Decrease In Electron Mobility Research Articles

60Co gamma radiation effects on NPN transistor at cryogenic temperature.

The 60Co gamma radiation effects on the DC electrical characteristics of silicon NPN transistor were studied in the dose range of 100krad to 6Mrad at room temperature (300K) and cryogenic temperature (77K). The measurements were carried out at both 300 and 77K temperature. The electrical characteristics such as Gummel characteristics, excess base current (ΔIB), current gain (hFE), transconductance (gm) and output characteristics were studied in situ as a function of total dose. The results show that there is a considerable degradation in the electrical parameters of the device irradiated both at 300 and 77K as a consequence of increase in excess base current (ΔIB) because of the formation of generation and recombination centers in the emitter-base spacer oxide (SiO2). At cryogenic temperature irradiation, the degradation in electrical characteristics is less because of the physical phenomena such as carrier freezeout effect, decreased recombination rate, reduced charge yield, decreased electron mobility, etc.

Characterization of trap evolution in GaN-based HEMTs under pulsed stress

In this paper, the evolution of traps in GaN-based high-electron-mobility transistors under pulsed thermal stress and the combined effect of thermal stress and electrical stress are studied. By applying impulse stresses of different powers to the device, collecting transient current response curves, and accurately extracting the time constant and absolute peak changes of the traps by employing the Bayesian inverse convolution theory time constant spectral technique, we analyzed the evolution of the traps under the action of impulse stress and clarified the influence and degradation mechanism of the thermal stress, as well as the combined action of thermal stress and electrical stress, on the evolution of traps. The results show that under pulsed stress, the decrease in electron mobility with increasing temperature leads to a decrease in device drain current, and the degradation of the device is exacerbated owing to channel hot-electron injection.

Integrated Bolometric Photodetectors Based on Transparent Conductive Oxides from Near‐ to Mid‐Infrared Wavelengths

On‐chip photodetectors are essential components in optical communications as they convert light into an electrical signal. Photobolometers are a type of photodetector that functions through a resistance change caused by electronic temperature fluctuations upon light absorption. They are widely used in the broad wavelength range from ultraviolet to mid‐infrared (MIR). In this work, a novel waveguide‐integrated bolometer that operates in a wide wavelength range from near‐infrared to MIR is introduced on the standard material platform with the transparent conductive oxides (TCOs) as the active material. This material platform enables the construction of both modulators and photodetectors using the same material, which is fully complementary metal–oxide–semiconductor compatible and easily integrated with passive on‐chip components. The photobolometers proposed here consist of a thin TCO layer placed inside the rib photonic waveguide to enhance light absorption and then heat the electrons in the TCO. This rise in electron temperature leads to decreasing electron mobility and consequential electrical resistance change. In consequence, a responsivity exceeding 10 A W−1 can be attained with a mere few microwatts of optical input power. Calculations suggest that further improvements can be expected with lower doping of the TCO, thus opening new doors in on‐chip photodetectors.

The Influence of Special Environments on SiC MOSFETs.

In this work, the influences of special environments (hydrogen gas and high temperature, high humidity environments) on the performance of three types of SiC MOSFETs are investigated. The results reveal several noteworthy observations. Firstly, after 500 h in a hydrogen gas environment, all the SiC MOSFETs exhibited a negative drift in threshold voltage, accompanied by an increase in maximum transconductance and drain current (@ VGS/VDS = 13 V/3 V). This phenomenon can be attributed to that the hydrogen atoms can increase the positive fixed charges in the oxide and increase the electron mobility in the channel. In addition, high temperature did not intensify the impact of hydrogen on the devices and electron mobility. Instead, prolonged exposure to high temperatures may induce stress on the SiO2/SiC interface, leading to a decrease in electron mobility, subsequently reducing the transconductance and drain current (@ VGS/VDS = 13 V/3 V). The high temperature, high humidity environment can cause a certain negative drift in the devices' threshold voltage. With the increasing duration of the experiment, the maximum transconductance and drain current (@ VGS/VDS = 18V (20 V)/3 V) gradually decreased. This may be because the presence of moisture can lead to corrosion of the devices' metal contacts and interconnects, which can increase the devices' resistance and lead to a decrease in the devices' maximum transconductance and drain current.

