Electron Mobility Research Articles

Review of Carbon Nanotube Field Effect Transistor for Nanoscale Regime

Background: The need for high performance, small size, low delay, low power consumption, and long battery backup of portable systems is increasing with the advancement of technology. Many features of portable systems can be improved using scaling methods. In the scaling process, reducing the size of devices causes serious difficulties, including the short channel effect (SCE) and leakage current, which degenerates the characteristics of the systems. Objective: In this review paper, a trending carbon nanotube field effect transistor (CNTFET) technology is discussed in detail. CNTFET can replace the conventional metal oxide semiconductor field effect transistor (MOSFET) technology to overcome the SCE problems in the nanoscale regime and also meet the requirements of portable systems. Methods: The CNTFET is an extremely good nanoscale technology due to its one-dimension band structure, high transconductance, high electron mobility, superior control over channel formation, and better threshold voltage. This technology is used to construct high-performance and low-power circuits by replacing the MOSFET technology. CNTFET in comparison to MOSFET takes the carbon nanotube (CNT) as a channel region. Results: The value of threshold voltage in CNTFET changes with the diameter of CNT. The threshold voltage of the devices controls many parameters at the circuit-level design. Hence, the detailed operation and the characteristics of CNTFET devices are presented in this review paper. The existing CNTFET-based ternary full adder (TFA) circuits are also described in this review paper for the performance evaluation of different parameters. Conclusion: CNTFET technology is the possible solution for the SCE in the nanoscale regime and is capable to design efficient logic circuits. The circuits using the CNTFET technology can provide better performance and various advantages, including fast speed, small area, and low power consumption, in comparison to the MOSFET circuits. Thus, CNTFET technology is the best choice for circuit designs at the nanoscale.

Graphite/h-BN van der Waals heterostructure as a gate stack for HgTe quantum wells

Two-dimensional topological insulators have attracted much interest due to their potential applications in spintronics and quantum computing. To access the exotic physical phenomena, a gate electric field is required to tune the Fermi level into the bulk band gap. Hexagonal boron nitride (h-BN) is a promising alternative gate dielectric due to its unique advantages such as flat and charge-free surface. Here we present a h-BN/graphite van der Waals heterostructure as a top gate on HgTe heterostructure-based Hall bar devices. We compare our results to devices with h-BN/Ti/Au and HfO2/Ti/Au gates. Devices with a h-BN/graphite gate show no charge carrier density shift compared to as-grown structures, in contrast to a significant n-type carrier density increase for HfO2/Ti/Au. We attribute this observation mainly to the comparable work function of HgTe and graphite. In addition, devices with h-BN gate dielectric show slightly higher electron mobility compared to HfO2-based devices. Our results demonstrate the compatibility between layered materials transfer and wet-etched structures and provide a strategy to solve the issue of significant shifts of the carrier density in gated HgTe heterostructures.

Understanding of multiway heat extraction using peripheral diamond in an AlGaN/GaN high electron mobility transistor by electrothermal simulations

High-power operation of high electron mobility transistors (HEMTs) is limited due to a variety of thermal resistances in HEMT devices that cause self-heating effects (SHEs). To reduce SHEs, diamond heat spreaders integrated in the device have proven efficient in extracting heat from the device. In this report, we use electrothermal technology computer-aided design simulations to demonstrate a qualitative understanding of multiway heat extraction utilizing diamond heat spreaders to improve HEMT thermal performance at high DC output power densities (∼40 W mm−1). The impact of each heat extraction pathway is understood while considering the thermal boundary resistance between the diamond/GaN heterointerface and optimization of the GaN buffer layer thickness. Using these findings, we simulate an AlGaN/GaN HEMT device operating at 40 W mm−1 DC output power and demonstrate significant reduction in the temperature.

