel lengths.

A Closed-Form Representation of Isofrequency Dispersion Curve and Group Velocity for Surface Waves Supported by Anisotropic and Spatially Dispersive Metasurfaces

A general closed-form representation is introduced for representing the isofrequency dispersion curve (IDC) of an anisotropic, spatially, and frequency dispersive metasurface (MTS) constituted by a dense periodic texture of metallic elements printed on a grounded substrate. The formulation is restricted to printed elements isolated from each other (namely, patches and not slots) whose geometry exhibits at least two axes of symmetry. The expression is valid for the dominant TM surface wave (SW) until the limit of the Floquet-Bloch (FB) region and generalizes our previous formulation to arbitrary direction of propagation. This generalization permits a closed-form representation of the IDCs and of the group velocity as a function of two parameters only; these are the equivalent quasi-static capacitances along the symmetry directions of the geometry. The limit of validity of the closed-form representation has been defined and the formulation has been tested by full-wave analysis. The present formulation simplifies the design of MTS antennas and flat transformation optics devices.

Critical Assessment on Modeling and Design of Nonfaradaic CMOS Electrochemical Sensing

To assist the design process of nonfaradaic electrochemical sensors in realistic biological media, we examine multiple types of sensing operations in polyelectrolytes. By comparing the quasi-static transconductance, impedance spectroscopy, and capacitance–voltage measurements, we assess the physical contributions from the double-layer composition, overall solution resistance, and sensing surface potential under various polyelectrolytic molarities. The mixture of NaCl and MgCl2 is chosen for illustration to provide insight into circuit model parameters for nonfaradaic sensing. Our finding also shed light on the dynamics of double-layer competition and correlation, which is critical for understanding the physical phenomena occurring at the sensing interface, and accurately interpreting sensor data.

Transition Function for Closed-Form Representation of Metasurface Reactance

Metasurfaces are thin metamaterials used for manipulating propagation of plane waves and surface-waves (SWs). They can be characterized by homogenized–boundary conditions, which, in absence of losses, can be represented through an equivalent reactance. In this paper, we introduce a general representation of isotropic frequency-dependent reactance which is valid along the dispersion curve of the relevant TM SW. This representation is written in terms of a transition function derived from a manipulation of the Cardano’s formula for third-degree algebraic equations. Throughout a large portion of the dispersion curve, this transition function depends on one parameter only, which is an equivalent quasi-static capacitance. Approaching the Floquet–Bloch region, where many higher order Floquet modes are excited, two additional parameters should be extracted from the full-wave data to complete the transitional representation of the reactance until the upper boundary of the Brillouin region. The final formula is valid for a generic isotropic reactance and for an anisotropic reactance when the direction of propagation is along a symmetry axis of the unit cell element.

Anomalous Staircase CV Characteristics of InGaSb-on-Insulator FET

Quasi-static capacitance voltage (CV) characteristics of In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> GaxSb-on-insulator field-effect transistor (FET) are investigated using 1-D coupled Schrödinger-Poisson equations. Here, we report for the first time the staircase trend in the CV characteristics of such ultrathin-body FET. This observation is well correlated with the gate-bias-dependent electron concentration in different subbands. It is revealed that the staircase trend tends to disappear as the channel thickness increases above 15 nm. While the channel thickness and doping concentration-dependent shifts in CV curves are found to be significant, the composition-dependent shift is almost negligible.

Switch the n-type to ambipolar transfer characteristics by illumination in n-type pentacene-based organic field-effect transistors

This work investigates the suppression of n-channel and the switch of transfer characteristics (from n-type to ambipolar) by illumination in n-type pentacene-based organic field-effect transistors (OFETs). The illumination outcomes differently on the output characteristics of OFETs, which markedly decreases the magnitude of drain current (n-channel) and shifts the turn-on voltage to a higher positive bias in the n-type regime, but induces the formation of p-channel in the p-type regime. We attribute that the trapped negative charges in the device as induced by illumination electrostatically shield the effective electrical field applied to the gate with source/drain electrodes and modulate the device performance. The result of quasi-static capacitance–voltage measurement agrees well with the modulations of the transfer characteristics for n-type OFETs by illumination. In addition, the de-trapping of charges recovers the n-type only output characteristics of pentacene-based OFETs. This study highlights the unique photo responses of n-type pentacene-based OFETs to the development of phototransistors of distinct output characteristics operated in n- and p-type regime.

