Carbon nanotube (CNT) electromechanical resonators have demonstrated unprecedented sensitivities for detecting small masses and forces. The detection speed in a cryogenic setup is usually limited by the CNT contact resistance and parasitic capacitance. We report the use of a heterojunction bipolar transistor (HBT) amplifying circuit near the device to measure the mechanical amplitude at microsecond timescales. A Coulomb rectification scheme, in which the probe signal is at much lower frequency than the mechanical drive signal, allows investigation of the strongly non-linear regime. The behaviour of transients in both the linear and non-linear regimes is observed and modeled by including Duffing and non-linear damping terms in a harmonic oscillator equation. We show that the non-linear regime can result in faster mechanical response times, on the order of 10 microseconds for the device and circuit presented, potentially enabling the magnetic moments of single molecules to be measured within their spin relaxation and dephasing timescales.
A vertical transistor based on a double gated, atomically thin heterojunction is theoretically examined. Both p-type and n-type transistor operations can be conveniently achieved by using one of the two gates as the switching gate. The transistor shows excellent saturation of output I-V characteristics due to drain-induced depletion and lack of tunneling barrier layers. The subthreshold slope could be below the thermionic limit due to band filtering as the switching mechanism. The atomically thin vertical PN heterojunction can be electrostatically modulated from a type II heterojunction to a b…
Recently it has been experimentally shown that a graphene nanoribbon (GNR) can be obtained by unzipping a carbon nanotube (CNT). This makes it possible to fabricate all-carbon heterostructures that have a unique interface between a CNT and a GNR. Here we demonstrate that such a heterojunction may be utilized to obtain a unique transistor operation. By performing a self-consistent non-equilibrium Green's function (NEGF) based calculation on an atomistically defined structure, we show that such a transistor may reduce energy dissipation below the classical limit while not compromising speed - th…
We propose a Heterojunction Vertical Tunneling FET and show using self-consistent ballistic quantum transport simulations that it can provide very steep subthreshold swings and high ON current, thereby improving the scalability of Tunnel FETs for high performance. The turn-on in pocket region of the device is dictated by modulation of heterojunction barrier height. The steepness of turn-on is increased because of simultaneous onset of tunneling in the pocket and the region underneath and also due to contribution to current by vertical tunneling in the pocket. These factors can be engineered by…
Vertical tunneling field-effect-transistor (FET) based on graphene heterojunctions with layers of hBN is simulated by self-consistent quantum transport simulations. It is found that the asymmetric p-type and n-type conduction is due to work function deference between the graphene contact and the tunneling channel material. Modulation of the bottom-graphene-contact plays an important role in determining the switching characteristic of the device. Due to the electrostatic short-channel-effects stemming from the vertical-FET structure, the output I-V characteristics do not saturate. The scaling b…
Comparing the results of transport measurements of strongly correlated two-dimensional holes in a GaAs heterojunction-insulated-gate field-effect-transistor obtained before and after a brief photo-illumination, the light-induced disorder is found to cause qualitative changes suggesting altered carrier states. For charge concentrations ranging from $3\times10^{10}$ $cm^{-2}$ down to $7\times10^{8}$ cm$^{-2}$, the post-illumination hole mobility exhibits a severe suppression for charge densities below $2\times10^{10}$ cm$^{-2}$, while almost no change for densities above. The long-ranged nature …
The electrical performance of a tunnel field-effect transistor depends critically on the band offset at their semiconductor heterojunction interface. Historically, it has been difficult to experimentally determine how the electronic bands align at the heterojunction interface. We report here on experimental methods to ascertain a complete energy band alignment of a broken-gap tunnel field-effect transistor based on an InAs/GaSb hetero-junction. By using graphene as an optically transparent electrode in a traditional internal photoemission measurement, both the electron and hole barrier heights…