Comprehensive empirical modeling of ScAlN/AlGaN/GaN ferroelectric HEMT

This correspondence introduces a concise depiction of a high electron mobility transistor (HEMT) utilizing a model that incorporates the effect of ferroelectric (FE) polarization on the empirical expression for drain current. This is accomplished by modifying the basic Shockley equation (S.E) for MESFET. The model employed in this investigation is particularly geared towards investigating the impact of the counterclockwise hysteresis effect due to the use of dielectric ScAlN on the transfer characteristics ( Id−Vgs ), which is mainly attributed to trapping and de-trapping of charge carriers at the dielectric/semiconductor interface. Modifying the S.E. made it possible to replicate the behavior observed in experimental HEMT accurately. We reported the modified drain current empirical expressions for the ScAlN/AlGaN/GaN HEMT that considers the FE polarization. The results of this study showed good agreement between the Id−Vgs characteristics obtained from the analytical and experimental data, with a root mean square deviation (RMSD) error of 0.97% and a mean squared error error of 0.94%. This analytical drain current model, presented herein for the first time, incorporates a limited number of fitting parameters. Therefore, enhancing the computational efficiency to assess the performance of FE GaN HEMTs for usage in wide-area electronics.

Contribution of Andreev Reflection to the Mobility of Surface State Electrons on Superfluid $$^3$$He-B

Contribution of Andreev Reflection to the Mobility of Surface State Electrons on Superfluid $$^3$$He-B

Janus TiSi Z 3H ( Z= N, P, As) monolayers with large out-of-plane piezoelectricity and high electron mobility: first-principles study

In this paper, we propose a series of novel two-dimensional Janus structures TiSiZ 3H ( Z= N, P, As) and investigate their electronic, mechanical, piezoelectric, and transport properties using the density functional theory. The examined results demonstrated that Janus TiSiZ 3H monolayers are structurally stable. At the ground state, TiSiN3H, TiSiP3H, and TiSiAs3H monolayers are all semiconductors with indirect bandgaps of 2.98, 1.06, 1.0 eV at the hybrid-functional level, respectively. The absence of vertical mirror symmetry in TiSiZ 3H results in the occurrence of both in-plane and out-of-plane piezoelectricity. Interestingly, TiSiAs3H monolayer exhibits large out-of-plane piezoelectric response with coefficient d 31 up to −0.45 pm V−1, that is appropriate for use in piezoelectric devices. The calculated results demonstrate that the carrier mobility in Janus TiSiZ 3H monolayers is high and directional anisotropic. Particularly, electron mobility in TiSiP3H monolayer is observed up to 772.31 cm2 V−1 s−1. Our results not only suggest and confirm some fundamental physical characteristics of new materials but also lay important premises for experimental studies as well as their application prospects in the future.

Optimizing charge transport in hybrid GaN-PEDOT:PSS/PMMADevice for advanced application

Organic–inorganic hybrid light-emitting devices have garnered significant attention in the last few years due to their potential. These devices integrate the superior electron mobility of inorganic semiconductors with the remarkable optoelectronic characteristics of organic semiconductors. The inquiry focused on analyzing the optical and electrical properties of a light-emitting heterojunction that combines p-type GaN with organic materials (PEDOT, PSS, and PMMA). This heterojunction is an organic–inorganic hybrid. The procedure entailed utilizing a spin-coating technique to apply a layer of either poly(methyl methacrylate) (PMMA) or a mixture of PMMA and poly(3,4ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT: PSS) onto an indium tin oxide (ITO) substrate. Subsequently, different Nd:YAG laser pulses (200, 250, and 300 pulses) were used to administer a GaN inorganic layer onto the prepared organic layer using a pulsed laser deposition approach. Subsequently, the thermal evaporation technique was employed to deposit an aluminum electrode on the top of the organic and inorganic layers, while laser pulses were fine-tuned for optimal performance. The Hall effect investigation verifies the p-type conductivity of the GaN material. The electroluminescence studies confirmed the production of blue light by the GaN-based devices throughout a range of voltage situations, spanning from 45 to 72 V.

Recent Advances in Metal Oxide Electron Transport Layers for Enhancing the Performance of Perovskite Solar Cells.

