Resonant Tunneling Transistor Research Articles

Double-Heterostructure Resonant Tunneling Transistors of Surface-Functionalized Sb and Bi Monolayer Nanoribbons

Zigzag nanoribbons tailored from chemically surface-modified Sb or Bi monolayers by methyl, amino or hydroxyl are investigated through first-principles electronic-structure calculations to explore their potential applications in topological transport nanoelectronics. It is verified by Dirac-point-like energy dispersion of band-edges near Fermi level that the scattering-forbidden edge-states of these nanoribbons can give a topological conductive channel with extremely high electron mobility. Accordingly, Sb/SbXHn/Sb and Bi/BiXHn/Bi nanoribbon double-heterostructures (SbXHn or BiXHn: XHn = CH3, NH2, OH) are designed as resonant tunneling transistors and modeled by bipolar transport devices with their electron transport characteristics being calculated by nonequilibrium Green’s function combined first-principles schemes. Ballistic equilibrium conduction spectra and current-voltage characteristics prove that quantum conductance currents of these nanoribbon double heterostructures originate from the electron resonant tunneling between the topological edge-states of the two constituent Sb or Bi monolayer nanoribbons through the central barrier of SbXHn or BiXHn nanoribbon segment. This renders a high resonant current peak with strong negative differential conductance, thus being competent for zero-loss and ultrahigh-frequency resonant tunneling nanotransistors.

Twisted monolayer and bilayer graphene for vertical tunneling transistors

We prepare twist-controlled resonant tunneling transistors consisting of monolayer and Bernal bilayer graphene electrodes separated by a thin layer of hexagonal boron nitride. The resonant conditions are achieved by closely aligning the crystallographic orientation of graphene electrodes, which leads to momentum conservation for tunneling electrons at certain bias voltages. Under such conditions, negative differential conductance can be achieved. Application of in-plane magnetic field leads to electrons acquiring additional momentum during the tunneling process, which allows control over the resonant conditions.

Resonant Tunneling and Negative Differential Resistance in Black Phosphorus Vertical Heterostructures

AbstractResonant tunneling diodes with negative differential resistance (NDR) have attracted significant attention due to their unique quantum resonant tunneling phenomena and potential applications in terahertz emission/detection and high‐density logic/memory. In this paper, resonant tunneling devices, where the carriers tunnel through a hexagonal boron nitride (hBN) barrier sandwiched by two black phosphorus (BP) layers, are explored. The resonance occurs when the energy bands of the two black phosphorus layers are aligned. The conductive atomic force microscopy (CAFM) measurements reveal prominent NDR peaks with large peak‐to‐valley ratios at room temperature. It is found that the positions of the NDR peaks are very sensitive to the amplitude and the shape of the voltage waveform used in CAFM, which can be explained by the charge trapping effect. Furthermore, resonant tunneling transistors are demonstrated based on BP/hBN/BP stacks in which the locations of the NDR peaks are tunable by the electrostatic gating. As compared to the traditional tunneling diodes based on bulk materials, the tunneling devices based on thin boron nitride tunneling barrier and high mobility black phosphorus offer ultra‐high‐speed response. This feature, together with the NDR characteristics, provides the potential for applications in THz oscillators and multi‐value logic devices.

Resonant Tunneling Spectroscopy to Probe the Giant Stark Effect in Atomically Thin Materials.

Each atomic layer in van der Waals heterostructures possesses a distinct electronic band structure that can be manipulated for unique device operations. In the precise device architecture, the subtle but critical band splits by the giant Stark effect between atomic layers, varied by the momentum of electrons and external electric fields in device operation, has not yet been demonstrated or applied to design original devices with the full potential of atomically thin materials. Here, resonant tunneling spectroscopy based on the negligible quantum capacitance of 2D semiconductors in resonant tunneling transistors is reported. The bandgaps and sub-band structures of various channel materials could be demonstrated by the new conceptual spectroscopy at the device scale without debatable quasiparticle effects. Moreover, the band splits by the giant Stark effect in the channel materials could be probed, overcoming the limitations of conventional optical, photoemission, and tunneling spectroscopy. The resonant tunneling spectroscopy reveals essential and practical information for novel device applications.

