Two-dimensional Field-effect Transistors Research Articles

Ultrathin calcium fluoride insulators for two-dimensional field-effect transistors

Two-dimensional semiconductors could be used to fabricate ultimately scaled field-effect transistors and more-than-Moore nanoelectronic devices. However, these targets cannot be reached without appropriate gate insulators that are scalable to the nanometre range. Typically used oxides such as SiO2, Al2O3 and HfO2 are, however, amorphous when scaled, and 2D hexagonal boron nitride exhibits excessive gate leakage currents. Here, we show that epitaxial calcium fluoride (CaF2), which can form a quasi van der Waals interface with 2D semiconductors, can serve as an ultrathin gate insulator for 2D devices. We fabricate scalable bilayer MoS2 field-effect transistors with a crystalline CaF2 insulator of ~2 nm thickness, which corresponds to an equivalent oxide thickness of less than 1 nm. Our devices exhibit low leakage currents and competitive device performance characteristics, including subthreshold swings down to 90 mV dec−1, on/off current ratios up to 107 and a small hysteresis. High-performance MoS2 transistors can be created using 2-nm-thick CaF2 as a gate insulator, which forms a quasi van der Waals interface with the 2D semiconductor.

Highly-sensitive gas sensor based on two-dimensional material field effect transistor

Atomically thin two-dimensional (2D) materials are ideal gas sensing materials for achieving an ultra-low detection limit, due to the high surface-to-volume ratio, low electronic noise and sensitively tunable Fermi level. However, the sensitivity of 2D materials to their surrounding environment may also severely degrade the long-term stability of sensing devices, since most of them use the same 2D material flake as both the sensing and conduction material. In this work, we report a gas sensor based on a 2D material field effect transistor (FET) which uses few-layer black phosphorus (BP), boron nitride (BN) and molybdenum disulfide (MoS2) as the top-gate, dielectric layer and conduction channel, respectively. In this device configuration, the top-gate of BP with a superior gas adsorption capability serves as the sensing material, while the conduction channel of MoS2 is isolated from ambient environment by the coverage of the BN dielectric layer. The separation of the sensing and conduction materials not only improves the long-term stability of the device, but also enables us to use different materials for gas adsorption and conduction purposes to achieve optimum sensing performances. In addition, the adsorption kinetics of the gas molecules on the sensing channel can be sensitively detected by the current/resistance variation of the conduction channel, since the adsorbed gas molecules can effectively tune the Fermi level of sensing and conduction materials (BP and MoS2, respectively) through band alignment. We experimentally demonstrated that the proposed 2D material FET not only achieved a detection limit of 3.3 ppb to NO2, but was also capable to differentiate oxidizing and reducing gases.

Materials with honeycomb structures for gate dielectrics in two-dimensional Field Effect Transistors – An ab initio study

The size of the Field Effect Transistor (FET) has been decreasing exponentially each year to keep up with the demands of the microelectronic industry. However, this scaling is now reaching its limits due to the performance of the materials currently used in the FET. Specifically, the leakage through silicon dioxide, the current gate dielectric, is beginning to inhibit the FET's performance. Recently, attention has been turning towards two-dimensional materials for electronic devices. However, insulating is especially challenging for two-dimensional materials due to the short diffusion length for charge carriers. We consider several materials that have been shown to have a stable two-dimensional honeycomb structure. In order to characterize these materials, we use Density Functional Theory (DFT) to calculate key properties that determine performance, including the dielectric function, band gap, and effective mass. We check the compatibility of these materials with Molybdenum Disulfide (MoS2), a two-dimensional material that has emerged as a promising candidate for the semiconducting channel of a FET. We have identified several promising candidates, including hBN, which is the common choice as a dielectric material, and LiF and BeO, which are materials that have not been used before as gate dielectrics. We anticipate that this study will contribute to advancing the development of transistors that are made completely of two-dimensional materials.