Perovskite solar cells (PSCs) have attracted considerable interest owing to their low processing costs and high efficiency. A crucial component of these devices is the electron transport layer (ETL), which plays a key role in extracting and transmitting light-induced electrons, modifying interfaces, and adjusting surface energy levels. This minimizes charge recombination in PSCs, a critical factor in their performance. Among the various ETL materials, titanium dioxide (TiO2) and tin dioxide (SnO2) stand out due to their excellent electron mobility, suitable band alignment, high transparency, and stability. TiO2 is widely used because of its appropriate conduction band position, easy fabrication, and favorable charge extraction properties. SnO2, on the other hand, offers higher electron mobility, better stability under UV illumination, and lower processing temperatures, making it a promising alternative. This paper summarizes the latest advancements in the research of electron transport materials, including material selection and a discussion of electron collection. Additionally, it examines doping techniques that enhance electron mobility and surface modification technologies that improve interface quality and reduce recombination. The impact of these parameters on the performance and passivation behavior of PSCs is also examined. Technological advancements in the ETL, especially those involving TiO2 and SnO2, are currently a prominent research direction for achieving high-efficiency PSCs. This review covers the current state and future directions in ETL research for PSCs, highlighting the crucial role of TiO2 and SnO2 in enhancing device performance.

Floquet nonadiabatic mixed quantum-classical dynamics in periodically driven solid systems.

In this paper, we introduce the Floquet mean-field dynamics and Floquet surface hopping approaches to study the nonadiabatic dynamics in periodically driven solid systems. We demonstrate that these two approaches can be formulated in both real and reciprocal spaces. Using the two approaches, we are able to simulate the interaction between electronic carriers and phonons under periodic drivings, such as strong light-matter interactions. Employing the Holstein and Peierls models, we show that strong light-matter interactions can effectively modulate the dynamics of electronic population and mobility. Notably, our study demonstrates the feasibility and effectiveness of modeling low-momentum carriers' interactions with phonons using a truncated reciprocal space basis, an approach impractical in real space frameworks. Moreover, we reveal that even with a significant truncation, carrier populations derived from surface hopping maintain greater accuracy compared to those obtained via mean-field dynamics. These results underscore the potential of our proposed methods in advancing the understanding of carrier-phonon interactions in various periodically driven materials.

High-speed independent polarization modulation based on dual channel 2DEG anisotropic terahertz metasurfaces

In communication and imaging systems, anisotropic metasurfaces bring new degrees of freedom for the independent control of electromagnetic waves with different polarizations. This paper proposes a dual-polarization modulation metasurface (DPMM) operating in the terahertz (THz) frequency range, which can effectively and independently modulate two orthogonally linearly polarized electromagnetic waves. The metasurface unit adopts an anisotropic structure, and by integrating multiple GaN-based high electron mobility transistors (GaN-HEMTs) into the anisotropic structure, independent control of the two-dimension electron gas (2DEG) carrier concentration in the dual-polarization channel is achieved, enabling selective modulation of x-polarization and y-polarization waves. Simulation and static experimental results demonstrate the excellent modulation isolation performance of the DPMM, with a maximum isolation exceeding 45 dB. Dynamic experiments further highlight the high-speed modulation effect, with modulation speeds exceeding 1 GHz for a single channel and an overall modulation speed of 2 GHz for dual polarization, highlighting its potential for enhancing channel capacity and information throughput in THz communication and imaging systems.

Preparation of Nb5+ Doped Na3V2(PO4)3 Cathode Material for Sodium Ion Batteries.