Resonant tunnelling into the two-dimensional subbands of InSe layers

Two-dimensional (2D) van der Waals (vdW) crystals have attracted considerable interest for digital electronics beyond Si-based complementary metal oxide semiconductor technologies. Despite the transformative success of Si-based devices, there are limits to their miniaturization and functionalities. Here we realize a resonant tunnelling transistor (RTT) based on a 2D InSe layer sandwiched between two multilayered graphene (MLG) electrodes. In the RTT the energy of the quantum-confined 2D subbands of InSe can be tuned by the thickness of the InSe layer. By applying a voltage across the two MLG electrodes, which serve as the source and drain electrodes to the InSe, the chemical potential in the source can be tuned in and out of resonance with a given 2D subband, leading to multiple regions of negative differential conductance that can be additionally tuned by electrostatic gating. This work demonstrates the potential of InSe and InSe-based RTTs for applications in quantum electronics.

Performance limitations of nanowire resonant-tunneling transistors with steep switching analyzed by Wigner transport simulation

We conducted a quantum transport simulation of nanowire resonant-tunneling field-effect transistors (NW-RTFETs) based on the Wigner function model. The current–voltage characteristics of the NW-RTFETs were compared with those of the nanowire transistors and nanowire resonant-tunneling diodes. For the selection of a gate with appropriate performance, symmetric and asymmetric gates with various lengths were tested, and a symmetric gate, covering the quantum well and barrier regions, was chosen as a main gate. The source-side asymmetric gates did not produce a negative differential resistance at low gate voltages in contrast to the symmetric or drain-side asymmetric gates. Although steep switching is achieved in the negative differential resistance region, the ON/OFF current ratio (ION/IOFF) is extremely low, compared to those of conventional transistors. In an attempt to increase the ION/IOFF ratio, the sizes of the semiconductor cylinder and the oxide tube were changed. This study discusses the requirements for increasing the applicability of steep switching.

First-principles study of graphyne/graphene heterostructure resonant tunneling nano-transistors

Resonant tunneling transistors have received wide attention because of their ability to reduce the complexity of circuits, and promise to be an efficient candidate in ultra-high speed and ultra-high frequency applications. The chemical compatibility between graphene and graphdiyne implies that they can be combined into various configurations to fulfill ultra-high frequency nanotransistor. In the present paper, two novel resonant tunneling transistors based on graphene/graphdiyne/graphene double-heterojunction are theoretically developed to model two new kinds of bipolar devices with two representative graphdiyne nanoribbons. The electronic structures of two pristine graphdiyne nanoribbons are investigated by performing the first-principles calculations with all-electron relativistic numerical-orbit scheme as implemented in Dmol3 code. The electronic transport properties including quantum conductance (transmission spectrum) and electrical current varying with bias-voltage for each of the designed graphdiyne nanoribbon transistors are calculated in combination with non-equilibrium Green function formalism. The calculated electronic transmission and current-voltage characteristics of these transistors demonstrate that the current is dominantly determined by resonant tunneling transition and can be effectively controlled by gate electric field thereby representing the favorable negative-differential-conductivity, which is the qualified attribute of ultra-high frequency nanotransistor. It follows from the <i>I</i>-<i>U</i><sub>b</sub> variations explained by electronic transmission spectra that quantum resonance tunneling can occur in the proposed star-like graphdiyne (SGDY) and net-like graphdiyne (NGDY) nanoribbon transistors, with the resonance condition limited to a narrow bias-voltage range, leading to a characteristic resonant peak in <i>I</i>-<i>U</i><sub>b</sub> curve, which means the strong negative differential conductivity. Under a gate voltage of 4 V, when the bias-voltage rises up to 0.6 V (0.7 V), the Fermi level of source electrode aligns identically to the quantized level of SGDY (NGDY) nanoribbon channel, causing electron resonance tunneling as illustrated by the considerable transmission peak in bias window; once the source Fermi level deviates from the quantized level of SGDY (NGDY) channels at higher bias-voltage, the resonance tunneling transforms into ordinary electron tunneling, which results in the disappearing of the substantial transmission peak in bias window and the rapid declining of current. The designed SGDY and NGDY nanotransistors will achieve high-level negative differential conductivity with the peak-to-valley current ratio approaching to 4.5 and 6.0 respectively, which can be expected to be applied to quantum transmission nanoelectronic devices.

