Electric Field Dependence Of Mobility Research Articles

Parameter extraction using the transfer characteristics of vertically stacked Si nanosheet MOSFETs

We present a critical assessment and discussion of the presence of parasitic source-and-drain series resistance and normal electric field-dependent mobility degradation in undoped Si nanosheet MOSFETs. A simple explicit Lambert W function-based closed-form model, continuously valid from sub-threshold to strong conduction, was used to clearly describe the transfer characteristics. The model was applied to experimental vertically stacked GAA undoped Si nanosheet MOSFETs using phenomenon-related model parameter values extracted from measured data through suitable numerical optimization procedures. The conducted analysis reveals and explains how these two effects produce analogous deleterious consequences on these devices’ transfer characteristics.

Analysis of Current–Voltage Characteristics in Insulating Polymers Using a Bipolar Charge Transport Model

A bipolar charge transport (BCT) model has first been used to compare the current density–voltage characteristics (J–V) for different applied voltage protocols and different times under voltage taking low-density polyethylene (LDPE) as a case study, as a steady state may take a very long time to be reached experimentally. Simulation results highlight the necessity to spend substantial time under voltage to reach a quasi-stable current. In a second part, evolutions of the model in terms of physical processes are proposed, to observe their impact on the current density–electric field (J–E) characteristics, and particularly the appearance of electric field thresholds, as proposed by the space-charge-limited current (SCLC) theory. To do so, different mathematical expressions are proposed for each physical process related to injection, mobility, and trapping. Field-dependent mobility equations allow obtaining electric field threshold values comparable to experimental data, while a constant mobility is the only way to observe a trap-free limit region. Moreover, all parameters linked to trapping are of most importance in the observation of an SCLC characteristic but are not enough to observe a trap-free limit region when the electric field-dependent mobility is considered.

Charge Carriers Density, Temperature, and Electric Field Dependence of the Charge Carrier Mobility in Disordered Organic Semiconductors in Low Density Region

We investigate the transport properties of charge carriers in disordered organic semiconductors using a model that relates a mobility with charge carriers (not with small polarons) hopping by thermal activation. Considering Miller and Abrahams expression for a hopping rate of a charge carrier between localized states of a Gaussian distributed energies, we employ Monte Carlo simulation methods, and calculate the average mobility of finite charge carriers focusing on a lower density region where the mobility was shown experimentally to be independent of the density. There are Monte Carlo simulation results for density dependence of mobility reported for hopping on regularly spaced states neglecting the role of spatial disorder, which does not fully mimic the hopping of charge carriers on randomly distributed states in disordered system as shown in recent publications. In this work we include the spatial disorder and distinguish the effects of electric field and density which are not separable in the experiment, and investigate the influence of density and electric field on mobility at different temperatures comparing with experimental results and that found in the absence of the spatial disorder. Moreover, we analyze the role of density and localization length on temperature and electric field dependence of mobility. Our results also give additional insight regarding the value of localization length that has been widely used as 0.1b where b is a lattice sites spacing.

Coexistence of band-like and thermally activated charge transport through nuclear tunneling effect in organic semiconductors

In this study, the coexistence of band-like and thermally activated charge transport in organic semiconductors is demonstrated through the first-principles calculation of the electric field dependent charge mobilities of TIPS-pentacene, which strongly deviates from the semi-classical Marcus theory. The nuclear tunneling hopping rates simultaneously exhibit band-like and thermally activated conduction for the downward and upward hoppings, respectively, which explains the puzzling observation of the transition from thermally activated transport to band-like transport with increasing field strength. As the hopping rates are very sensitive to the site-energy fluctuations induced by defect configurations, the transition from band-like transport to thermally activated transport with increasing energetic disorder is found. The results suggest that the interplay of the quantum-mechanical effect and disorder is of particular importance in the understanding of the temperature and field dependence of mobility in organic materials.