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs) due to the abundance and low cost of sodium resources. Cathode material plays a crucial role in the performance of sodium ion batteries determining the capacity, cycling stability, and rate capability. Na3V2(PO4)3 (NVP) is a promising cathode material due to its stable three-dimensional NASICON structure, but its discharge capacity is low and its decay is serious with the increase of cycle period. We focused on modifying NVP cathode material by coating carbon and doping Nb5+ ions for synergistic electrochemical properties of carbon-coated NVP@C as a cathode material. X-ray diffraction analysis was performed to confirm the phase purity and crystal structure of the Nb5+ doped NVP material, which exhibited characteristic diffraction peaks that matched well with the NASICON structure. Nb5+-doped NVP@C@Nbx materials were prepared using the sol-gel method and characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Raman and Brunauer -Emmett-Teller (BET) analysis. First-principles calculations were performed based on density functional theory. VASP and PAW methods were chosen for these calculations. GGA in the PBE framework served as the exchange-correlation functional. The results showed the NVP unit cell consisted of six NVP structural motifs, each containing octahedral VO6 and tetrahedral PO4 groups to form a polyanionomer [V2(PO4)3] along with the c-axis direction by PO4 groups, which had Na1(6b) and Na2(18e) sites. And PDOS revealed that after Nb doping, the d orbitals of the Nb atoms also contributed electrons that were concentrated near the Fermi surface. Additionally, the decrease in the effective mass after Nb doping indicated that the electrons could move more freely through the material, implying an enhancement of the electron mobility. The electrochemical properties of the Nb5+ doped NVP@C@Nb cathode material were evaluated through cyclic voltammetry (CV), galvanostatic charge-discharge tests, electrochemical impedance spectroscopy (EIS), and X-ray photoelectric spectroscopy (XPS). The results showed that NVP@C@Nb0.15 achieved an initial discharge capacity as high as 114.27 mAhg-1, with a discharge capacity of 106.38 mAhg-1 maintained after 500 cycles at 0.5C, and the retention rate of the NVP@C@Nb0.15 composite reached an impressive 90.22%. NVP@C@Nb0.15 exhibited low resistance and high capacity, enabling it to create more vacancies and modulate crystal structure, ultimately enhancing the electrochemical properties of NVP. The outstanding performance can be attributed to the Nb5+-doped carbon layer, which not only improves electronic conductivity but also shortens the diffusion length of Na+ ions and electrons, as well as reduces volume changes in electrode materials. These preliminary results suggested that the as-obtained NVP@C@Nb0.15 composite was a promising novel cathode electrode material for efficient sodium energy storage.

Dual-Module Ultrawide Dynamic-Range High-Power Rectifier for WPT Systems

Rectifier plays a pivotal role in wireless power transfer systems. While numerous studies have concentrated on enhancing efficiency and bandwidth at specific high-power levels, practical scenarios often involve unpredictable power inputs. Consequently, a distinct need arises for a rectifier that demonstrates superior efficiency across a broad range of input power levels. This paper introduces a high-power RF-to-DC rectifier designed for WPT applications, featuring an ultrawide dynamic range of input power. The rectification process leverages a GaN (gallium nitride) high electron mobility transistor (HEMT) to efficiently handle high power levels up to 12.6 W. The matching circuit was designed to ensure that the rectifier will operate in class-F mode. A Schottky diode is incorporated into the design for relatively lower-power rectification. Seamless switching between the rectification modes of the two circuits is accomplished through the integration of a circulator. The proposed rectifier exhibits a 27.5 dB dynamic range, achieving an efficiency exceeding 55% at 2.4 GHz. Substantial improvement in power handling and dynamic range over traditional rectifiers is demonstrated.

Synthesis and Electron Transporting Properties of Diblock Copolymers Consisting of Polyfluorene and Polystyrene.

Poly(9,9-di-n-octylfluorene) (PFO) is a promising material for polymer light-emitting diodes (PLEDs) due to its advantageous properties. To enhance its electron transporting capabilities, diblock polymers were synthesized by attaching polystyrene (PSt) chains of varying lengths to one end of the PFO molecule. In a comparative study with PFO homopolymer, the diblock polymers maintained similar thermal properties, absorption spectra, and photoluminescent stability, while exhibiting slightly deeper lowest unoccupied molecular orbital (LUMO) levels and higher crystallinity. Notably, diblock polymers with shorter polystyrene blocks demonstrated higher electron mobility than the PFO homopolymer and diblock polymers with excessively long polystyrene blocks. These findings suggest that the optimal chain length of the polystyrene block is crucial for maximizing electron mobility, thus offering valuable insights for designing high-performance PLED materials.