The availability of transistors capable of operating at low supply voltage is essential to improve the key performance metric of computing circuits, i.e., the number of operations per unit energy. In this paper, we propose a new device concept for energy-efficient, steep-slope transistors based on heterojunctions of 2D materials. We show that by injecting electrons from an isolated and weakly dispersive band into a strongly dispersive one, subthermionic subthreshold swings can be obtained, as a result of a cold-source effect and of a reduced thermalization of carriers. This mechanism is implem…
We examine a silicon-germanium heterojunction bipolar transistor (HBT) for cryogenic pre-amplification of a single electron transistor (SET). The SET current modulates the base current of the HBT directly. The HBT-SET circuit is immersed in liquid helium, and its frequency response from low frequency to several MHz is measured. The current gain and the noise spectrum with the HBT result in a signal-to-noise-ratio (SNR) that is a factor of 10-100 larger than without the HBT at lower frequencies. The transition frequency defined by SNR = 1 has been extended by as much as a factor of 10 compared …
A distinctive approach for forming a lateral Bipolar Charge Plasma Transistor (BCPT) is explored using 2-D simulations. Different metal work-function electrodes are used to induce n- and p-type charge plasma layers on undoped SOI to form the emitter, base and collector regions of a lateral NPN transistor. Electrical characteristics of the proposed device are simulated and compared with that of a conventionally doped lateral bipolar junction transistor with identical dimensions. Our simulation results demonstrate that the BCPT concept will help us realize a superior bipolar transistor in terms …
Black phosphorus (BP) tunneling transistors (TFETs) using heterojunction (He) are investigated by atomistic quantum transport simulations. It is observed that edge states have a great impact on transport characteristics of BP He-TFETs, which result in the potential pinning effect and deteriorate the gate control. While, on-state current can be effectively enhanced by using hydrogen to saturate the edge dangling bonds in BP He-TFETs, in which edge states are quenched. By extending layered BP with a smaller band gap to the channel region and modulating the BP thickness, device performance of BP …
We report a novel method to reduce the collector-emitter offset-voltage of the wide bandgap SiC-P-emitter lateral HBTs using a dual-bandgap emitter. In our approach, the collector-emitter offset-voltage VCE(offset) is reduced drastically by eliminating the built-in potential difference between the emitter-base (EB) junction and collector-base (CB) junction by using a SiC-on-Si P-emitter. We demonstrate that the proposed dualbandgap P-emitter HBT together with the SiGe base and Schottky collector, not only has a very low VCE(offset) but also exhibits high current gain, reduced Kirk effect, exce…
In this paper, we report an alternative approach of implementing a Schottky collector bipolar transistor without doping the ultrathin SOI film. Using different metal work function electrodes, the electrons and holes are induced in an intrinsic silicon film to create the n emitter and the p base regions, respectively. Using two-dimensional device simulation, the performance of the proposed device has been evaluated. Our results demonstrate that the charge plasma based bipolar transistor with Schottky collector exhibits a high current gain and a better cut-off frequency compared to its conventio…
Graphane is a semiconductor with an energy gap, obtained from hydrogenation of the two-dimensional grapheme sheet. Together with the two-dimensional geometry, unique transport features of graphene, and possibility of doping graphane, p and n regions can be defined so that p-n junctions become feasible with small reverse currents. Our recent analysis has shown that an ideal I-V characteristic for this type of junctions may be expected. Here, we predict the behavior of bipolar juncrion transistors based on graphane. Profiles of carriers and intrinsic parameters of the graphane transistor are cal…