High-Performance n-Type Organic Thermoelectrics Enabled by Synergistically Achieving High Electron Mobility and Doping Efficiency.

n-Doped polymers with high electrical conductivity (σ) are still very scarce in organic thermoelectrics (OTEs), which limits the development of efficient organic thermoelectric generators. A series of fused bithiophene imide dimer-based polymers, PO8, PO12, and PO16, incorporating distinct oligo(ethylene glycol) side-chain to optimize σ is reported here. Three polymers show a monotonic electron mobility decrease as side-chain size increasing due to the gradually lowered film crystallinity and change of backbone orientation. Interestingly, polymer PO12 with a moderate side-chain size delivers a champion σ up to 92.0 S cm-1 and a power factor (PF) as high as 94.3 µW m-1 K-2 in the series when applied in OTE devices. The PF value is among the highest ones for the solution-processing n-doped polymers. In-depth morphology studies unravel that the moderate crystallinity and the formation of 3D conduction channel derived from bimodal orientation synergistically contribute to high doping efficiency and large charge carrier mobility, thus resulting in high performance for the PO12-based OTEs. The results demonstrate the great power of simple tuning of side chain in developing n-type polymers with substantial σ for improving organic thermoelectric performance.

Junctionless Electric-Double-Layer MoS2 Field-Effect Transistor with a Sub-5 nm Thick Electrostatically Highly Doped Channel.

Junctionless transistors are suitable for sub-3 nm applications because of their extremely simple structure and high electrical performance, which compensate for short-channel effects. Two-dimensional semiconductor transition-metal dichalcogenide materials, such as MoS2, may also resolve technical and fundamental issues for Si-based technology. Here, we present the first junctionless electric-double-layer field-effect transistor with an electrostatically highly doped 5 nm thick MoS2 channel. A double-gated MoS2 transistor with an ionic-liquid top gate and a conventional bottom gate demonstrated good transfer characteristics with a 104 on-off current ratio, a 70 mV dec-1 subthreshold swing at a 0 V bottom-gate bias, and drain-current versus top-gate-voltage characteristics were shifted left significantly with increasing bottom-gate bias due to an electrostatically increased overall charge carrier concentration in the MoS2 channel. When a bottom-gate bias of 80 V was applied, a shoulder and two clear peak features were identified in the transconductance and its derivative, respectively; this outcome is typical of Si-based junctionless transistors. Furthermore, the decrease in electron mobility induced by a transverse electric field was reduced with increasing bottom-gate bias. Numerical simulations and analytical models were used to support these findings, which clarify the operation of junctionless MoS2 transistors with an electrostatically highly doped channel.

Monte Carlo Simulation of Flash Memory Elements’ Electrophysical Parameters

Operation of modern flash memory elements is based on electron transport processes in the channel of silicon MOSFETs with floating gate. The aim of this work was calculation of electron mobility and study of the influence of phonon and ionized impurity scattering mechanisms on the mobility, as well as calculation of parasitic tunneling current and channel current in the conductive channel of flash memory element. Numerical simulation during the design stage of flash memory element allows working out guidelines for optimization of device parameters defining its performance and reliability.In the work such electrophysical parameters, characterizing electron transport, as mobility and average electron energy, as well as tunneling current and current in the channel of the flash memory element are studied via the numerical simulation by means of Monte Carlo method. Influence of phonon and ionized impurity scattering processes on electron mobility in the channel has been analyzed. It is shown that in the vicinity of drain region a sufficient decrease of electron mobility defined by phonon scattering processes occurs and the growth of parasitic tunneling current is observed which have a negative influence on device characteristics.The developed simulation program may be used in computer-aided design of flash memory elements for the purpose of their structure optimization and improvement of their electrical characteristics.