Metal‐Organic Chemical Vapor Deposition Regrowth of Highly Doped n+ (In)GaN Source/Drain Layers for Radio Frequency Transistors

Herein, epitaxially regrown n+ (In)GaN source/drain layers for radio frequency high electron mobility transistors, addressing material and electrical characterization, are reported. A range of n+ GaN and n+ InGaN layers with indium 4–12 at% and silicon 0–5.1 × 1020 cm−3 are evaluated. The active carrier concentration in n+ InGaN exceeds 2 × 1020 cm−3. The layers exhibit so‐called V‐defects, observed by atomic force microscope and transmission electron microscope (TEM), which are associated with local composition changes. In addition to the high Si doping levels, nitrogen vacancies are also considered to contribute toward their net carrier concentration. Due to its relevance for device processing, selectivity analysis is performed, and the optimal process conditions for selective regrowth are identified. Regrowth under high temperature (800 °C) is found to be conducive to improved selectivity. However, a high thermal budget negatively impacts the overall regrowth process, as reported here for In0.17Al0.83N and Al0.26Ga0.74N barriers: whereas the InAlN barrier suffers from intermixing, the AlGaN barrier demonstrates high‐temperature stability. The impact of intermixing is studied from complementary TEM and DC electrical measurements. A low overall contact resistance of 75 Ω μm is obtained with the regrown n+ InGaN layers.

Deuterated chloroform replaces ultra-dry chloroform to achieve high-efficient organic solar cells

Chloroform is a common and excellent solvent for preparing high-efficient organic solar cells (OSCs), however, it is toxic and poisonable chemical. In comparisons, deuterated chloroform (DC) is less toxic and costly, and particularly, it is non-poisonable chemical. In this paper, we use DC to replace ultra-dry chloroform (UC) as the processing solvent for preparation of active layers of organic solar cells. First, we selected PM6:BTP-eC9 as the basic binary and counted 100 solar cells’ data, from which comparable device performance were obtained with use of DC and UC. Interestingly, DC showed better reproducibility, superior storage under a nitrogen atmosphere and a little better performance than UC. Both DC and UC gave rise of comparable hole and electron mobilities and similar charge recombination losses. Second, we based PM6:Y6 and D18-Cl:Y6 as the binaries and similar effects were obtained from both UC and DC when counting 30 devices for each binary. Third, the universality of the use of DC for preparing high-efficient OSCs were again checked with several binary and ternary systems. In all, this study demonstrate that DC can replace UC for use in the field of OSCs.

Multiphysics simulation study of thermal stress effects in nanoscale FinFETs heterogeneously integrated with GaN high-power device on silicon substrate

Heterogeneous integration enables high-density integration to improve system functionality, but meanwhile brings reliability concerns due to the intensification of temperature and thermal stress. However, research on thermal stress effects in nanoscale transistors within heterogeneous integrated circuit is still very limited up to now. Multiphysics computation for the heterogeneous integration of nanoscale p-FinFET with GaN high electron mobility transistor (HEMT) on silicon substrate is performed. Firstly, self-heating of GaN HEMT induced thermal stress is simulated. Then, thermal stress effects induced by the GaN HEMT on quantum transport in the FinFET at different locations are compared and analyzed, including the hole effective mass, hole average velocity, on/off-state current, and on-off ratio. Moreover, the impacts of crystal orientation configurations are further discussed. Simulation results show that the impact of thermal stress on FinFET is highly dependent on the Si crystal orientation and its location relative to GaN HEMT. Taking the FinFET with (001)/[110] confinement/transport orientation, which is at a distance of RFET and an angle of φ from the GaN HEMT as an example, thermal stress induces the significant reductions in Ion and Ioff of FinFET at φ = π/2 and φ = 3π/2, and the increases of Ion and Ioff in FinFET at φ = 0 and φ = π, but has small impact on FinFETs located diagonally to the GaN HEMT.