A novel bipolar transistor structure, namely, a SiC emitter lateral NPM Schottky collector bipolar transistor (SCBT) with a silicon-on-insulator (SOI) substrate is explored using two-dimensional (2-D) simulation. A comprehensive comparison of the proposed structure with its equivalent Si lateral NPN BJT and an SiC emitter lateral NPN HBT is presented. Based on simulation results, the authors demonstrate for the first time that the proposed SiC emitter lateral NPM transistor shows superior performance in terms of high current gain and cut-off frequency, reduced collector resistance, negligible …
In-plane heterojunction tunnel field effect transistors based on monolayer transition metal dichalcogenides are studied by means of self-consistent non-equilibrium Green's functions simulations and an atomistic tight-binding Hamiltonian. We start by comparing several heterojunctions before focusing on the most promising ones, i.e WTe2-MoS2 and MoTe2-MoS2. The scalability of those devices as a function of channel length is studied, and the influence of backgate voltages on device performance is analysed. Our results indicate that, by fine-tuning the design parameters, those devices can yield ex…
High-fidelity single-shot readout of spin qubits requires distinguishing states much faster than the T1 time of the spin state. One approach to improving readout fidelity and bandwidth (BW) is cryogenic amplification, where the signal from the qubit is amplified before noise sources are introduced and room-temperature amplifiers can operate at lower gain and higher BW. We compare the performance of two cryogenic amplification circuits: a current-biased heterojunction bipolar transistor circuit (CB-HBT), and an AC-coupled HBT circuit (AC-HBT). Both circuits are mounted on the mixing-chamber sta…
Heterojunction, a photoelectric conversion center, plays a critical role in photo detection. A compare study is performed on field effect phototransistors (FEpT) with two typical heterojunctions based on graphene and PbSe quantum dots (QDs) hybrid, including layer-heterojunction (LH) and bulk-heterojunction (BH). LH-FEpT exhibits a higher mobility (μn) 677 cm2V-1s-1, whereas μn of BH-FEpT is as low as 314 cm2V-1s-1, however the higher responsivity (106 A/W) is achieved by the former. Because R is proportional to the product of mobility and transfer rate, and high transfer rate of LH-FEpT overc…
We use a simultaneous flow of ethylene and hydrogen gases to grow single wall carbon nanotubes by chemical vapor deposition. Strong coupling to the gate is inferred from transport measurements for both metallic and semiconducting tubes. At low-temperatures, our samples act as single-electron transistors where the transport mechanism is mainly governed by Coulomb blockade. The measurements reveal very rich quantized energy level spectra spanning from valence to conduction band. The Coulomb diamonds have similar addition energies on both sides of the semiconducting gap. Signatures of subbands po…
A new Schottky-gate Bipolar Mode Field Effect Transistor (SBMFET) is proposed and verified by two-dimensional simulation. Unlike in the case of conventional BMFET, which uses deep diffused p+-regions as the gate, the proposed device uses the Schottky gate formed on the silicon planar surface for injecting minority carriers into the drift region. The SBMFET is demonstrated to have improved current gain, identical breakdown voltage and ON-voltage drop when compared to the conventional BMFET. Since the fabrication of the SBMFET is much simpler and obliterates the need for deep thermal diffusion o…
A general self-aligned fabrication scheme is reported here for a diverse class of electronic devices based on van der Waals materials and heterojunctions. In particular, self-alignment enables the fabrication of source-gated transistors in monolayer MoS2 with near-ideal current saturation characteristics and channel lengths down to 135 nm. Furthermore, self-alignment of van der Waals p-n heterojunction diodes achieves complete electrostatic control of both the p-type and n-type constituent semiconductors in a dual-gated geometry, resulting in gate-tunable mean and variance of anti-ambipolar Ga…
A small scale bipolar transistor with polysilicon emitter will, depending on the emitter window size, display suppression of the hole tramsport due to single electron effects. In this paper the resulting base current suppression is computed in terms of the orthodox theory of single electron tunneling and a recombination time approximation. The possible application of the transistor as readout system for Coulomb blockade device circuits is discussed.
