Ballistic Transport Research Articles

Recent Experimental Breakthroughs on 2D Transistors: Approaching the Theoretical Limit

AbstractSince Si‐based Moore's law is physically limited, 2D semiconductors are proposed as successors to continue shrinking the transistor size for more Moore electronics. However, limited by experimental technology bottlenecks, the theoretical predicted superiorities of the 2D transistors over the state‐of‐the‐art Si transistors have been lacking concrete evidence for a decade. In this review, recent exciting experimental breakthroughs for 2D transistors are presented, including gate length miniaturization to a sub‐1 nm limit, electrode contact optimization to the resistance quantum limit, high‐quality dielectric fabrication with an equivalent oxide thickness to sub‐0.5 nm, novel architecture form (2D fin field‐effect transistor), and back‐end‐of‐line integration of directly grown 2D materials on Si complementary metal‐oxide‐semiconductor circuits. Remarkably, an ultrashort channel, Ohmic contact, ballistic transport, and ultrathin dielectric layer are simultaneously satisfied in the 2D InSe transistor, and device performances approaching the theoretical limit are observed. The measured key figures of merit of the ideal 2D InSe transistor are comparable to or even surpass those of the Si transistors. Finally, the challenges and outlook on more Moore electronics based on 2D transistors are highlighted.

Consideration of electronic mean heat transport via a low dimension system

The object of research is the complex realm of energy localization and coherent ballistic electronic transport within low-dimensional silicon quantum wires, specifically those doped with germanium atoms. Unlike their three-dimensional counterparts, low-dimensional systems exhibit unique electronic transport behaviors, necessitating novel analytical approaches for a comprehensive understanding. The core of this investigation leverages the Phase Field Matching Theory (PMFT) and the tight-binding (TB) approximation, sophisticated methodologies that enable a deep dive into the quantum mechanical nuances of these systems. Through this lens, we examine the intricate dynamics of dispersion relationships, phase factors, group velocities, and notably, the impact of defects introduced by the germanium doping. This research meticulously analyzes how these defects affect electronic and thermal conductivities, along with densities of states, offering new insights into the role of Fano resonances in the fluctuation of transmission and reflection spectra. These resonances, we find, are crucially dependent on the nature of the defects, their configuration, and the electronic parameters in their vicinity, underscoring the nuanced interplay between material composition and electronic properties in low-dimensional systems. The implications of our findings extend far beyond the theoretical. They pave the way for significant advancements in nanotechnology and the design of electronic devices, highlighting the potential for creating more efficient, high-performance components. Furthermore, this work proposes a framework for developing non-destructive testing methodologies that could revolutionize material science by enabling the precise analysis of defects in low-dimensional systems without causing damage. This is particularly critical for the ongoing development of materials with optimized properties for various applications, from electronics to energy storage. In essence, this research not only enriches our understanding of the physics governing low-dimensional systems but also offers practical insights into leveraging these properties for technological innovation. By bridging the gap between theoretical physics and material science, our study sets the stage for the next generation of electronic components and non-destructive evaluation techniques, marking a significant step forward in the application of quantum mechanics to real-world challenges.

G-C3N4 as ballistic electron transport “Tunnel” in CsPbBr3-based ternary photocatalyst for gas phase CO2 reduction

Perovskite CsPbBr3 quantum dot shows great potential in artificial photosynthesis, attributed to its outstanding optoelectronic properties. Nevertheless, its photocatalytic activity is hindered by insufficient catalytic active sites and severe charge recombination. In this work, a CsPbBr3@Ag-C3N4 ternary heterojunction photocatalyst is designed and synthesized for high-efficiency CO2 reduction. The CsPbBr3 quantum dots and Ag nanoparticles are chemically anchored on the surface of g-C3N4 sheets, forming an electron transfer tunnel from CsPbBr3 quantum dots to Ag nanoparticles via g-C3N4 sheets. The resulting CsPbBr3@Ag-C3N4 ternary photocatalyst, with spatial separation of photogenerated carriers, achieves a remarkable conversion rate of 19.49 μmol·g−1·h−1 with almost 100 % CO selectivity, a 3.13-fold enhancement in photocatalytic activity as compared to CsPbBr3 quantum dots. Density functional theory calculations reveal the rapid CO2 adsorption/activation and the decreased free energy (0.66 eV) of *COOH formation at the interface of Ag nanoparticles and g-C3N4 in contrast to the g-C3N4, leading to the excellent photocatalytic activity, while the thermodynamically favored CO desorption contributes to the high CO selectivity. This work presents an innovative strategy of constructing perovskite-based photocatalyst by modulating catalyst structure and offers profound insights for efficient CO2 conversion.