Insights into the hole transport properties of LiTFSI-doped spiro-OMeTAD films through impedance spectroscopy

Hole transport materials are crucial for efficient charge extraction in perovskite solar cells to achieve high power conversion efficiency and stability. Herein, the hole transport properties of the 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) thin films with a dopant lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) are investigated through impedance spectroscopy. Upon doping, we observe a dispersive hole transport with nearly a 100-fold increase in the hole mobility compared with the pristine spiro-OMeTAD films. The hole mobilities slightly decrease with increasing electric fields for both films, exhibiting a negative electric field dependence of mobility due to the positional disorder. Subsequently, the charge carrier density of the LiTFSI-doped spiro-OMeTAD film is three orders of magnitude higher than that of the pristine film. The LiTFSI dopant induces two different electrical regions in the doped thin film, which can be reflected through impedance spectroscopy. The presented investigation through impedance spectroscopy is of high practical interest for the development of hole transport materials and the optimization of the transport layer doping in perovskite solar cells.

Impact of Device Parameter Variation on the Electrical Characteristic of N-type Junctionless Nanowire Transistor with High-k Dielectrics

Metallurgical junction and thermal budget are serious constraints in scaling and performance of conventional metal-oxide-semiconductor field-effect transistor (MOSFET). To overcome this problem, junctionless nanowire field-effect transistor (JLNWFET) was introduced. In this paper, we investigate the impact of device parameter variation on the performance of n-type JLNWFET with high-k dielectrics. The electrical characteristic of JLNWFET and the inversion-mode transistor of different gate length (L G ) and nanowire diameter (d NW ) was compared and analyzed. Different high-k dielectrics were used to get an optimum device structure of JLNWFET. The device was simulated using SDE Tool of Sentaurus TCAD and the I-V characteristics were simulated using Sdevice Tools. Lombardi mobility model and Philips unified mobility model were applied to define its electric field and doping dependent mobility degradation. A thin-film heavily doped silicon nanowire with a gate electrode that controls the flow of current between the source and drain was used. The proposed JLNWFET exhibits high ON-state current (I ON ) due to the high doping concentration (N D ) of 1 x 10 19 cm -3 which leads to the improved ON-state to OFF-state current ratio (I ON /I OFF ) of about 10% than the inversion-mode device for a L G of 7 nm and the silicon d NW of 6 nm. Electrical characteristics such are drain induced barrier lowering (DIBL) and subthreshold slope (SS) were extracted which leads to low leakage current as well as a high I ON /I OFF ratio. The performance was improved by introducing silicon dioxide (SiO 2 ) with high-k dielectric materials, hafnium oxide (HfO 2 ) and silicon nitrate (Si 3 N 4 ). It was found that JLNWFET with HfO 2 exhibits better electrical characteristics and performance.

Impact of Device Parameter Variation on the Electrical Characteristic of N-type Junctionless Nanowire Transistor with High-k Dielectrics

Metallurgical junction and thermal budget are serious constraints in scaling and performance of conventional metal-oxide-semiconductor field-effect transistor (MOSFET). To overcome this problem, junctionless nanowire field-effect transistor (JLNWFET) was introduced. In this paper, we investigate the impact of device parameter variation on the performance of n-type JLNWFET with high-k dielectrics. The electrical characteristic of JLNWFET and the inversion-mode transistor of different gate length (L G ) and nanowire diameter (d NW ) was compared and analyzed. Different high-k dielectrics were used to get an optimum device structure of JLNWFET. The device was simulated using SDE Tool of Sentaurus TCAD and the I-V characteristics were simulated using Sdevice Tools. Lombardi mobility model and Philips unified mobility model were applied to define its electric field and doping dependent mobility degradation. A thin-film heavily doped silicon nanowire with a gate electrode that controls the flow of current between the source and drain was used. The proposed JLNWFET exhibits high ON-state current (I ON ) due to the high doping concentration (N D ) of 1 x 10 19 cm -3 which leads to the improved ON-state to OFF-state current ratio (I ON /I OFF ) of about 10% than the inversion-mode device for a L G of 7 nm and the silicon d NW of 6 nm. Electrical characteristics such are drain induced barrier lowering (DIBL) and subthreshold slope (SS) were extracted which leads to low leakage current as well as a high I ON /I OFF ratio. The performance was improved by introducing silicon dioxide (SiO 2 ) with high-k dielectric materials, hafnium oxide (HfO 2 ) and silicon nitrate (Si 3 N 4 ). It was found that JLNWFET with HfO 2 exhibits better electrical characteristics and performance.