Recent development in fabrication technology of planar two-dimensional (2D) materials has brought up possibilities of numerous novel applications. Our recent analysis has revealed that by definition of p-n junctions through appropriate patterned doping of 2D semiconductors, ideal exponential I-V characteristics may be expected. However, the theory of 2D junctions turns out to be very much different to that of the standard bulk junctions. Based on this theory of 2D diodes, here we construct for the first time, a model to describe the 2D Bipolar Junction Transistors (2D-BJTs). We derive the smal…
We extend the analogy between charge-based bipolar semiconductor electronics and spin-based unipolar electronics by considering unipolar spin transistors with different equilibrium spin splittings in the emitter, base, and collector. The current of base majority spin electrons to the collector limits the performance of ``homojunction'' unipolar spin transistors, in which the emitter, base, and collector all are made from the same magnetic material. This current is very similar in origin to the current of base majority carriers to the emitter in homojunction bipolar junction transistors. The cu…
The single particle tunneling in a vertical stack consisting of monolayers of two-dimensional semiconductors is studied theoretically and its application to a novel Two-dimensional Heterojunction Interlayer Tunneling Field Effect Transistor (Thin-TFET) is proposed and described. The tunneling current is calculated by using a formalism based on the Bardeen's transfer Hamiltonian, and including a semi-classical treatment of scattering and energy broadening effects. The misalignment between the two 2D materials is also studied and found to influence the magnitude of the tunneling current, but hav…
We propose and investigate the intrinsically thinnest transistor concept: a monolayer ballistic heterojunction bipolar transistor based on a lateral heterostructure of transition metal dichalcogenides. The device is intrinsically thinner than a Field Effect Transistor because it does not need a top or bottom gate, since transport is controlled by the electrochemical potential of the base electrode. As typical of bipolar transistors, the collector current undergoes a tenfold increase for each 60 mV increase of the base voltage over several orders of magnitude at room temperature, without sophis…
We investigate the crossed Andreev reflections between two graphene leads connected by a narrow superconductor. When the leads are respectively of the n-and p- type, we find that electron elastic cotunneling and local Andreev reflection are both eliminated even in the absence of any valley-isospin or spin polarizations. We further predict oscillations of both diagonal and cross conductances as a function of the distance between the graphene-superconductor interfaces.
Valley degree of freedom in the 2D semiconductor is a promising platform for the next generation optoelectronics. Electrons in different valleys can have opposite Berry curvature, leading to the valley Hall effect (VHE). However, VHE without the plasmonic structure's assistance has only been reported in cryogenic temperature, limiting its practical application. Here, we report the observation of VHE at room temperature in the MoS2/WSe2 heterostructures. We also uncover that both the magnitude and the polarity of the VHE in the 2D heterostructure is gate tunable. We attribute this to the opposi…
This paper projects the enhanced drive current of a n-type electrostatically doped (ED) tunnel field-effect transistor (ED-TFET) based on heterojunction and band-gap engineering via TCAD 2-D device simulations. The homojunction ED-TFET device utilizes the electrostatic doping in order to create the source/drain region on an intrinsic silicon nanowire that also felicitates dynamic re-configurability. The ED-TFET offers good electrostatic control over the channel with reduced thermal budget and process complexity. However, device exhibits low ON current, therefore, in this work, we elaborate on …
In this paper we study the device physics of a technique for realizing an n-p-n bipolar transistor action in the source side of a junctionless nanowire tunneling FET (BJN-TFET). In the on-state, tunneling of electrons from valence band of the source to conduction band of the channel enhances the hole concentration as well as the potential in the source region which drives a built-in BJT transistor by forward biasing the base-emitter junction, with the source acting as a p-type region. Owing to the sharp switching of the JN-TFET and high BJT current gain, the overall performance is improved, in…
