Field Dependent Mobility Model Research Articles

Effect of synthesis temperature on the charge transport properties of poly(3-methylthiophene) devices

Low temperature-dependent I-V measurement was adopted to investigate an insight into the effect of synthesis temperature on the field and temperature-dependent charge transport properties of electrochemically polymerized poly(3-methylthiophene) devices. Devices having different doping levels were fabricated at two different temperatures, one at room temperature (25oC) and another at a lower temperature (5oC). The transport mechanism in the devices were studied in the temperature range 40 K to 300 K. Our studies revealed that trap controlled space charge limited conduction (TCSCLC) mechanism dominated the transport mechanism at higher temperature(T>200K) and changed to field dependent mobility model at lower temperature(T<200K). It was observed that the synthesis temperature influenced the transport parameters. Devices prepared at low temperature showed an increase in resistivity and a corresponding decrease in the charge carrier mobility compared to those prepared at high temperature. Moreover, the distribution of the trap states in the devices was also affected by the synthesis temperature, with devices fabricated at low temperature exhibiting a wider trap state distribution.

Dopant induced anomalous field dependent mobility behavior of poly(3-octylthiophene) devices

Temperature dependent DC current-voltage (I-V) down to 50 K and field-dependent AC impedance measurements were carried out to study the electric field and temperature dependence of charge carrier mobility in Poly(3-octylthiophene) based metal/polymer/metal device structures of different charge carrier concentrations. In highly doped device, a negative field-dependent mobility (NFDM) which vanishes below 150 K was observed whereas a positive field-dependent mobility (PFDM) was observed in moderately doped devices. The observed NFDM is attributed to the low energetic disorder and high positional disorder in the highly doped devices. With further lowering of carrier concentration, charge carrier mobility first decreases with the increase in applied field attaining a minimum value and then increases with the applied field. At higher temperatures (T>150K), all the three devices show thermally-activated process and activation energy increases with lowering the charge carrier concentrations indicating that doping level plays important role in thermally activated transport mechanism. However, in lower temperature (T<100K), activation energy is almost independent of doping level suggesting that injected carrier plays an important role in the lower temperature region. Data from AC impedance measurement also supports the field-dependent mobility model of conduction mechanism in the devices. Analysis of the AC impedance data of the three devices reveals that the equivalent circuit of highly and moderately doped devices is different from that of the low carrier concentration device. The presence of highly energetic disorder in low carrier concentration devices is also confirmed from AC impedance analysis.

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.

Simulation and Compact Modeling of Organic Thin Film Transistors (OTFTs) for Circuit Simulation

In this paper we present TCAD simulation and compact modeling of OTFTs. Finite element method (FEM) based numerical simulations have been performed using density of state model and field dependent mobility model to simulate the electrical behavior of the OTFT devices. Further Universal Organic Thin Film Transistor (UOTFT) compact model has been applied for simulating the DC I-V characteristics of the device using device modeling software UTMOST-IV. TCAD simulated characteristics and compact model based simulated characteristics were compared and contrasted. Device model parameters were extracted using UOTFT model. Extracted model is applied in designing the OTFT based PMOS-like inverter and hybrid operational amplifier.

A charge carrier transport model for donor-acceptor blend layers

Highly efficient organic solar cells typically comprise donor-acceptor blend layers facilitating effective splitting of excitons. However, the charge carrier mobility in the blends can be substantially smaller than in neat materials, hampering the device performance. Currently, available mobility models do not describe the transport in blend layers entirely. Here, we investigate hole transport in a model blend system consisting of the small molecule donor zinc phthalocyanine (ZnPc) and the acceptor fullerene C60 in different mixing ratios. The blend layer is sandwiched between p-doped organic injection layers, which prevent minority charge carrier injection and enable exploiting diffusion currents for the characterization of exponential tail states from a thickness variation of the blend layer using numerical drift-diffusion simulations. Trap-assisted recombination must be considered to correctly model the conductivity behavior of the devices, which are influenced by local electron currents in the active layer, even though the active layer is sandwiched in between p-doped contacts. We find that the density of deep tail states is largest in the devices with 1:1 mixing ratio (Et = 0.14 eV, Nt = 1.2 × 1018 cm−3) directing towards lattice disorder as the transport limiting process. A combined field and charge carrier density dependent mobility model are developed for this blend layer.

Influence of mobility on the dissociation probability of electron-hole pair in hopping system

We propose a model to describe the dissociation probability of electron-hole pair in the presence of external field in the hopping systems. The influence of field dependent mobility on the dissociation probability has been extensively discussed. The results show that the field dependent mobility model has a major influence on the electron-hole dissociation probability. The application of the dissociation model has also been presented.