Nonreciprocal ballistic transport in asymmetric bands

Nonreciprocal ballistic transport in asymmetric bands

Evolution and performance analysis of quantum well FinFET for 3 nm technology node with type-II strained tri-layered hetero-channel system

3D FinFETs are meticulously scaled down to sub-14 nm leading to reemerging undesirable characteristics namely increased Drain Induced Barrier Leakage (DIBL), higher subthreshold swing and excessive leakage currents. This inhibits the scaling of FinFETs and research suggests probable utilization of strained silicon technology in FinFETs to improve the on currents and transconductance of the nano devices. The emergence of quantum effects including velocity overshoot and carrier confinement severely affects the electrical characteristics at sub-10 nm channel length devices. Therefore, amalgamation of strained silicon prove to be a boon in FinFETs while being at par with the proposed 3 nm technology node of IRDS 2018, and designing to develop reliable devices at 08 nm gate length is the requisite. Thus, exploring the design and performance investigation of novel 08 nm Quantum Well FinFETs (QW-FinFETs) incorporating a tri-layered strained silicon Heterostructure-On-Insulator (HOI)are proposed with distinct channel dimensions which are analyzed and compared with existing devices. The optimum QW-FinFET device developed for 3 nm technology node of IRDS 2018 achieved a ∼25% enhancement in drain currents with Device D2 portraying almost ∼103% escalations in electron mobility on account of ballistic transport of charge carriers without scattering and enriching the performance for the future generation of device resulting in faster switching operation in sub-nano regime.

Investigation of thermal stress effects on subthreshold conduction in nanoscale p-FinFET from Multiphysics perspective

The rising temperature due to a self-heating or thermal environment not only degrades the subthreshold performance but also intensifies thermal stress, posing a severe challenge to device performance and reliability design. The thermal stress effects on the ON-state performance of the p-type fin field-effect transistor were previously studied. However, as far as we know, how thermal stress affects its subthreshold conduction remains unclear, which is studied in this manuscript. The impact of thermal stress due to the self-heating of adjacent devices on subthreshold conduction is investigated by solving the quantum transport, thermal conduction, and force balance equations for ballistic transport and dissipative transport with phonon scattering. Then, the thermal stress effects at different ambient temperatures are further discussed and analyzed. The simulation results show that the OFF-state leakage current can be reduced by thermal stress, even up to 9.28% for the (110)/[001] device operating at an ambient temperature of 550 K, and its reduction is the comprehensive result of the thermal stress effects on the band structure, potential profile, carrier distribution, and source-to-drain tunneling. In addition, the thermal stress has no significant effects on subthreshold swing although it can change the magnitude of the subthreshold current. Moreover, the effect of thermal stress on subthreshold conduction is highly dependent on the thermal environment of the device and the crystal orientation of the channel semiconductor material.

Gap engineering effects on transport and tunneling magnetoresistance properties in phosphorene ferromagnetic/normal/ferromagnetic junction

Tuning the band gap is of utmost importance for the practicality of two-dimensional materials in the semiconductor industry. In this study, we investigate the ballistic transport and the tunneling magnetoresistance (TMR) properties within a modulated gap in a ferromagnetic/normal/ferromagnetic (F/N/F) phosphorene junction. The theoretical framework is established on a Dirac-like Hamiltonian, the transfer matrix method, and the Landauer–Büttiker formalism to characterize electron behavior and obtain transmittance, conductance and TMR. Our results reveal that a reduction in gap energy leads to an enhancement of conductance for both parallel and anti-parallel magnetization configurations. In contrast, a significant reduction and redshift in TMR have been observed. Notably, the application of an electrostatic field in a gapless phosphorene F/N/F junction induces a blueshift and a slight increase in TMR. Furthermore, we found that introducing an asymmetrically applied electrostatic field in this gapless junction results in a significant reduction and redshift in TMR. Additionally, intensifying the applied magnetic field leads to a substantial increase in TMR. These findings could be useful for designing and implementing practical applications that require precise control over the TMR properties of a phosphorene F/N/F junction with a modulated gap.