We present, here, advanced DFT-NEGF techniques that we have implemented in our ATOmistic MOdelling Solver, ATOMOS, to explore transport in novel materials and devices and in particular in van-der-Waals heterojunction transistors. We describe our methodologies using plane-wave DFT, followed by a Wannierization step, and linear combination of atomic orbital DFT, that leads to an orthogonal and non-orthogonal NEGF model, respectively. We then describe in detail our non-orthogonal NEGF implementation including the Sancho-Rubio and electron-phonon scattering within a non-orthogonal framework. We al…
The superior intrinsic properties of graphene have been a key research focus for the past few years. However, external components, such as metallic contacts, serve not only as essential probing elements, but also give rise to an effective electron cavity, which can form the basis for new quantum devices. In previous studies, quantum interference effects were demonstrated in graphene heterojunctions formed by a top gate. Here phase coherent transport behavior is demonstrated in a simple two terminal graphene structure with clearly-resolved Fabry-Perot oscillations in sub-100 nm devices. By aggr…
Low-dimensional metallic transport is at the heart of modern high-performance transistors. While the best devices are currently based on III-V heterojunction quantum-well channels, we demonstrate that much greater performance gains reside in the unique properties of one-dimensional carbon nanotubes. Using recent experimental data, we show how the specific features of degenerate carrier kinetics in metallic nanotubes enable a novel class of quantum-confined signal detector whose current gain exceeds present transistor technology. We analyze the key interplay of degeneracy and scattering dynamic…
The current integrated circuit (IC) technology based on conventional MOS-FET (metal-oxide-semiconductor field-effect transistor) is approaching the limit of miniaturization with increasing demand on energy. Several analog circuit applications based on graphene FETs have been demonstrated with less components comparing to the conventional technology. However, low on/off current ratio caused by the semimetal nature of graphene has severely hindered its practical applications. Here we report a graphene/MoTe2 van der Waals (vdW) vertical transistor with V-shaped ambipolar field effect transfer cha…
Group-IV monochalcogenides are emerging as a new class of layered materials beyond graphene, transition metal dichalcogenides (TMDCs), and black phosphorus (BP). In this paper, we report experimental and theoretical investigations of the band structure and transport properties of GeSe and its heterostructures. We find that GeSe exhibits a markedly anisotropic electronic transport, with maximum conductance along the armchair direction. Density functional theory calculations reveal that the effective mass is 2.7 times larger along the zigzag direction than the armchair direction; this mass aniso…
Van der Waals (vdW) semiconductors are attractive for highly scaled devices and heterogeneous integration since they can be isolated into self-passivated, two-dimensional (2D) layers that enable superior electrostatic control. These attributes have led to numerous demonstrations of field-effect devices ranging from transistors to triodes. By exploiting the controlled, substitutional doping schemes in covalently-bonded, three-dimensional (3D) semiconductors and the passivated surfaces of 2D semiconductors, one can construct devices that can exceed performance metrics of 'all-2D' vdW heterojunct…
The p-n junction diode and field-effect transistor (FET) are the two most ubiquitous building blocks of modern electronics and optoelectronics. In recent years, the emergence of reduced dimensionality materials has suggested that these components can be scaled down to atomic thicknesses. Although high performance field-effect devices have been achieved from monolayered materials and their heterostructures, a p-n heterojunction diode derived from ultrathin materials is notably absent and constrains the fabrication of complex electronic and optoelectronic circuits. Here, we demonstrate a gate-tu…
In the past decade graphene has been one of the most studied material for several unique and excellent properties. Due to its two dimensional nature, physical and chemical properties and ease of manipulation, graphene offers the possibility of integration with the exiting semiconductor technology for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to t…
We report a bipolar field effect tunneling transistor that exploits to advantage the low density of states in graphene and its one atomic layer thickness. Our proof-of-concept devices are graphene heterostructures with atomically thin boron nitride acting as a tunnel barrier. They exhibit room temperature switching ratios ~50, a value that can be enhanced further by optimizing the device structure. These devices have potential for high frequency operation and large scale integration.