In-Situ Cyclized Polyacrylonitrile as an Electron Selective Layer for n-i-p Perovskite Solar Cell with Enhanced Efficiency and Stability.

In situ cyclized polyacrylonitrile (CPAN) is developed to replace n-type metal oxide semiconductors (TiO2 or SnO2) as an electron selective layer (ESL) for highly efficient and stable n-i-p perovskite solar cells (PSCs). The CPAN layer is fabricated via facile in situ cyclization reaction of polyacrylonitrile (PAN) coated on a conducting glass substrate. The CPAN layer is robust and insoluble in common solvents, and possesses n-type semiconductor properties with a high electron mobility of 4.13×10-3 cm2 V-1 s-1. With the CPAN as an ESL, the PSC affords a power conversion efficiency (PCE) of 23.12 %, which is the highest for the n-i-p PSCs with organic ESLs. Moreover, the device with the CPAN layer holds superior operational stability, maintaining over 90 % of their initial efficiency after 500 h continuous light soaking. These results confirm that the CPAN layer would be a desirable low-cost and efficient ESL for n-i-p PSCs and other photoelectronic devices with high performance and stability.

Thermally stable radio frequency power and noise behaviors of AlGaN/GaN high electron mobility transistor without voltage-blocking buffer layer design

Thermally stable radio frequency power and noise behaviors of AlGaN/GaN high electron mobility transistor without voltage-blocking buffer layer design

In‐Situ Cyclized Polyacrylonitrile as an Electron Selective Layer for n‐i‐p Perovskite Solar Cell with Enhanced Efficiency and Stability

AbstractIn situ cyclized polyacrylonitrile (CPAN) is developed to replace n‐type metal oxide semiconductors (TiO2 or SnO2) as an electron selective layer (ESL) for highly efficient and stable n‐i‐p perovskite solar cells (PSCs). The CPAN layer is fabricated via facile in situ cyclization reaction of polyacrylonitrile (PAN) coated on a conducting glass substrate. The CPAN layer is robust and insoluble in common solvents, and possesses n‐type semiconductor properties with a high electron mobility of 4.13×10−3 cm2 V−1 s−1. With the CPAN as an ESL, the PSC affords a power conversion efficiency (PCE) of 23.12 %, which is the highest for the n‐i‐p PSCs with organic ESLs. Moreover, the device with the CPAN layer holds superior operational stability, maintaining over 90 % of their initial efficiency after 500 h continuous light soaking. These results confirm that the CPAN layer would be a desirable low‐cost and efficient ESL for n‐i‐p PSCs and other photoelectronic devices with high performance and stability.

High Mobility n‐Type Imide‐Based Semiconductor with Unusual Single‐Crystal Packing Structure in Solution‐Processed Thin Film

AbstractSolid‐state molecular arrangement has been recognized as the most important role in the charge transport properties of organic semiconductors. Although highly ordered molecular stacking is achieved in single crystals, maintaining single‐crystal molecular packing in solution‐processed thin films remains a significant challenge. Herein, a new type of n‐type organic semiconductors based on an asymmetric fluoranthene imide unit is reported, whose intermolecular packing and aggregation behavior in the thin film state can be effectively controlled by regulating the cyano substitution sites and alkyl chain types in the imide group. F10 with cyano groups at 4,9‐sites and branched 2‐ethylhexyl chain encouragingly shows a highly ordered single‐crystal‐like molecular packing in solution‐processed thin film after thermal annealing, and thus the resulting organic field‐effect transistors exhibit impressive charge transport performance, with the electron mobility as high as 0.116 cm2 V−1 s−1. This work opens a new avenue for developing high‐performance solution‐processed n‐type semiconductors.