Unraveling the mechanisms of thermal boundary conductance at the graphene-silicon interface: Insights from ballistic, diffusive, and localized phonon transport regimes

Unraveling the mechanisms of thermal boundary conductance at the graphene-silicon interface: Insights from ballistic, diffusive, and localized phonon transport regimes

Diffusive and ballistic transport in thin InSb nanowire devices using a few-layer-graphene-AlO x gate

Quantum devices based on InSb nanowires (NWs) are a prime candidate system for realizing and exploring topologically-protected quantum states and for electrically-controlled spin-based qubits. The influence of disorder on achieving reliable quantum transport regimes has been studied theoretically, highlighting the importance of optimizing both growth and nanofabrication. In this work, we consider both aspects. We developed InSb NW with thin diameters, as well as a novel gating approach, involving few-layer graphene and atomic layer deposition-grown AlO x . Low-temperature electronic transport measurements of these devices reveal conductance plateaus and Fabry–Pérot interference, evidencing phase-coherent transport in the regime of few quantum modes. The approaches developed in this work could help mitigate the role of material and fabrication-induced disorder in semiconductor-based quantum devices.

Ballistic performance and overshoot effects in gallenene nanoribbon field-effect transistors

Gallenene is a novel metallic 2D material that can provide a semiconducting counterpart if patterned into quasi-one-dimensional (quasi-1D) nanostructures, i.e., gallenene nanoribbons (GaNRs). We investigate semiconducting GaNRs as a potential channel material for future ultrascaled field-effect transistors (FETs) by employing quantum transport simulations based on Green's functions and tight-binding Hamiltonians with the orbital resolution calibrated on ab initio calculations. The impact of GaNR width downscaling from ∼6 nm down to ∼0.2 nm on the electronic, transport, and ballistic device properties is investigated for the FET channel length of 15 nm. We report current enhancement and injection velocity overshoot effects for sub-1.2 nm-wide nFETs and pFETs, with a maximum current increase of 53% in the 1.2 nm-wide GaNR pFET in comparison to the widest device. In addition, promising current-driving capabilities of n- and p-channel GaNR FETs are observed with top ballistic currents of more than 2.2 mA/μm and injection velocities of up to 2.4 × 107 cm/s. The reported data are explained by analyzing the evolution of band structure and related parameters such as injection velocity, quantum capacitance, effective transport mass etc., with increasing quantum confinement effects in ultranarrow GaNRs. Generally, we find that quasi-1D gallenene is a promising channel material for future nanoscale FETs, especially for transistor architectures based on stacked nanosheets.

Exploration of underlap induced high-k spacer with gate stack on strain channel cylindrical nanowire FET for enriched performance

This research explores a comprehensive examination of gate underlap incorporated strained channel Cylindrical Gate All Around Nanowire FET having enriched performances above the requirement of the 2 nm technology node of IRDS 2025. The device installs a combination of strain engineering based quantum well barrier system in the channel region with high-k spacers sandwiching the device underlaps and stack high-k gate-oxide. The underlaps are prone to parasitic resistance and various short channel effects (SCEs) hence, are sandwiched by HfO2 based high-k. This SCE degradations and a strong electric field in the drain-channel region is rendered controlling the leakages. The strain based Nanosystem engineering is incorporated with Type-II heterostructure band alignment inducing quantum well barrier mechanism in the ultra-thin cylindrical channel region creating an electrostatic charge centroid leading to energy band bending and splitting among the two-fold and four-fold valleys of the strained Silicon layer. This provides stupendous electron mobility instigating high current density and electron velocity in the channel. Thereby, the device is susceptible to on-current enhancement via ballistic transport of carriers and carrier confinement via succumbing of quantum charge carriers. The device transconductance, Ion, Ioff, Ion/Ioff ratio are measured and the output performance (ID-VDS) characteristics is determined providing emphatic enrichments in contrast to the existing gate all-around FETs as well as the 2 nm technology node data of IRDS 2025. Hence, the strained channel Nanowire FET device developed here is presented here as the device of the future for various digital applications, RF applications and faster switching speed.