AlN passivation by plasma-enhanced atomic layer deposition for GaN-based power switches and power amplifiers

Developing an effective passivation technique for AlGaN/GaN high-electron-mobility transistors (HEMTs) is of primary importance to keep pace with the advancement of GaN-based high-voltage power switches and high-power amplifiers. In this work, effective passivation of AlGaN/GaN HEMT has been developed using a monocrystal-like AlN thin film grown by plasma-enhanced atomic layer deposition (ALD) (PEALD). Significant current collapse suppression and dynamic ON-resistance reduction are demonstrated in the PEALD-AlN-passivated AlGaN/GaN HEMTs, especially under high-drain-bias switching conditions. An output power density of 2.64 W mm−1 at 2 GHz is achieved on the PEALD-AlN-passivated GaN-on-Si HEMTs without the use of the field-plate. The physical mechanism of the PEALD-AlN passivation of AlGaN/GaN HEMTs is systematically investigated with high-frequency and quasi-static capacitance–voltage (C–V) characterizations, and a novel passivation scheme using polarization charges in passivation materials to compensate the slow-response interface trap states is proposed.

Quasi-static capacitance–voltage characteristics of pentacene-based metal–oxide–semiconductor structures

Quasi-static capacitance–voltage (QSCV) measurements are performed on modified metal–oxide–semiconductor capacitors in order to investigate the capacitance characteristics of the active channel region of organic thin-film transistors (OTFTs). Devices are fabricated on highly doped Si wafer with SiO2 as the gate dielectric and pentacene as the organic semiconductor. Hysteresis is observed in the QSCV characteristics, which is explained with the charge trapping in the organic semiconductor. In addition, we show that surface charges generated by Au penetration into pentacene can cause plateaus in the QSCV curves. A behavioral PSpice model is also presented which allows QSCV simulations of OTFTs to be performed.

Quasistatic compact modelling of organic thin-film transistors

A compact model for the quasistatic charge and capacitances in organic thin-film transistor (OTFT) is derived and implemented for the simulation of organic circuits. A model for organic ring oscillator circuits is also developed. Comparing the models to experimental data, the simulation qualitatively reproduces the experimental data for frequency, amplitudes and waveforms. The simulation results indicate that the quasistatic model underestimates the capacitances in organic circuits, and it is shown that the geometrical capacitances originating from layout and gate overlap dominate in organic ring oscillators.

Characterization of gold nanodots arrangements in SiO2/SiO2+Au nanostructured metamaterials

Effective thermoelectric materials have a low thermal conductivity and a high electrical conductivity. For this study, we have prepared a thermoelectric generator device of SiO2/SiO2+Au multi-nano-layered thin film systems using ion beam-assisted deposition followed by 5 MeV Si ion bombardment. The ion bombardment causes the Au atoms to nucleate into metallic nanoclusters. However, as the kinetic energy of the Si ions decreases with the depth of the sample, so does the electronic stopping power responsible for the Au nucleation. This produces variations in the size and spacing of the nanoclusters. Here, we are investigating the effects of the size and arrangement variations within the device on the electrical and thermal transports within the system. We characterized the thin film system, using I–V characterization, conductance measurement, quasi-static capacitance, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy.

Generation of very fast states by nitridation of the SiO2/SiC interface

Fast states at SiO2/SiC interfaces annealed in NO at 1150–1350 °C have been investigated. The response frequency of the interface states was measured by the conductance method with a maximum frequency of 100 MHz. The interface state density was evaluated based on the difference between quasi-static and theoretical capacitances (C−ψS method). Very fast states, which are not observed in as-oxidized samples, were generated by NO annealing, while states existing at an as-oxidized interface decreased by approximately 90%. The response frequency of the very fast states was higher than 1 MHz and increased when the energy level approaches the conduction band edge. For example, the response frequency (time) was 100 MHz (5 ns) at EC−ET = 0.4 eV and room temperature. The SiO2/SiC interface annealed in NO at 1250 °C showed the lowest interface state density, and NO annealing at a temperature higher than 1250 °C is not effective because of the increase in the very fast states.