Enhanced phonon scattering and thermoelectric performance for N-type Bi2Te2.7Se0.3 through incorporation of conductive polyaniline particles

Bi2Te3 based alloys are the state-of-art thermoelectric materials at/near room temperatures. However, the performance of n-type Bi2Te3 is much difficult to improve as compared to p-type Bi2Te3. Here, we show that via incorporation of 1.5 wt% conductive polyaniline nanoparticles in Bi2Te2.7Se0.3 the lattice thermal conductivity of the composite system reduces by 49 % at 300 K mainly due to enhanced phonon scattering by the polymer inclusions. Moreover, energy-dependent carrier scattering occurs owing to the interfacial potentials formed at the inorganic/organic boundaries, leading to 8 % elevation of thermopower in the sample, which counteracts the decrease of electron mobility to a large degree. As a result, both a maximum figure of merit Zmax = 3.57 × 10−3 K−1 (300 K) and a maximum dimensionless figure of merit ZTmax = 1.22 (345 K) are achieved for the composite sample with 1.5 wt% PANI inclusions, which are respectively ∼ 42 % and ∼ 46 % larger than those of pristine Bi2Te2.7Se0.3, demonstrating that incorporation of nanophase polyaniline in Bi2Te2.7Se0.3 is an effective strategy to enhance its thermoelectric performance.

The Influence of Humidity on Electron Transport Parameters and Insulation Performance of Air

The environmental conditions affect the external insulation performance of power equipment. In order to study the physical characteristics of air discharge, the microscopic process of electron–molecule collision in the air based on the Boltzmann equation has been studied in this paper. The influence of humidity on the air gap insulation performance was also analyzed. The calculation results show that with the temperature 300 K and the pressure 1.0 atm, the electron energy distribution function and electron transport parameters varied with the air relative humidity. As the air relative humidity is increased by each 30%, the average electron energy decreases by about 0.2 eV, the reduced electron mobility decreases by about 0.25 × 1023 [1/(V·m·s)], the reduced electron diffusion coefficient decreases by about 0.2 × 1024 [1/(m s)], and the effective ionization coefficient decreases by about 4 × 10−24 m2. As the air relative humidity increases from 0% to 60%, the critical breakdown electric field increases by 1.22 kV/cm.

Impact of n-Doping Mechanisms on the Molecular Packing and Electron Mobilities of Molecular Semiconductors for Organic Thermoelectrics

Electrical conductivity is one of the key parameters for organic thermoelectrics and depends on both the concentration and mobility of charge carriers. To increase the carrier concentration, molecular dopants have to be added into organic semiconductor materials, whereas the introduction of dopants can influence the molecular packing structures and hence carrier mobility of the organic semiconductors. Herein, we have theoretically investigated the impact of different n-doping mechanisms on molecular packing and electron transport properties by taking (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI-H) and quinoid-dicyanomethylene-dipyrrolo-[3,4-c]pyrrole-1,4-diylidene)bis(thieno[3,2-b]thiophene (Q-DCM-DPPTT) respectively as representative n-dopant and molecular semiconductor. The results show that when the doping reactions and charge transfer spontaneously occur in the solution at room temperature, the oppositely charged dopant and semiconductor molecules will be tightly bound to disrupt the semiconductor to form long-range molecular packing, leading to a substantial decrease of electron mobility in the doped film. In contrast, when the doping reactions and charge transfer are activated by heating the doped film, the molecular packing of the semiconductor is slightly affected and hence the electron mobility remains quite high. This work indicates that thermally activated n-doping is an effective way to achieve both high carrier concentration and high electron mobility in n-type organic thermoelectric materials.

Abnormally weak intervalley electron scattering in MoS2 monolayer: insights from the matching between electron and phonon bands.