Nano-electronic integrated circuit technology is exclusively based on MOSFET transistor due to its scalability down to the nanometer range. On the other hand, Bipolar Junction Transistor (BJT), which provides unmatched analog characteristics and frequency response, cannot be scaled to nanometer regime without the loss of transistor action. Here a versatile nanoscale transistor is introduced that provides identical BJT behavior and expands its capabilities. The new transistor uses CMOS fabrication technology and creates BJT emitter, base, and collector via electric fields. By allowing carrier m…
A heterojunction tunneling field effect transistor based on armchair graphene nanoribbons is proposed and studied using ballistic quantum transport simulation based on 3D real space nonequilibrium Green's function formalism. By using low band gap nanoribbons as the source/drain material, the hetero- structure shows better performance including higher on current, lower off current, and improved steep subthreshold swings compared with homostructure. It is also found the device performance greatly depends on the source/drain dopping density. High doping density leads to fewer density states in th…
Transition metal dichalcogenides (TMDCs) have recently shown much promise as thin layer semiconductors in application to transistor technology. TMDCs provide a unique vantage point for studying the properties of phosphorene, a highly efficient and tunable super-material. Phosphorene-TMDC heterostructure nanodevices have remained largely unexplored due to the lack of air stability observed in phosphorene under ambient conditions. This study investigates a novel nanofabrication technique that effectively enhances the air stability and practical scalability of graphene-analogous phosphorene. A ph…
Graphene/silicon (G/Si) heterojunction based devices have been demonstrated as high responsivity photodetectors that are potentially compatible with semiconductor technology. Such G/Si Schottky junction diodes are typically in parallel with gated G/silicon dioxide (SiO$_2$)/Si areas, where the graphene is contacted. Here, we utilize scanning photocurrent measurements to investigate the spatial distribution and explain the physical origin of photocurrent generation in these devices. We observe distinctly higher photocurrents underneath the isolating region of graphene on SiO$_2$ adjacent to the…
Champions of spintronics often claim that spin based signal processing devices will vastly increase speed and/or reduce power dissipation compared to traditional charge based electronic devices. Yet, not a single spintronic device exists today that can lend credence to this claim. Here, I show that no spintronic device that clones conventional electronic devices, such as field effect transistors and bipolar junction transistors, is likely to reduce power dissipation significantly. For that to happen, spin-based devices must forsake the transistor paradigm of switching states by physical moveme…
Low power consumption in both static and dynamic modes of operation is a key requirement in modern, highly scaled nanoelectronics. Tunneling field-effect transistors (TFETs) that exploit direct band-to-band tunneling of charges and exhibit steep sub-threshold slope (SS) transfer characteristics are an attractive option in this regard. However, current generation of Si and III-V heterojunction based TFETs while suffer from low ON current density and ON/OFF current ratios for < 60 mV/dec operation. Semiconducting two-dimensional (2D) layers have recently renewed enthusiasm in novel device design…
We investigate the design of steep-slope metal-oxide-semiconductor field-effect transistors (MOSFETs) exploiting monolayers of transition metal dihalides as channel materials. With respect to other previously proposed steep-slope transistors, these devices require simplified manufacturing processes, as no confinement of the 2D material is needed, nor any tunneling heterojunction or ferroelectric gate insulators, and only n- or p-type contacts are demanded. We demonstrate their operation by studying an implementation based on monolayer CrI$_2$ through quantum transport simulations. We show that…
This paper studies the mutual coupling in trench isolated multi emitter bipolar transistors fabricated in a Si/SiGe:C HBT technology STMicroelectronics featuring fT and fmax of ~300GHz and ~400GHz, respectively. Thermal coupling parameters are extracted using three dimensional (3D) thermal TCAD simulations. The obtained parameters are implemented in a distributed transistor model that considers self-heating as well as thermal coupling between emitter fingers. Very good agreement is achieved between circuit simulations and DC measurements carried out on an in house designed test structure.
Developing van der Waals (vdW) homojunction devices requires materials with narrow bandgaps and simultaneously high hole and electron mobilities for bipolar transport, as well as methods to image and study spatial variations in carrier type and associated conductivity with nanometer spatial resolution. Here we demonstrate the general capability of near-field scanning microwave microscopy (SMM) to image and study the local carrier type and associated conductivity in operando by studying ambiploar field effect transistors (FETs) of the 1D vdW material tellurium in 2D form. To quantitatively unde…