Mobility Models Based on Forward Current-Voltage Characteristics of P-type Pseudo-Vertical Diamond Schottky Barrier Diodes.

Compared with silicon and silicon carbide, diamond has superior material parameters and is therefore suitable for power switching devices. Numerical simulation is important for predicting the electric characteristics of diamond devices before fabrication. Here, we present numerical simulations of p-type diamond pseudo-vertical Schottky barrier diodes using various mobility models. The constant mobility model, based on the parameter μconst, fixed the hole mobility absolutely. The analytic mobility model resulted in temperature- and doping concentration-dependent mobility. An improved model, the Lombard concentration, voltage, and temperature (CVT) mobility model, considered electric field-dependent mobility in addition to temperature and doping concentration. The forward voltage drop at 100 A/cm2 using the analytic and Lombard CVT mobility models was 2.86 and 5.17 V at 300 K, respectively. Finally, we used an empirical mobility model based on experimental results from the literature. We also compared the forward voltage drop and breakdown voltage of the devices, according to variations in p- drift layer thickness and cathode length. The device successfully achieved a low specific on-resistance of 6.8 mΩ∙cm2, a high breakdown voltage of 1190 V, and a high figure-of-merit of 210 MW/cm2.

Electrothermal Characteristics of Delta-Doped $\beta$ -Ga2O3 Metal–Semiconductor Field-Effect Transistors

A 2-D electrothermal model of delta-doped b eta-gallium oxide ( $\beta $ -Ga2O3) metal–semiconductor field-effect transistor (MESFET) is developed by using TCAD Sentaurus to investigate its electrical and thermal characteristics. The temperature and electric field-dependent electron mobility model is incorporated to predict I – V characteristics of the FET, which are in good agreement with the measured I – V characteristics. We investigated the effect of bias voltages on an electric field, a current path, a volumetric heat generation profile, and a peak temperature at a given total heat dissipation. The peak temperature is observed to be significantly different at the same power dissipation but different bias conditions, e.g., the peak temperature is ~9 K higher when the drain-to-source voltage ( V ds) = 25 V and the gate-to-source voltage ( V gs) = −6 V compared with V ds = 8.3 V and V gs = 2 V at the total power dissipation of 1.16 W/mm. This difference is attributed to the change in the electron Joule heating profile with the applied bias voltage. The effects of the location of the delta-doping layer, the gate–drain spacing, and the source–drain spacing are investigated to guide a future device fabrication. The variation in these parameters mainly affects electrical characteristics such as ON-resistance, saturation current, threshold voltage, and so on. The thermal characteristics, such as peak temperature, temperature profile, and so on, are not significantly affected at a low-power dissipation.

Balanced Electric Field Dependent Mobilities: A Key to Access High Fill Factors in Organic Bulk Heterojunction Solar Cells

The compositions of most lab‐based bulk‐heterojunction (BHJ) solar cells are optimized by their power conversion efficiencies (PCEs). In this report, we suggest that the compositions should be optimized by their fill‐factors (FFs) instead. With the optimized‐FF approach, BHJ cells tend to have higher acceptor contents and possess better thermal and operational stabilities. Three model systems, namely, PTB7:PC71BM, PTB7‐Th:ITIC, and PBDB‐T:ITIC BHJs, are chosen as case studies. Charge carrier transport measurements are used to reveal the origin of the enhanced FFs of these BHJ solar cells. We demonstrate that these acceptor‐rich BHJs possess better balanced field‐dependent electron‐to‐hole mobility ratios due to improved electron mobilities near open‐circuit conditions. We introduce a new parameter, known as charge imbalance factor (Δ), to quantify the impact of field dependent mobilities on the FF of the OPV cells. The improved mobility ratio (reduced Δ) suppresses carrier recombinations (especially at the open‐circuit conditions). Despite having slightly reduced PCEs, the FF‐optimized cells enjoy a much better stability. Our results suggest that FF‐optimized BHJ cells with higher acceptor contents should be considered for practical applications, due to better thermal and operational stability.