Exact extraction method of trap densities at insulator interfaces using quasi-static capacitance–voltage characteristics and numerical solutions of physical equations

An exact extraction method of trap densities at insulator interfaces (Dit) has been developed using quasi-static capacitance–voltage (C–V) characteristics and numerical solutions of physical equations. First, the surface potential (ϕs) is calculated from the C–V characteristic by applying Q=CV to the insulator. Next, the flat-band voltage (Vfb) is determined by utilizing the fact that the total change of ϕs is equal to the bandgap energy (Eg). Subsequently, the electric potential (ϕ), electron density (n), and hole density (p) are calculated in the entire semiconductor by numerically solving the Poisson equation and carrier density equations so that the calculated surface potential (ϕs′) is equal to the measured ϕs. Finally, Dit is extracted by applying Gauss’s law to the insulator interface. Dit at an interface between a SiNx film deposited at low temperature and a Si wafer is extracted as an example.

Accurate evaluation of interface state density in SiC metal-oxide-semiconductor structures using surface potential based on depletion capacitance

We propose a method to accurately determine the surface potential (ψS) based on depletion capacitance, and the interface state density (DIT) was evaluated based on the difference between quasi-static and theoretical capacitances in SiC metal-oxide-semiconductor capacitors (C−ψS method). We determined that this method gives accurate values for ψS and DIT. From the frequency dependence of the capacitance measured at up to 100 MHz, a significant fast-interface-state response exists at 1 MHz, which results in the overestimation of ψS if it is determined based on the flatband capacitance at 1 MHz. The overestimation of ψS directly affects the accuracy of the energy level. DIT at a specific energy level is underestimated by the overestimation of ψS. Furthermore, the fast interface states that respond at 1 MHz cannot be detected by the conventional high(1 MHz)-low method. The C−ψS method can accurately determine the interface state density including the fast states without high-frequency measurements.

Transition temperature from band to hopping direct current conduction in crystalline semiconductors with hydrogen-like impurities: Heat versus Coulomb attraction

For nondegenerate bulk semiconductors, we have used the virial theorem to derive an expression for the temperature Tj of the transition from the regime of “free” motion of electrons in the c-band (or holes in the υ-band) to their hopping motion between donors (or acceptors). Distribution of impurities over the crystal was assumed to be of the Poisson type, while distribution of their energy levels was assumed to be of the Gaussian type. Our conception of the virial theorem implementation is that the transition from the band-like conduction to hopping conduction occurs when the average kinetic energy of an electron in the c-band (hole in the υ-band) is equal to the half of the absolute value of the average energy of the Coulomb interaction of an electron (hole) with the nearest neighbor ionized donor (acceptor). Calculations of Tj according to our model agree with experimental data for crystals of Ge, Si, diamond, etc. up to the concentrations of a hydrogen-like impurity, at which the phase insulator-metal transition (Mott transition) occurs. Under the temperature Th ≈ Tj /3, when the nearest neighbor hopping conduction via impurity atoms dominates, we obtained expressions for the electrostatic field screening length Λh in the Debye-Hückel approximation, taking into account a nonzero width of the impurity energy band. It is shown that the measurements of quasistatic capacitance of the semiconductor in a metal-insulator-semiconductor structure in the regime of the flat bands at the temperature Th allow to determine the concentration of doping impurity or its compensation ratio by knowing Λh.

Design and Characterization of Novel Millimeter-Wave MEMS CPW Transmission Lines

The design and development of novel microelectromechanical systems’ (MEMS) coplanar waveguide (CPW) transmission lines, using microshield and groove, are presented in the paper to operate between 5–60 GHz. The quasi-static capacitances of CPW are calculated using the conformal mapping technique to express the propagation properties, i.e., the characteristic impedance and effective permittivity. Simulation results have shown a considerable loss reduction to levels that compare favorably with the conventional CPW. These transmission lines can be widely used in the development of phase shifters, filters, and antennas, because of their advantages in loss reduction and improvement in the performance.