Analytical Modeling of [001] Orientation in Silicon Trigate Rectangular Nanowire Using a Tight-Binding Model

AbstractIn the realm of electronics, the performance of Silicon Trigate Rectangular Nanowires (Si-TRNW) and the structural characteristics of <001> orientation using tight-binding models have been analyzed. The fast algorithm based on the tight-binding model for Trigate Silicon nanowires yielded a remarkable ION/IOFF ratio of 1.49 × 1010 and leakage current (ILeak or IOFF) of 3.7 × 10−17μA. Furthermore, a maximum conduction band energy level (Ecmax) of −0.003 eV and a Subthreshold Slope (SS) of 120 mV has been obtained for a channel length of 15 nm. At an energy level of 3 eV, a high Transmission coefficient, T(ε), of 4 has been attained using the E-k dispersion method. This analysis also involved the calculation of three ∆ valleys pertinent to the channel’s effectiveness in <001> orientation, with proximity nearer to 1 m0. The Schrodinger-Poisson equation has been analyzed with the Ballistic transport along the [001] z-direction in channel potential. A comparative assessment has been also performed between the lateral dimensions of rectangular nanowires with equal energy levels, utilizing both the tight-binding model and Density Functional Theory (DFT) techniques. In some high-frequency applications, a high transmission coefficient is beneficial to maximize the amount of energy or information that gets transmitted. Reducing leakage current would offer a technological pathway for performance improvement of high-frequency applications. The high ON-current (ION) has been obtained through the DFT approach between source and drain terminals is particularly desirable for applications demanding for fast switching speeds and high-performance computing. The strengths of both methods in hybrid approaches is a common strategy to achieve simulations that are both accurate and efficient. Notably, the nanowires subjected to hydrostatic strain, exhibiting enhanced mobility and exceptional electrostatic integrity, emerged as pivotal components for forthcoming technology nodes. This research augments the potential feasibility of strain-based Si nanowires, even at the 3 nm scale, in subsequent technological advancements.

Ballistic Conductance through Porphyrin Nanoribbons.

The search for long molecular wires that can transport charge with maximum efficiency over many nanometers has driven molecular electronics since its inception. Single-molecule conductance normally decays with length and is typically far below the theoretical limit of G0 (77.5 μS). Here, we measure the conductances of a family of edge-fused porphyrin ribbons (lengths 1-7 nm) that display remarkable behavior. The low-bias conductance is high across the whole series. Charging the molecules in situ results in a dramatic realignment of the frontier orbitals, increasing the conductance to 1 G0 (corresponding to a current of 20 μA). This behavior is most pronounced in the longer molecules due to their smaller HOMO-LUMO gaps. The conductance-voltage traces frequently exhibit peaks at zero bias, showing that a molecular energy level is in resonance with the Fermi level. This work lays the foundations for long, perfectly transmissive, molecular wires with technological potential.

Combined effect of rashba spin-orbit interaction, hydrostatic pressure and temperature on energy dispersion based ballistic conductance of InAs tunnel-coupled (double) quantum wire under exterior magnetic and electric field

In this work the change in energy dispersion curve and ballistic conductance for InAs tunnel-coupled double quantum wire (DQWR) under the influence of the external magnetic field, electric field, hydrostatic pressure, temperature, and Rashba spin-orbit interaction (SOI) are studied theoretically in detail. A constant magnetic field transverse to the fundamental plane is applied to the DQWR. The Landuer-Büttiker formalism is used to determine the transport characteristics, and the energy eigenvectors and eigenvalues are derived within the parameters of the diagonalization approach. The numerical results indicate that the energy spectra of the system are influenced by external factors such as magnetic field, electric field, temperature, hydrostatic pressure, and Rashba SOI, resulting in variations in lateral/vertical and downward/upward shifts. These factors also cause oscillatory patterns in ballistic conductance due to the oddity of energy subbands. These results have significant implications for the advancement of double-quantum wire-based devices.