Mobility of holes and polarons in polyaniline films assessed by frequency-dependent impedance and charge extraction by linearly increasing voltage

Thin films of semiconducting polyaniline hydrochloride were deposited on glass or silicon substrates. Frequency spectra of impedance measured at several voltages display regions of holes and polarons contributions and enable to evaluate their mobility 2.7×10−5cm2V−1s−1 at the electric field of 1×105Vcm−1. Good agreement of the average value 2.4×10−5cm2V−1s−1 of holes mobility was obtained by another method-charge extraction by linearly increasing voltage (CELIV). A Poole-Frenkel type of electric field dependence of mobility assessed by frequency- dependent impedance points to trapping of the charge carriers. From the value of the work function measured by the Kelvin probe and from the position of HOMO level, the hole concentration 2.9×1018cm−3 was obtained. Using this concentration together with the mobility measured by the impedance spectroscopy and the CELIV method, the conductivity 1.2×10−5Scm−1 carried by holes was estimated. Our results widen the work dealing with the mixed electron and proton conductivity of polyaniline films in aqueous solution of acids published in the literature.

Energetic fluctuations in amorphous semiconducting polymers: Impact on charge-carrier mobility

We present a computational approach to model hole transport in an amorphous semiconducting fluorene-triphenylamine copolymer (TFB), which is based on the combination of molecular dynamics to predict the morphology of the oligomeric system and Kinetic Monte Carlo (KMC), parameterized with quantum chemistry calculations, to simulate hole transport. Carrying out a systematic comparison with available experimental results, we discuss the role that different transport parameters play in the KMC simulation and in particular the dynamic nature of positional and energetic disorder on the temperature and electric field dependence of charge mobility. It emerges that a semi-quantitative agreement with experiments is found only when the dynamic nature of the disorder is taken into account. This study establishes a clear link between microscopic quantities and macroscopic hole mobility for TFB and provides substantial evidence of the importance of incorporating fluctuations, at the molecular level, to obtain results that are in good agreement with temperature and electric field-dependent experimental mobilities. Our work makes a step forward towards the application of nanoscale theoretical schemes as a tool for predictive material screening.

Charge transport characterization in a squaraine-based photodetector by means of admittance spectroscopy

Thanks to their strong absorption in the near infrared, squaraine dyes have raised interest in the field of organic photovoltaics and photodetection. We develop photodetectors based on the bulk heterojunction of an indolic squaraine and of a fullerene derivative, obtaining an external quantum efficiency as high as 12% at 590nm. To gain insight into carrier transport and recombination phenomena, devices are characterized in the dark by means of admittance spectroscopy. At low applied voltages carrier transport is dominated by holes, which show dispersive transport and an electric field dependent mobility of about 10−5–10−4cm2V−1s−1. At higher voltages electron injection occurs as well and a bipolar, recombination-limited regime sets in. The modeling of the low-frequency negative capacitance contribution suggests a trap-mediated carrier recombination mechanism.

Improved model for diffusion-limited current in organic metal-insulator-metal diodes

The analytic J–V formula for organic diodes recently derived by Bruyn et al. is improved by Ammar and Sun by considering the electric field dependence of mobility.

Electrical transport mechanism of single monolayer pentacene film employing field-effect characterization

We employed a novel electrode-contact architecture to enable operation of single monolayer pentacene-based field-effect transistor with a high electrical performance, whose mobility reaches as high as 0.31cm2V−1s−1, the highest among any pentacene-based monolayer devices ever reported. A temperature dependent charge transport was systematically carried out to elucidate the carrier transport mechanisms within even single layer of molecules. The carrier mobilities are found to exhibit a pure Arrhenius type at the high temperature regime (above 170K), by contrast, a pronounced turning point has been observed when the measurement temperature is below 170K, possible mechanisms were ascribed to the distribution attribute of trapped states among the grain boundary. Furthermore, an electric field dependent mobility characterization shows a unique non-Poole–Frenkel type behavior at room temperature, which shows much different from the case occurred in multiple-layer devices where the quantity of permitted percolation routes among different monolayers guarantees an mobility enhancement as an electric field increasing. But for the case of single monolayer, the electric field-induced potential reduction effect is competing with a drop of percolation path arising from the directional movement of carrier under a strong electric field. The depth understanding of carrier transport within one monolayer may be helpful for optimizing the design of OFETs for better device applications.