A double gate resonant tunneling transistor scheme based on silicene nanotube

In this paper, we propose a scheme of double gate resonant tunneling field effect transistor based on two gates wrapped on a silicene nanotube. By applying a vertical electric field in the side gates, the energy gap in these regions is opened. Due to different bandgap in these regions and other parts of the nanotube, a quantum well is created in the middle which results in confined energy states in the well region. Hence by applying an electric field via lateral gates, we can change the left and right barrier height simultaneously. Moreover, confined states in the well region are tuned by the voltage applied to the central gate. The transmission coefficient of the incident electrons is calculated using the transfer matrix method. Our theoretical results illustrate that the proposed structure has an oscillatory behavior in the transfer characteristics. On the other hand, output characteristic displays a step-like behavior. We also investigate the influence of the barriers and well thicknesses as well as the perpendicular electric field exerted by lateral gates on the current of the proposed device.

Comparison of the I–V characteristics of a topological insulator quantum well resonant tunneling transistor and its conventional counterpart: A TCAD simulation study

In this study, we investigated and simulated the structure of a quantum well resonant tunneling (QWRT) transistor based on two barriers and a well in the middle. The barriers were formed of semiconductors with a higher bandgap and the well separating the barriers comprised a semimetal-like material with a very small bandgap, i.e., close to zero. The device was positioned on the oxide to exploit the benefits of SOI technology, e.g., improving leakage and the immunity of the device against environment effects. When the well thickness was a few nanometers, the electron-bound states appeared in the quantum well region. By tuning the voltage and work function of the top gate, the bound states moved in the potential well and the carriers were transferred through the well via the resonant tunneling phenomenon. Our simulations showed the step-like behavior of the output characteristics and the oscillatory quantum switching of the transfer characteristics for the proposed device. We also investigated the influence of the well thickness and gate voltage on the device characteristics, and we compared the optimal case with a theoretical analysis of a QWRT transistor based on a topological insulator.

Vertical Transistors Based on 2D Materials: Status and Prospects

Two-dimensional (2D) materials, such as graphene (Gr), transition metal dichalcogenides (TMDs) and hexagonal boron nitride (h-BN), offer interesting opportunities for the implementation of vertical transistors for digital and high-frequency electronics. This paper reviews recent developments in this field, presenting the main vertical device architectures based on 2D/2D or 2D/3D material heterostructures proposed so far. For each of them, the working principles and the targeted application field are discussed. In particular, tunneling field effect transistors (TFETs) for beyond-CMOS low power digital applications are presented, including resonant tunneling transistors based on Gr/h-BN/Gr stacks and band-to-band tunneling transistors based on heterojunctions of different semiconductor layered materials. Furthermore, recent experimental work on the implementation of the hot electron transistor (HET) with the Gr base is reviewed, due to the predicted potential of this device for ultra-high frequency operation in the THz range. Finally, the material sciences issues and the open challenges for the realization of 2D material-based vertical transistors at a large scale for future industrial applications are discussed.

DFT-NEGF simulation of graphene-graphdiyne-graphene resonant tunneling transistor

The chemical stability of graphene and graphdiyne means that they can be stacked in different combinations to produce a new nanotransistor for ultra-high frequency applications. Here we report a resonant tunneling transistor through a graphene-graphdiyne-graphene heterojunctions sandwiched between two graphene electrodes. The characteristics of this transistor, which were modeled on the basis of density functional theory, revealed that the current is dominated by tunneling transitions.