Non-fourier phonon heat transport in graphene nanoribbon field effect transistors based on modified phonon hydrodynamic lattice Boltzmann method

Graphene, with its exceptional thermal conductivity and electron mobility, is widely recognized as a potential candidate for next-generation transistor materials. Despite the lack of pronounced phonon hydrodynamic phenomena at room temperature in graphene materials, momentum-conserving phonon normal scattering predominantly drives phonon hydrodynamic transport. Current methodologies, including the Single Mode Relaxation Time (SMRT) approximation and Fourier's law, inadequately capture phonon transport within the ballistic and hydrodynamic regimes of graphene. In light of these limitations, this study introduces a Non-gray phonon hydrodynamic lattice Boltzmann method in combination with the Temperature Jump Boundary Condition to provide a more accurate depiction of phonon heat transport in graphene nanoribbon field-effect transistors (GNRFETs) spanning these regimes. The findings reveal that our proposed model significantly mitigates heat transfer errors in GNRFETs, primarily driven by ballistic transport compared to the SMRT model. This suggests that the model's applicability extends beyond scenarios dominated by phonon hydrodynamics and is effective even in cases dominated by ballistic transport. In conclusion, our study emphasizes the impact of hydrodynamic transport on heat transfer in GNRFETs and underscores the importance of adjusting parameters related to the heating zone width and specular parameter to enhance thermal stability.

Electrostatically Doped Junctionless Graphene Nanoribbon Tunnel Field-Effect Transistor for High-Performance Gas Sensing Applications: Leveraging Doping Gates for Multi-Gas Detection.

In this paper, a new junctionless graphene nanoribbon tunnel field-effect transistor (JLGNR TFET) is proposed as a multi-gas nanosensor. The nanosensor has been computationally assessed using a quantum simulation based on the self-consistent solutions of the mode space non-equilibrium Green's function (NEGF) formalism coupled with the Poisson's equation considering ballistic transport conditions. The proposed multi-gas nanosensor is endowed with two top gates ensuring both reservoirs' doping and multi-gas sensing. The investigations have included the IDS-VGS transfer characteristics, the gas-induced electrostatic modulations, subthreshold swing, and sensitivity. The order of change in drain current has been considered as a sensitivity metric. The underlying physics of the proposed JLGNR TFET-based multi-gas nanosensor has also been studied through the analysis of the band diagrams behavior and the energy-position-resolved current spectrum. It has been found that the gas-induced work function modulation of the source (drain) gate affects the n-type (p-type) conduction branch by modulating the band-to-band tunneling (BTBT) while the p-type (n-type) conduction branch still unaffected forming a kind of high selectivity from operating regime point of view. The high sensitivity has been recorded in subthermionic subthreshold swing (SS < 60 mV/dec) regime considering small gas-induced gate work function modulation. In addition, advanced simulations have been performed for the detection of two different types of gases separately and simultaneously, where high-performance has been recorded in terms of sensitivity, selectivity, and electrical behavior. The proposed detection approach, which is viable, innovative, simple, and efficient, can be applied using other types of junctionless tunneling field-effect transistors with emerging channel nanomaterials such as the transition metal dichalcogenides materials. The proposed JLGNRTFET-based multi-gas nanosensor is not limited to two specific gases but can also detect other gases by employing appropriate gate materials in terms of selectivity.

Molecular dynamics study of thermal transport across diamond/cubic boron nitride interfaces