It is known that carrier mobility in layered semiconductors generally increases from two-dimensions (2D) to three-dimensions due to fewer scattering channels resulting from decreased densities of electron and phonon states. In this work, we find an abnormal decrease of electron mobility from monolayer to bulk MoS2. By carefully analyzing the scattering mechanisms, we can attribute such abnormality to the stronger intravalley scattering in the monolayer but weaker intervalley scattering caused by few intervalley scattering channels and weaker corresponding electron-phonon couplings compared to the bulk case. We show that it is the matching between the electronic band structure and phonon spectrum rather than their densities of electronic and phonon states that determines scattering channels. We propose, for the first time, the phonon-energy-resolved matching function to identify the intra- and inter-valley scattering channels. Furthermore, we show that multiple valleys do not necessarily lead to strong intervalley scattering if: (1) the scattering channels, which can be explicitly captured by the distribution of the matching function, are few due to the small matching between the corresponding electron and phonon bands; and/or (2) the multiple valleys are far apart in the reciprocal space and composed of out-of-plane orbitals so that the corresponding electron-phonon coupling strengths are weak. Consequently, the searching scope of high-mobility 2D materials can be reasonably enlarged using the matching function as useful guidance with the help of band edge orbital analysis.

Enhancement of H2 gas sensing properties of ZnO films by Mg alloying

To examine the effect of Mg addition on the gas sensing properties of ZnO, MgxZn1-xO epitaxial films with a constant thickness, in-plane crystal grain size, and surface morphology and varying Mg contents were prepared by pulse laser deposition, and their gas sensing properties were investigated. The results showed that the response to H2 gas improved as the number of Mg atoms substituted into Zn sites increased. In addition, as the thickness of MgxZn1-xO films with the optimal Mg content decreases, the sensor response increases remarkably. A sensor with a thickness of 9 nm showed a maximum response value of approximately 3000 to 800 ppm H2. To determine why the sensor response was improved by the addition of Mg, MgO-particle-deposited undoped ZnO films and c-axis-oriented MgxZn1-xO films with random in-plane rotation domains were prepared, and the characteristics of these sensors were compared with those of the epitaxial MgxZn1-xO films. We identified two causes for the improved sensing properties: a reduced residual electron concentration in the films due to the substitution of Mg and the chemical sensitization effect of Mg atoms. This study also suggested that a decrease in electron mobility results in a decreased sensor response value.

Particle charge distributions in the effluent of a flow-through atmospheric pressure low temperature plasma

Atmospheric pressure low-temperature plasmas are often utilized to perform particle synthesis, treatment, and removal. It is well-known that dust particles are highly negatively charged in these plasmas; however, little is known about dust particle charging behavior as particles leave the plasma volume and pass through the spatial afterglow region. In this work, monodisperse particles of various sizes and work functions were introduced into an atmospheric pressure radiofrequency capacitively coupled flow-through plasma. Dust particle electrical mobility distributions downstream of the flow-through plasma were measured utilizing a differential mobility analyzer in conjunction with a condensation particle counter at various gas flow velocities. Charge distributions were determined from the measured electrical mobility distributions. Experiments confirm that particles become less negatively charged, and even net-positively charged after leaving the plasma volume, with a distribution that follows a shifted Boltzmann charge distribution. Additionally, particle charge in the effluent of the flow-through plasma is negligibly dependent on work function but highly size and flow velocity dependent. Larger particles were shown to have a higher magnitude of charge under all studied conditions; however, particle polarity was switchable by varying gas flow velocity. The charging dynamics were simulated utilizing a constant number Monte Carlo model that accounts for electron temperature decay and the transition from ambipolar to free diffusion of electrons and ions in the spatial afterglow. Simulation results also suggest that, at the same flow velocity, larger particles obtain a greater magnitude of charge, negative or positive. The decrease in electron mobility and the difference between ion and electron convective loss rates create an ion-rich region in the plasma effluent that promotes ion–particle collisions and drives particle charge removal and even reversal of polarity. Larger particles more favorably collide with energetic species in these environments, which results in higher charge states.