A surface potential based compact model for two-dimensional field effect transistors with disorders induced transition behaviors

A surface potential based compact model for two-dimensional field effect transistors (2D-FETs) is proposed to incorporate the structural disorders induced transition behaviors among variable range hopping (VRH), nearest neighbor hopping (NNH), and band-like transport in most 2D materials. These functions coupled with effective transport energy and multiple trapping and releasing theory enable our developed model to predict the temperature and carrier density dependent current characteristics. Its validity is confirmed by the experimental results such as the metal insulator transition (MIT) in transition metal dichalcogenides and VRH-NNH transition in black phosphorus nanoribbon. Based on this model, the band-tail effects on the crossover gate voltage of MIT behavior are quantitatively investigated. It is found that the transition behavior is closely related to the distribution of the band-tail states. Furthermore, this model is implemented in Verilog-A for circuit-level prediction and evaluation of 2D-FETs to provide deeper insight into the relationship between material properties, device physics, and circuit performances.

Transverse Detection of DNA in a MoS2 Nanopore

Solid-state nanopores can be used for single-molecule detection and potentially provide a platform for rapid DNA sequencing. DNA molecules along with water and ions are electrophoretically driven through a small orifice in a membrane. The ionic current blockade is measured and provides information about the size and the length of the molecule. Typically, solid-state nanopores are fabricated in relatively thick material (20nm), thus not providing single-base resolution. Recently, monolayer materials such as MoS2 have proven very useful in detecting and differentiating single nucleotides. Compared to e.g. graphene, monolayer MoS2 is an intrinsic semiconductor with a bandgap of 1.8eV and can thus be used to fabricate a field effect transistor. Here we show that a nanopore in monolayer MoS2 can be integrated with a two-dimensional field-effect transistor. We report simultaneous and correlated detection of the ionic current trace and the transverse sheet current through suspended MoS2 during DNA translocation. Signal-to-noise ratios in the sheet current are superior to ionic current and seem to follow a different sensing principle. During translocation through the nanopore, the negative charge of DNA decreases the drain-source conductance of the n-type semiconductor, effectively gating the semiconductor. Such a device configuration might overcome resolution limitations due to the dominating access-resistance in ionic current through ultra-thin membranes. Furthermore, this technology provides a simple way of parallelization: An array of nanopores can be created on the same membrane without the need of fabricating individually addressable microfluidic chambers. The ionic voltage would then just act as the driving force of bringing the molecules to the field-effect transistor and all detection is done on the transistors. Further work is required in order to better understand the sensing principle and establish the resolution of these new devices.

Magnetic two-dimensional field effect transistor

We study experimentally low-temperature current-voltage characteristics of n-type GaAs/GaAlAs modulation doped quantum wells under the influence of an external magnetic field. In particular, we use samples additionally doped in the well with Be acceptors. As showed previously, negatively charged acceptor ions can localize conduction electrons by a joint effect of a quantum well and an external magnetic field. It is found that, in the acceptor-doped samples, the Hall electric field resulting from the presence of magnetic field plays the role of the gate voltage. At sufficiently high magnetic fields, the drain current has a constant value independent of the drain voltage. It is argued that the above phenomenon is due to the electron localization with the resulting decrease of conducting electron density in the crossed-field configuration. We propose to exploit the observed unusual behaviour as a device called “magnetic two-dimensional electron gas field effect transistor” operating at low temperatures.

Fabrication of Two-dimensional MoS2 Films-based Field Effect Transistor for High Mobility Electronic Device Application

Fabrication of Two-dimensional MoS2 Films-based Field Effect Transistor for High Mobility Electronic Device Application

Highly anisotropic mobility in solution processed TIPS-pentacene film studied by independently driven four GaIn probes

We have studied in-plane anisotropy in the field-effect mobility of solution-processed organic semiconductor 6,13-bis(triisopropylsilylethynyl)pentacene by using independently driven four gallium indium (Ga-In) probes. Liquid-metal Ga-In probes are highly effective for reproducible conductivity measurements of organic thin films. We demonstrated that a high mobility anisotropy of 44 was obtained by using a square four-probe method and a feedback circuit to keep the channel potential constant. The present method minimized the influences of the contact resistance and the insensitivity of anisotropy in a linear arrangement in two-dimensional field-effect transistors.