Vertical resonant tunneling transistors with molecular quantum dots for large-scale integration.

Quantum molecular devices have a potential for the construction of new data processing architectures that cannot be achieved using current complementary metal-oxide-semiconductor (CMOS) technology. The relevant basic quantum transport properties have been examined by specific methods such as scanning probe and break-junction techniques. However, these methodologies are not compatible with current CMOS applications, and the development of practical molecular devices remains a persistent challenge. Here, we demonstrate a new vertical resonant tunneling transistor for large-scale integration. The transistor channel is comprised of a MOS structure with C60 molecules as quantum dots, and the structure behaves like a double tunnel junction. Notably, the transistors enabled the observation of stepwise drain currents, which originated from resonant tunneling via the discrete molecular orbitals. Applying side-gate voltages produced depletion layers in Si substrates, to achieve effective modulation of the drain currents and obvious peak shifts in the differential conductance curves. Our device configuration thus provides a promising means of integrating molecular functions into future CMOS applications.

Stacking transition in bilayer graphene caused by thermally activated rotation

Crystallographic alignment between two-dimensional crystals in van der Waals heterostructures brought a number of profound physical phenomena, including observation of Hofstadter butterfly and topological currents, and promising novel applications, such as resonant tunnelling transistors. Here, by probing the electronic density of states in graphene using graphene-hexagonal boron nitride-graphene tunnelling transistors, we demonstrate a structural transition of bilayer graphene from incommensurate twisted stacking state into a commensurate AB stacking due to a macroscopic graphene self-rotation. This structural transition is accompanied by a topological transition in the reciprocal space and by pseudospin texturing. The stacking transition is driven by van der Waals interaction energy of the two graphene layers and is thermally activated by unpinning the microscopic chemical adsorbents which are then removed by the self-cleaning of graphene.

Probing barrier transmission in ballistic graphene

We derive the local density of states from itinerant and boundary states around transport barriers and edges in graphene and show that the itinerant states lead to mesoscale undulations that could be used to probe their scattering properties in equilibrium without the need for lateral transport measurements. This finding will facilitate vetting of extended structural defects such as grain boundaries or line defects as transport barriers for switchable graphene resonant tunneling transistors. We also show that barriers could exhibit double minima and that the charge density away from highly reflective barriers and edges scales as $x^{-2}$.

Nonequilibrium green function simulations of graphene-nanoribbon resonant-tunneling transistors

We have performed nonequilibrium Green function simulations on the transport characteristics in ultra-small graphene nanoribbon resonant-tunneling transistors (RTTs). For an ultra-narrow nanoribbon transistor, the current–voltage characteristics resemble those observed in large-size graphene-based RTTs. For a wider nanoribbon transistor, we find that two types of structure due to inter-subband transitions appear in addition to the main peak: one originates from the resonance at kx = 0, while the other is due to resonance at finite kx.

Graphene nanoribbon tunneling field effect transistors

The electron-hole symmetry characteristic of graphene nanoribbons (GNRs) gives rise to the electron (hole) tunneling through valence (conduction) band states. By employing this property we have numerically investigated GNR field effect transistors with p+-type source and drain in the presence of a gate voltage-induced n-type channel using the non-equilibrium Green's function formalism.For long channels, the traditional FET-like I-V behavior is achieved, but at short channels, the sub threshold current opens up an oscillatory dependence on the gate voltage with a considerable amount of current of over 10−6A. This is the characteristic current behavior of resonant tunneling transistors that exhibit regions of negative differential resistance. The calculated discrete density of states in the channel attributes this behavior to the constructed n-type channel island between p-type source and drain with thin barriers formed by the energy gap.