Normal and superconducting state properties of Cu-doped FeSe single crystals

We report the evolution of the physical properties of FeSe single crystals with Cu substitution. We show that introducing Cu suppresses bulk superconductivity quickly, much faster than the decrease in structural transition temperature, and further doping Cu induces a metal-insulator transition. In contrast, zero-field $ab$-plane resistivity ${\ensuremath{\rho}}_{xx}$ exhibits an unusual temperature dependence ${\ensuremath{\rho}}_{xx}(T)\ensuremath{\sim}A{T}^{n}$, with $n\ensuremath{\sim}$ 1, that is almost unchanged with the variation of Cu content. This result implies that magnetic fluctuations in FeSe are insensitive to the Cu substitution. The field dependence of the Hall resistivity of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{Se}$ with $x>$ 0 shows a positive slope in the low-temperature region which can be ascribed to the relatively higher hole mobility than the electron one. Therefore, the low-concentration Cu dopant as a strong scattering source can lead to the significant decrease in electron mobility which is detrimental to superconductivity, but it has minor effects on the carrier densities of electrons and holes as well as the shapes of the Fermi surfaces. Correspondingly, the structural transition and magnetic fluctuations in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{Se}$ change slowly with $x$.

Hydrogen roles approaching ideal electrical and optical properties for undoped and Al doped ZnO thin films

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3 carrier concentrations for HAl co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

Resonant electron–plasmon interactions in drifting electron gas

In this paper, we investigate the resonant electron–plasmon interactions in a drifting electron gas of arbitrary degeneracy. The kinetic-corrected quantum hydrodynamic model is transformed into the effective Schrödinger–Poisson model, and the driven coupled pseudoforce system is obtained via separation of variables from the appropriately linearized system. It is noted that in the low phase–speed kinetic regime, the characteristic particle-like plasmon branch is significantly affected by the correction factor, which is a function of electron number density and temperature. It is shown that the electron current density of drifting electron gas sharply peaks at two distinct drift wavenumbers for a given value of electron density, temperature, plasmon energy, and damping parameter. The Fano-resonance of current density profile confirms the electron–plasmon resonant interaction in the presence of underlying interference effect. The electron drift current density shows fundamentally different resonance effects for plasmon energies with a wavenumber below and above a critical wavenumber. Moreover, an extension to the multistream model is presented, and the total current density of drifting electron gas in the presence of resonant electron–plasmon interactions is obtained. We further investigate the kinetic correction effect on matter-wave energy dispersion of the electron gas. It is also found that the increase in the electron number density leads to an increase in effective mass and consequently a decrease in electron mobility, whereas the increase in electron temperature has the converse effect. The kinetic correction is noted to significantly lower the quasiparticle conduction band minimum. The current model may be further elaborated to investigate the electron beam–plasma interactions.

Enhanced Gas Sensing Properties of Graphene Transistor by Reduced Doping with Hydrophobic Polymer Brush as a Surface Modification Layer.

Surface modification layer of a silicon substrate has been used to enhance the performance of graphene field-effect transistors (FETs). In this report, ultrathin and chemically robust polymer brush was used as a surface modification to enhance the gas sensing properties of graphene FETs. The insertion of the polymer brush decreased substrate-induced doping of graphene. This leads to a huge increase in field-effect mobility as well as a minimum shift of the Dirac point voltage. The use of the polymer brush enables fast detection of target gas molecules because graphene sensing modality can be maximized at the undoped state of graphene. The increase of source-drain current, as well as the abrupt decrease of electron mobility upon NO2 exposure, was utilized for the instantaneous detection, and a limit of detection of 4.8 ppb was achieved with graphene FETs on PS brush. We also showed excellent cross-sensitivity of graphene gas sensors to NH3, CO2, and relative humidity condition; the source-drain current decreases upon NH3 exposure, while response to CO2 or relative humidity condition is extremely low. Our results prove that reducing the substrate-induced doping of graphene with a polymer brush is a direct method for boosting the gas sensing properties of graphene FETs.