Highly-stable black phosphorus field-effect transistors with low density of oxide traps

Black phosphorus is considered a very promising semiconductor for two-dimensional field-effect transistors. Initially, the main disadvantage of this material was thought to be its poor air stability. However, recent studies have shown that this problem can be solved by suitable encapsulation. As such, long-term studies of the outstanding properties of black phosphorus devices have become possible. In particular, here we examine highly-stable black phosphorus field-effect transistors and demonstrate that they can exhibit reproducible characteristics for at least 17 months. Furthermore, we notice some improvement in the performance of black phosphorus devices after this long time, i.e., positive aging. Although our black phosphorus devices are stable at room temperature, we show that their performance is affected by thermally activated charge trapping by oxide traps into the adjacent SiO2 substrate layer. Aiming to analyze the dynamics of these defects in detail, we perform an accurate mapping of oxide traps with different time constants using the ‘extended incremental hysteresis sweep method’. Our results show that at room temperature the extracted oxide trap densities are (i) few orders of magnitude lower than for MoS2/SiO2 transistors and (ii) close to those reported for more mature Si/SiO2 devices (~1017 cm−3 eV−1). Taking into account the novelty of black phosphorus and recent issues with its stability, these values must be considered unexpectedly low.

(Invited) Dynamic Bandgap Engineering in 2D Materials for Beyond Boltzmann Devices

In this talk I will discuss a disruptive device concept based on dynamic bandgap engineering in two dimensional layered seminconductors in order to achieve sub-60mV/decade subthrshold swing [1]. This device is refer to as two-dimensional electrostrictive field effect transistor or 2D-EFET and it works on the principle of voltage induced stress transduction. It uses an electrostrictive material as gate insulator which expands when a gate bias is applied and thereby transduces an out-of-plane stress on the 2D semiconducting channel. The stress decreases the inter-layer distance between the consecutive layers of the 2D material and dynamically reduces its bandgap. Thus the device operates with a large bandgap in the OFF state and a small or zero bandgap in the ON state. As a consequence of this transduction mechanism, internal voltage amplification takes place which results in sub-60mV/decade subthreshold swing (SS). 1. Das, S. "Two Dimensional Electrostrictive Field Effect Transistor (2D-EFET): A sub-60mV/decade Steep Slope Device with High ON current", Scientific Reports 2016, 6 (34811).

S2DS: Physics-based compact model for circuit simulation of two-dimensional semiconductor devices including non-idealities

We present a physics-based compact model for two-dimensional (2D) field-effect transistors (FETs) based on monolayer semiconductors such as MoS2. A semi-classical transport approach is appropriate for the 2D channel, enabling simplified analytical expressions for the drain current. In addition to intrinsic FET behavior, the model includes contact resistance, traps and impurities, quantum capacitance, fringing fields, high-field velocity saturation, and self-heating, the latter being found to play an important role. The model is calibrated with state-of-the-art experimental data for n- and p-type 2D-FETs, and it can be used to analyze device properties for sub-100 nm gate lengths. Using the experimental fit, we demonstrate the feasibility of circuit simulations using properly scaled devices. The complete model is implemented in SPICE-compatible Verilog-A, and a downloadable version is freely available at the nanoHUB.org.

Two Dimensional Electrostrictive Field Effect Transistor (2D-EFET): A sub-60mV/decade Steep Slope Device with High ON current.

This article proposes a disruptive device concept which meets both low power and high performance criterion for post-CMOS computing and at the same time enables aggressive channel length scaling. This device, hereafter refer to as two-dimensional electrostrictive field effect transistor or 2D-EFET, allows sub-60 mV/decade subthreshold swing and considerably higher ON current compared to any state of the art FETs. Additionally, by the virtue of its ultra-thin body nature and electrostatic integrity, the 2D-EFET enjoys scaling beyond 10 nm technology node. The 2D-EFET works on the principle of voltage induced strain transduction. It uses an electrostrictive material as gate oxide which expands in response to an applied gate bias and thereby transduces an out-of-plane stress on the 2D channel material. This stress reduces the inter-layer distance between the consecutive layers of the semiconducting 2D material and dynamically reduces its bandgap to zero i.e. converts it into a semi-metal. Thus the device operates with a large bandgap in the OFF state and a small or zero bandgap in the ON state. As a consequence of this transduction mechanism, internal voltage amplification takes place which results in sub-60 mV/decade subthreshold swing (SS).