A New Full Adder Cell for Molecular Electronics

Due to high power consumption and difficulties with minimizing the CMOS transistor size, molecular electronics has been introduced as an emerging technology. Further, there have been noticeable advances in fabrication of molecular wires and switches and also molecular diodes can be used for designing different logic circuits. Considering this novel technology, we use molecules as the active components of the circuit, for transporting electric charge. In this paper, a full adder cell based on molecular electronics is presented. This full adder is consisted of resonant tunneling diodes and transistors which are implemented via molecular electronics. The area occupied by this kind of full adder would be much times smaller than the conventional designs and it can be used as the building block of more complex molecular arithmetic circuits.

Physics of Gate Modulated Resonant Tunneling (RT)-FETs: Multi-barrier MOSFET for steep slope and high on-current

A new concept of nanoscale MOSFET, the Gate Modulated Resonant Tunneling Transistor (RT-FET), is presented and modeled using 3D Non-Equilibrium Green’s Function simulations enlightening the main physical mechanisms. Owing to the additional tunnel barriers and the related longitudinal confinement present in the device, the density of state is reduced in its off-state, while remaining comparable in its on-state, to that of a MOS transistor without barriers. The RT-FET thus features both a lower RT-limited off-current and a faster increase of the current with V G , i.e. an improved slope characteristic, and hence an improved I on/ I off ratio. Such improvement of the slope can happen in subthreshold regime, and therefore lead to subthreshold slope below the kT/q limit. In addition, faster increase of current and improved slope occur above threshold and lead to high thermionic on-current and significant I on/ I off ratio improvement, even with threshold voltage below 0.2 V and supply voltage V dd of a few hundreds of mV as critically needed for future technology nodes. Finally RT-FETs are intrinsically immune to source–drain tunneling and are therefore promising candidate for extending the roadmap below 10 nm.

Gate-Induced Cross-Over between Fabry–Perot Interference and Coulomb Blockade in a Single-Walled Carbon Nanotube Transistor with Double-Gate Structure

The gate-induced cross-over between Fabry–Perot interference and Coulomb blockade characteristics in a single-walled carbon nanotube (SWNT) transistor are observed. The behaviors of Fabry–Perot interference and Coulomb blockade are controlled by the control-gate and observed by the sweep-gate. When a negatively high voltage is applied to the control-gate electrode, the SWNT transistor showed a resonant tunneling transistor (RTT) characteristic, e.g., a Fabry–Perot quantum interference pattern. However, when a negatively low voltage is applied to the control gate electrode, the SWNT transistor showed a single-hole transistor (SHT) characteristic, e.g., a Coulomb diamond characteristic. The transition between RTT and SHT is achieved by modulating the coupling strength between the source and drain electrodes and the quantum island using control-gate voltage change. Schottky barriers at the contact between the SWNT and the source and drain electrodes act as tunneling barriers. The thicknesses of the tunneling barriers are modulated by the control-gate voltage change, and the strength of the coupling between the source and drain electrode and the quantum island is also modulated.

Gate-Induced Cross-Over between Fabry–Perot Interference and Coulomb Blockade in a Single-Walled Carbon Nanotube Transistor with Double-Gate Structure

The gate-induced cross-over between Fabry–Perot interference and Coulomb blockade characteristics in a single-walled carbon nanotube (SWNT) transistor are observed. The behaviors of Fabry–Perot interference and Coulomb blockade are controlled by the control-gate and observed by the sweep-gate. When a negatively high voltage is applied to the control-gate electrode, the SWNT transistor showed a resonant tunneling transistor (RTT) characteristic, e.g., a Fabry–Perot quantum interference pattern. However, when a negatively low voltage is applied to the control gate electrode, the SWNT transistor showed a single-hole transistor (SHT) characteristic, e.g., a Coulomb diamond characteristic. The transition between RTT and SHT is achieved by modulating the coupling strength between the source and drain electrodes and the quantum island using control-gate voltage change. Schottky barriers at the contact between the SWNT and the source and drain electrodes act as tunneling barriers. The thicknesses of the tunneling barriers are modulated by the control-gate voltage change, and the strength of the coupling between the source and drain electrode and the quantum island is also modulated.