A new field dependent mobility model for high frequency channel thermal noise of deep submicron RFCMOS

In this paper, a new field dependent effective mobility model including the drain-induced vertical field effect (DIVF) is presented to calculate the channel thermal noise of short channel MOSFETs operating at high frequencies. Based on the new channel thermal noise model, the simulated channel thermal noise spectral densities have been compared to the channel thermal noise directly extracted from noise measurements on devices fabricated using GLOBALFOUNDRIES’ 0.13 μm RFCMOS technology. The comparison has been done across different channel length, finger width and number of finger for different frequencies, gate biases and drain biases. Excellent agreement between simulated and extracted noise data has shown that the proposed model is scalable over different dimensions and operating conditions. The proposed model is simple and can be easily implemented in a circuit simulation environment.

A Physics-Based Model for a SiC JFET Accounting for Electric-Field-Dependent Mobility

In this paper, a physical model for a SiC Junction Field Effect Transistor (JFET) is presented. The novel feature of the model is that the mobility dependence on both temperature and electric field is taken into account. This is particularly important for high-current power devices where the maximum conduction current is limited by drift velocity saturation in the channel. The model equations are described in detail, emphasizing the differences introduced by the field-dependent mobility model. The model is then implemented in Pspice. Both static and dynamic simulation results are given. The results are validated with experimental results under static conditions and under resistive and inductive switching conditions.

Simulation based performance comparison of transistors designed using standard photolithographic and coarse printing design specifications

In this work a simulation based comparative study of organic field effect transistors designed using standard lithographic and printing designs is presented. The device simulations were performed using two-dimensional drift-diffusion equations with a Poole–Frenkel field dependent mobility model. Both photolithographic and coarse printing transistor designs employed common materials such as 150nm thick pentacene, 150nm thick parylene gate insulator, gold source-drain electrodes and aluminum gate electrodes. The major differences between the two fabrication specifications are the minimum source/drain line width and the transistor channel length. The typical specifications for the minimum line width and channel length were 2μm and 5μm for photolithography and 25μm and 20μm for coarse printing techniques, respectively. The gate, source, and drain capacitances and channel on-resistances at various channel lengths and gate overlaps have been extracted and presented specifically for both process schemes. Due to increased channel length and gate-source/drain overlap of printed electrodes relative to lithographic design, the resulting on-resistance and capacitances for coarse printing are significantly higher. These results demonstrate a substantial operating frequency reduction for printing design relative to photolithographic design. For the tested materials and designs it is shown that the cut-off frequency for the photolithographic process was 400kHz but decreased to a much lower 26kHz for the coarse printing process. Since printing technology uses various other materials, which typically have less performance than the ones used for this simulation, the actual printed device might have even lower performance than predicted here.

The effects of the writing voltage on the electrical bistability properties of organic memory devices consisting of a single layer

Electrical bistability properties of organic memory devices consisting of a single layer were theoretically investigated by using a drift-diffusion model combined with a field dependent mobility model and a single level trap model. After application of a writing voltage, the current under a reading voltage was larger than that without a writing voltage. The behavior in the current bistability was affected from the trapped electron density near the metal/organic interface. The increasing rate of the trapped electron density by increasing a writing voltage was relatively small, but it causes the abrupt increment to the current density, resulting in the bistable characteristics in the model device.

Transport study of a novel polyfluorene/poly(p-phenylenevinylene) copolymer by various mobility models

The polyfluorene/poly(p-phenylenevinylene) copolymer based hole-only devices are fabricated and the current–voltage characteristics are measured as a function of temperature. The hole current is fitted well with space-charge limited and field-dependent mobility model, which provides a direct measurement of the hole mobility μ as a function of electric field E and temperature. The mobility is fitted with existing Gill’s model, Gaussian disorder model, correlated Gaussian disorder model and Brownian motion model. Energy hopping time and activation energy are obtained from Brownian motion model. Microscopic transport parameters are derived and a consistent picture of the influence of the molecular structure of the polymer on the charge transport is depicted. For the polyfluorene/poly(p-phenylenevinylene) copolymer, although with a high degree of irregularity in structure and larger energetic disorder, the two bulky structure favors charge delocalization and remove defect sites, results in a higher mobility. The results suggest space-charge limited and field-dependent mobility model combine with various mobility model, include Brownian motion model, is a useful technique to study charge transport in thin films with thicknesses close to those used in real devices.

Analysis of magnetic field dependent mobility in ferromagnetic Ga 1− xMn xAs layers

The magneto-transport properties of ferromagnetic Ga 1− x Mn x As epilayers with Mn mole fractions in the range of x≈2.2–4.4% were investigated through Hall effect measurements. The magnetic field-dependent Hall mobility for a metallic sample with x≈2.2% in the temperature range of T=0–300 K was analyzed by magnetic field-dependent mobility model including an activation energy of Mn acceptor level. This model provides outstanding fits to the measured data up to T=300 K. It was found that the acceptor levels with activation energies of 112 meV at B=0 Oe decreased to 99 meV at B=5 kOe in the ferromagnetic region. The decrease in acceptor activation energy was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for increase in carrier concentration.