Theoretical investigation of spin–orbit coupling on structural, electronic and optical properties for CuAB2 (A = Sb, Bi; B = S, Se) compounds using Tran–Blaha-modified Becke–Johnson method: A first-principles approach

We investigate the structural, electronic and optical properties of CuAB2 (A = Sb, Bi; B = S, Se) compounds within density functional theory (DFT) using the full potential linear augmented plane wave plus local orbital (FP-LAPW + lo) method. For these total energy calculations, exchange correlation potentials, such as Perdew–Burke–Ernzerhof (PBE) and Tran–Blaha-modified Becke–Johnson (TB-mBJ) functionals, were executed with and without spin–orbit coupling (SOC). The CuSbSe2 compound shows a direct band gap nature and the other compounds of CuSbS2, CuBiS2 and CuBiSe2 show an indirect band gap nature. The calculated bandgap of these chalcogenides are in good agreement with the reported experimental results. The inclusion of SOC causes band degeneracies, which are confirmed from the electronic structure. The mobility of electrons and holes is higher in CuSbS2 and CuBiS2, whereas electron mobility decreases and hole mobility increases from the Sb-to the Bi-containing compounds. The calculated absorption coefficients of these compounds were greater than 105 cm−1, which make them suitable for optoelectronic applications.

Sensitivity of 2DEG-based Hall-effect sensors at high temperatures.

The magnetic sensitivity of Hall-effect sensors made of InAlN/GaN and AlGaN/GaN heterostructures was measured between room temperature and 576 °C. Both devices showed decreasing voltage-scaled magnetic sensitivity at high temperatures, declining from 53 mV/V/T to 8.3 mV/V/T for the InAlN/GaN sample and from 89 mV/V/T to 8.5 mV/V/T for the AlGaN/GaN sample, corresponding to the decreasing electron mobility due to scattering effects at elevated temperatures. Alternatively, current-scaled sensitivities remained stable over the temperature range, only varying by 13.1% from the mean of 26.3 V/A/T and 10.5% from the mean of 60.2 V/A/T for the InAlN/GaN and AlGaN/GaN samples, respectively. This is due to the minimal temperature dependence of the electron sheet density on the 2-dimensional electron gas (2DEG). Both devices showed consistency in their voltage- and current-scaled sensitivity over multiple temperature cycles as well as nearly full recovery when returned to room temperature after thermal cycling. Additionally, an AlGaN/GaN sample held at 576 °C for 12 h also showed nearly full recovery at room temperature, further suggesting that GaN-based Hall-effect sensors are a good candidate for use in high temperature applications.

Theoretical evaluation of electronic density-of-states and transport effects on field emission from n-type ultrananocrystalline diamond films

In the nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] films, representing an n-type highly conductive two-phase material comprised of sp3 diamond grains and sp2-rich graphitic grain boundaries, current is carried by a high concentration of mobile electrons within large-volume grain-boundary networks. Fabricated in a simple thin-film planar form, (N)UNCD was found to be an efficient field emitter capable of emitting a significant amount of charge starting at the applied electric field as low as a few volts per micrometer, which makes it a promising material for designing electron sources. Despite semimetallic conduction, field emission (FE) characteristics of this material demonstrate a strong deviation from the Fowler–Nordheim law in a high-current-density regime when (N)UNCD field emitters switch from a diodelike to a resistorlike behavior. Such a phenomenon resembles the current-density saturation effect in conventional semiconductors. In the present paper, we adapt the formalism developed for conventional semiconductors to study current-density saturation in (N)UNCD field emitters. We provide a comprehensive theoretical investigation of (i) partial penetration of the electric field into the material, (ii) transport effects (such as electric-field-dependent mobility), and (iii) features of a complex density-of-states structure (position and shape of π−π∗ bands, controlling the concentration of charge carriers) on the FE characteristics of (N)UNCD. We show that the formation of the current-density saturation plateau can be explained by the limited supply of electrons within the impurity π−π∗ bands and decreasing electron mobility in a high electric field. Theoretical calculations are consistent with the experiment.