Effect of traps' adjacency on the electric field dependence of mobility in organic systems

In some organic materials, varying the finite distance between adjacent carrier traps modifies the Coulomb potential around each trap, resulting in a more complex field-dependence of mobility, differing from (but not incompatible with) the usually considered relationship of ln μ ∝ √E, a relationship which has been successfully explained by the Poole—Frenkel effect. To investigate the influence of the adjacency of traps, a model system is proposed, which consists of two traps separated by distance α. Our numerical calculation shows that with increasing α, the dependence of mobility on the electric field changes from linear to exponential. Moreover, beyond a certain large α, i.e., as the distance to the nearest trap approaches infinity, the proposed model is essentially the same as the Poole—Frenkel effect. The proposed model accounts for the effect of the energy barrier shape, especially the effect of the location of the potential-energy maximum, a phenomenon which is not accommodated in the Poole—Frenkel model. Because the model assumes the Coulomb interaction between the adjacent traps, it applies to those charged traps which may exist in organic materials for various reasons.

Temperature and field dependent effective hole mobility and impact ionization at extremely high fields in amorphous selenium

An analytical model for the electric field and temperature dependent effective drift mobility of holes in amorphous selenium (a-Se) has been developed by considering density of states distribution near the valence band, field enhancement release rate from the shallow traps, and carrier heating. The models for the field-dependent microscopic mobility and momentum relaxation mean free path considering carrier heating are also proposed. The models are fitted with the published experimental results on effective hole mobility and impact ionization with wide variations of applied electric fields and temperatures. The fittings of the model with the published experimental data on the effective hole mobility reveal that, while the effective hole drift mobility increases with increasing temperature and field, the microscopic mobility and momentum relaxation mean free path in a-Se decreases with increasing electric field. A better fitting considering thermally activated tunneling for the field-enhancement release rate indicates that the shallow hole traps in a-Se are neutral defects.

Temperature dependent charge transport in organic field-effect transistors with the variation of both carrier concentration and electric field

We present experimental evidence of combined effects of temperature, carrier concentration, electric field as well as disorder on charge transport in an organic field-effect transistor (OFET). Transfer characteristics of an OFET based on sexithiophene active layer were measured from 80 to 300 K. Thermally activated carrier mobility followed Arrhenius law with two activation energies. Carrier density variation led to finite extrapolated Meyer–Neldel (MN) temperature (780 K) at low fields. Negative electric field-dependent mobility was observed in available field range. MN temperature shifted towards higher temperature when the electric field increased, and did not retain its finite character above the field of 4 × 103 V cm−1.

Effect of parabolic potential well profile on multisubband electron mobility in a coupled quantum well in presence of an external electric field

We study the effect of parabolic potential well profile on the electric field dependent multisubband electron mobility in a double quantum well structure. We consider a barrier delta doped GaAs–AlxGa1−xAs parabolic coupled double quantum well subject to an external electric field perpendicular to the interface plane. Variation of the electric field transfers the system from double subband occupancy to single subband occupancy at which an enhancement of mobility is obtained due curtailment of the intersubband effects. It is gratifying to show that enhancement of curvature of the potential profile i.e., replacement of a square quantum well into a parabolic quantum well structure enhances the mobility through the intersubband effects. We vary the structure parameters such as well width, barrier width, doping concentration and height of the parabolic potential to analyze the effect of the parabolic well structure on the field dependent multisubband electron mobility, which shows interesting features.

Transit Phenomena in Organic Field‐Effect Transistors Through Kelvin‐Probe Force Microscopy

The temporal evolution of the surface-potential distribution in the channel of pentacene based field-effect transistors is investigated during the charge reversal from the electron to the hole dominated operation. This measurement allows the determination of the carrier density and electric field dependent hole mobility in the sub-threshold regime of the transistor.