Monte Carlo calculation of two-dimensional electron dynamics in GaN–AlGaN heterostructures

We present detailed Monte Carlo based calculations of the electron dynamics in GaN–AlGaN heterostructures in the presence of strain polarization fields. The model consists of a fully numerical self-consistent solution of the Schrödinger–Poisson equation with a Monte Carlo transport model. The two-dimensional sub-band energies, wave functions and carrier scattering mechanisms are computed numerically and included within a Monte Carlo simulation. The electron energy, steady-state and transient drift velocity and band occupancy are calculated as a function of electric field for different AlGaN–GaN heterostructure compositions. The effect of piezoelectrically induced strain fields on the transport dynamics is examined. A field dependent mobility model is also developed from the Monte Carlo results.

Monte Carlo simulation of electron transport in the III-nitride wurtzite phase materials system: binaries and ternaries

We present a comprehensive study of the transport dynamics of electrons in the ternary compounds, Al/sub x/Ga/sub 1-x/N and In/sub x/Ga/sub 1-x/N. Calculations are made using a nonparabolic effective mass energy band model. Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models. The effects of alloy scattering on the electron transport physics are examined. The steady state velocity field curves and low field mobilities are calculated for representative compositions of these alloys at different temperatures and ionized impurity concentrations. A field dependent mobility model is provided for both ternary compounds AlGaN and InGaN. The parameters for the low and high field mobility models for these ternary compounds are extracted and presented. The mobility models can be employed in simulations of devices that incorporate the ternary III-nitrides.

Monte Carlo Calibrated Drift-Diffusion Simulation of Short Channel HFETs

In this paper we present a methodology to use drift diffusion (DD) simulations in the design of short channel heterojunction FETs (HFETs) with well pronounced velocity overshoot. In the DD simulations the velocity overshoot in the channel is emulated by forcing the saturation velocity in the field dependent mobility model to values corresponding to the average velocity in the channel obtained from Monte Carlo (MC) simulation. To illustrate our approach we compare enhanced DD and MC simulation results for a pseudomorphic HEMTs with 0.12 μm channel length, which are in good agreement. The usefulness of the described methodology is illustrated in a simulation example of self aligned gamma gate pseudomorphic HEMTs. The effect of the gamma gate shape and the self aligned contacts on the overall device performance has been investigated.

Hybrid field-dependent mobility model for the simulationofpower VDMOSTs

An experimentally verified hybrid model of field-dependent mobility is proposed for the 2-D numerical simulation of power VDMOSTs. With the new scheme, the 2-D simulation of the DC output characteristics agrees very well with the experimental results with a single value of the surface degradation factor.

Delay time analysis for 0.4- to 5- mu m-gate InAlAs-InGaAs HEMTs

InAlAs-InGaAs HEMTs with 0.4- to 5- mu m gate lengths have been fabricated and a maximum f/sub T/ of 84 GHz has been obtained by a device with a 0.4- mu m gate length. A simple analysis of their delay times was performed. It was found that gradual channel approximation with a field-dependent mobility model with E/sub c/ of 5 kV/cm holds for long-channel devices (L/sub g/>2 mu m), while a saturated velocity model with a saturated velocity of 2.7*10/sup 7/ cm/s holds for short-channel devices (L/sub g/<1 mu m).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A new approach to model d.c. and a.c. characteristics of junction gate field effect transistors

A field dependent mobility model for use in quasi-unidimensional numerical analysis of JG FET is proposed. This model takes into account the electrical data (mobility and diffusion laws vs electric field) obtained on Si bulk material, the technological data given by the manufacturer (geometry of the mask) and the assumption of an isotropic diffusion of the gates. The static and the dynamic behaviour of the JG FET is obtained. The validity of the present model is demonstrated using very different geometrical JG FET structures. Reasonable agreement is found between calculated and experimental results up to the saturation range.

Field-dependent mobility model for two-dimensional numerical analysis of MOSFET's

A field-dependent mobility model for use in two-dimensional numerical analysis of MOSFET's is proposed. This model takes into account two field components: One is the gate field which induces carriers in the inversion layer and the other is the drain field which transports carriers to the drain. Mobility is assumed to be a product of two functions, each of which includes only a field component perpendicular or parallel to the current flow ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_{\perp} or E_{\parallel}</tex> ), that is \micro = \micro_{0} f (N_{B}, E_{\parallel})g(E_{\perp}), with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\micro_{0}</tex> and N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> being a constant and the impurity concentration, respectively. This equation is applied to calculate the mobility at each mesh point in the numerical analysis. The validity of the present model is demonstrated using several MOSFET structures, for example, conventional structures with short and long gate lengths, and offset-gate structrues with channel-doped layers. Reasonable agreement is found between calculated and experimental results under moderate bias conditions. Flat-band voltage, V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FB</inf> , is assumed to be the only fitting parameter and is adjusted once for each set of samples. Quantitative agreement for short-channel MOSFET's is improved, typically by a factor of 2, and errors are within 20 percent with respect to the experiments.