The thermal transport properties at the interface of diamond and cubic boron nitride (c-BN) heterostructures significantly influence heat dissipation in high-power electronic and optoelectronic devices. However, a fundamental understanding of the various parameters modulating the interfacial thermal conductance is still lacking. In this work, we employ non-equilibrium molecular dynamics (NEMD) simulations to systematically investigate the effects of system size, temperature, and defect density on the interfacial thermal conductance across diamond/c-BN interfaces. The results indicate a positive correlation between system length and interface thermal conductance when below the phonon mean free path threshold, attributable to ballistic phonon transport regimes in smaller domains. Additionally, we observe an incremental enhancement in interface thermal conductance with increasing temperature, stemmed from intensified phonon-phonon interactions and reduced boundary scattering of thermal energy carriers. The introduction of vacancy and twinning defects is found to hinder interfacial thermal transport due to heightened phonon scattering processes that impede phononic transmission. The interatomic interactions and lattice dynamics are analyzed to provide insights into the underlying thermal transport physics at the atomistic scale. By tuning the system length from 4 to 16 nm, temperature from 300 to 500 K, and defect density from 0 to 0.4%, we achieve tunable control of the interfacial thermal conductance. Our study elucidates the multiscale mechanisms governing thermal transport across diamond/c-BN and provides potential pathways to actively tailor interfacial thermal properties through structural and temperature engineering. The fundamental understandings are valuable for optimizing heat dissipation and enabling thermal management solutions in next-generation power electronics leveraging these materials.

STEM in situ thermal wave observations for investigating thermal diffusivity in nanoscale materials and devices.

Practical techniques to identify heat routes at the nanoscale are required for the thermal control of microelectronic, thermoelectric, and photonic devices. Nanoscale thermometry using various approaches has been extensively investigated, yet a reliable method has not been finalized. We developed an original technique using thermal waves induced by a pulsed convergent electron beam in a scanning transmission electron microscopy (STEM) mode at room temperature. By quantifying the relative phase delay at each irradiated position, we demonstrate the heat transport within various samples with a spatial resolution of ~10 nm and a temperature resolution of 0.01 K. Phonon-surface scatterings were quantitatively confirmed due to the suppression of thermal diffusivity. The phonon-grain boundary scatterings and ballistic phonon transport near the pulsed convergent electron beam were also visualized.

Ballistic transport and spin-dependent anomalous quantum tunneling in Rashba–Zeeman and bilayer graphene hybrid structures

In this work, we have studied the spin-dependent ballistic transport and anomalous quantum tunneling in bilayer graphene horizontally placed in between two Rashba–Zeeman (RZ) leads under external electric biasing. We investigated the transmission and conductance for the proposed system using scattering matrix formalism and the Landauer–Büttiker formula considering a double delta-like barrier under a set of experimentally viable parameters. We found that the transmission characteristics are notably different for up- and down-spin incoming electrons depending upon the strength of magnetization. Moreover, the transmission of up- and down-spin electrons is found to be magnetization orientation dependent. The maximum tunneling conductance can be achieved by tuning biasing energy and magnetization strength and choosing a material with suitable Rashba spin–orbit coupling (RSOC). This astonishing property of our system can be utilized in fabricating devices, such as spin filters. We found that the Fano factor of our system is 0.4 under strong magnetization conditions, while it reduces to 0.3 under low magnetization conditions. Moreover, we also noticed that the transmission and conductance significantly depend on the Rashba–Zeeman effect. Therefore, considering a suitable RZ material, the tunneling of the electrons can be tuned and controlled. Our result suggests that considering suitable strength and orientation of magnetization with moderate RSOC, one can obtain a different transmission probability for spin species under suitable biasing energy. These results indicate the suitability of the proposed system in fabrication of spintronic devices, such as spin filter, spin transistor, etc.

Quasi-ballistic thermal transport in silicon carbide nanowires

Silicon carbide (SiC) is an important industrial material that enables the thermal stability of power electronics. However, the nanoscale phenomenon of ballistic thermal conduction, which may further improve the thermal performance, remains unexplored in SiC. Here, we reveal the length and temperature scales at which SiC exhibits quasi-ballistic thermal conduction. Our time-domain thermoreflectance measurements probe the thermal conductivity of SiC nanowires as a function of their length and temperature. The deviation of the thermal conductivity from the diffusive limit in nanowires shorter than a few micrometers indicates the transition into a quasi-ballistic thermal conduction regime. Naturally, the deviation is greater at lower temperatures, yet the effect persists even above room temperature. Our Monte Carlo simulations of phonon transport support our experimental results and show how phonons with long mean free paths carry a substantial amount of heat, causing quasi-ballistic conduction. These findings show that quasi-ballistic heat conduction can persist at the microscale at operating temperatures of power devices, and thus may help improve the thermal design in electronics based on SiC.