Comparative study of quasi-static and normal capacitance–voltage characteristics in amorphous Indium-Gallium-Zinc-Oxide thin film transistors

Capacitance–voltage (C–V) characteristics of amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) thin film transistors (TFTs) are comparatively investigated with two different measurement configurations. Normal gate-to-source/drain (S/D) C–V and quasi-static C–V curves are employed to characterize physical mechanisms with equivalent circuit models for a-IGZO TFTs. The difference between the normal C–V and the quasi-static C–V (QSCV) characteristics is investigated by the dependence on the gate voltage (VG), measurement configuration, and optical illumination. The discrepancy is analyzed to be due to a high hole barrier in the S/D contact region and a slow response of active bulk charges (Qloc and Qfree) in the a-IGZO active layer.

Quasi-static modelling of carbon nanotube interconnects for gigahertz applications

Quasi-static numerical analysis of carbon nanotube (CNT) interconnects using meshless method is presented. Reproducing kernel particle method (RKPM) approach – a meshless technique is more efficient as compared with other regular discretisation approaches because of its adaptive nature. Numerical results for characteristic impedances (Zo) of CNT interconnects obtained using quasi-static capacitance are presented. Validation is done by comparing the obtained results with Luttinger liquid theory model and good agreement is obtained. The radio frequency performance of the CNT interconnects is completed using Agilent ADS with the quasi-static parameters obtained through RKPM. The simulation results in a return loss of −5.25 dB for a single CNT and −10.64 dB for CNT bundles accounting a power loss of 8.67% making it suitable for gigahertz applications. CNT bundles are the preferred candidates as interconnects to reduce the power loss in the circuit.

Experimental Investigation of Effect of Channel Doping Concentration on Random Telegraph Signal Noise

Random telegraph signal (RTS) noise has become one of the most important problems in the continuous scaling down of field-effect transistors (FETs). In this study, we investigate experimentally the relationship between RTS amplitude and channel doping concentration (NA), which is a key parameter related to threshold voltage adjustment and short-channel effects, in 393,216 n-type FETs by a high-speed measurement method. We demonstrate that NA has a significant effect on RTS amplitude even at approximately the same interface and bulk trap densities of the gate insulator films, which are evaluated by the charge pumping method, by the quasi-static capacitance–voltage method, and on the basis of 1/f noise characteristics. An increase in RTS amplitude may arise from a more advanced channel nonuniformity as NA increases.

Passivation by N Implantation of the SiO2/SiC Acceptor Interface States: Impact on the Oxide Hole Traps and the Gate Oxide Reliability

AbstractThis study compares p-MOS capacitors fabricated on N+ implanted and on virgin 4H-SiC. The former sample have N at the SiO2/SiC interface, the latter have not. To investigate the presence of deep and shallow hole traps at the SiO2/SiC interface, high frequency and quasi-static capacitance voltage measurements under dark have been compared for bias sweeping from accumulation to depletion and from depletion to accumulation, the latter after white light illumination. The presence of N has an effect on the density of the shallow donor like traps but none effect on the deep ones. The positive charge trapped in the oxide and/or at the oxide interface after equivalent tunneling hole injection have been compared and are equivalent. Time dependent dielectric breakdown tests have been compared too. The oxide grown on N+implanted SiC broken at lower electric field.

Analytical expression for the quantum dot contribution to the quasistatic capacitance for conduction band characterization

This paper demonstrates an analytical expression for the quasistatic capacitance of a quantum dot layer embedded in a junction, where the reverse bias is used to discharge the initially occupied energy levels. This analysis can be used to determine the position and the Gaussian homogeneous broadening of the energy levels in the conduction band, and is applied for an InGaAs/GaAs quantum dot structure grown by metal organic chemical vapor deposition. It is shown that the Gaussian broadening of the conduction band levels is significantly larger than the broadening of the interband photoluminescence (PL) transitions involving both conduction and hole states. The analysis also reveals a contribution from the wetting layer both in PL and modeled C-V profiles which is much stronger than in typical molecular beam epitaxy grown dots. The presence of a built-in local field oriented from the apex of the dot toward its base, contrary to the direction expected for a strained dot with uniform composition (negative dipole), is also derived from fitting of the C-V experimental data.