Modeling of anisotropic two-dimensional materials monolayer HfS2 and phosphorene metal-oxide semiconductor field effect transistors

Ballistic transport characteristics of metal-oxide semiconductor field effect transistors (MOSFETs) based on anisotropic two-dimensional materials monolayer HfS2 and phosphorene are explored through quantum transport simulations. We focus on the effects of the channel crystal orientation and the channel length scaling on device performances. Especially, the role of degenerate conduction band (CB) valleys in monolayer HfS2 is comprehensively analyzed. Benchmarking monolayer HfS2 with phosphorene MOSFETs, we predict that the effect of channel orientation on device performances is much weaker in monolayer HfS2 than in phosphorene due to the degenerate CB valleys of monolayer HfS2. Our simulations also reveal that at 10 nm channel length scale, phosphorene MOSFETs outperform monolayer HfS2 MOSFETs in terms of the on-state current. However, it is observed that monolayer HfS2 MOSFETs may offer comparable, but a little bit degraded, device performances as compared with phosphorene MOSFETs at 5 nm channel length.

Electrical characterization of multilayer HfSe2 field-effect transistors on SiO2 substrate

We fabricated and characterized two-dimensional field-effect transistors (FETs) based on hafnium diselenide (HfSe2) crystalline nanoflakes. The HfSe2 FET exhibits an n-type semiconductor behavior with a high on/off current ratio exceeding 7.5 × 106. In the temperature range of 120 K–280 K, the thermally activated transport is observed at high carrier concentrations, while at low concentrations and low temperatures hopping conduction dominates the transport mechanism. We also observed the metal insulator transition at carrier density of ∼1.8 × 1012 cm−2. This initial report on the physical and electrical characterization of two dimensional HfSe2 material demonstrates the feasibility of this semiconducting material for electronic devices.

Contact research strategy for emerging molybdenum disulfide and other two-dimensional field-effect transistors

Layered two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have been widely isolated, synthesized, and characterized recently. Numerous 2D materials are identified as the potential candidates as channel materials for future thin film technology due to their high mobility and the exhibiting bandgaps. While many TMD filed-effect transistors (FETs) have been widely demonstrated along with a significant progress to clearly understand the device physics, large contact resistance at metal/semiconductor interface still remain a challenge. From 2D device research point of view, how to minimize the Schottky barrier effects on contacts thus reduce the contact resistance of metals on 2D materials is very critical for the further development of the field. Here, we present a review of contact research on molybdenum disulfide and other TMD FETs from the fundamental understanding of metal-semiconductor interfaces on 2D materials. A clear contact research strategy on 2D semiconducting materials is developed for future high-performance 2D FETs with aggressively scaled dimensions.

Chemical vapor sensing of two-dimensional MoS2 field effect transistor devices

MoS2, in single to few-layer format, is of interest because of its potential for advanced transistor and sensor applications. Its sizable bandgap enables single layer transistors with large on/off current ratios, and the large surface-to-volume ratio provides sensitive transduction of surface physisorption to the channel conductivity. Here, we discuss aspects of transistor device fabrication and of chemical vapor sensing experiments. We expose MoS2 chemical sensors to a variety of analytes, find the largest response to triethylamine, a nerve gas by-product, and explain our results based on a donor–acceptor model. We show that our MoS2 sensors provide comparable sensitivity and much higher selectivity than other low-dimensional sensors such as carbon nanotube and graphene chemical sensors. We present results for back-gated sensing and light sensitivity for our monolayer MoS2 sensors, and compare the results with multilayer MoS2 sensors.

Superconductivity in two-dimensional NbSe2 field effect transistors

We describe investigations of superconductivity in few molecular layer NbSe2 field effect transistors. While devices fabricated from NbSe2 flakes less than eight molecular layers thick did not conduct, thicker flakes were superconducting with an onset Tc that was only slightly depressed from the bulk value for 2H-NbSe2 (7.2 K). The resistance typically showed a small, sharp high temperature transition followed by one or more broader transitions which usually ended in a wide tail to zero resistance at low temperatures. We speculate that these multiple resistive transitions are related to disorder in the layer stacking. The behavior of several flakes has been characterized as a function of temperature, applied field and back-gate voltage. We find that the conductance in the normal state and transition temperature depend weakly on the gate voltage, with both conductivity and Tc decreasing as the electron concentration is increased. The application of a perpendicular magnetic field allows the evolution of different resistive transitions to be tracked and values of the zero temperature upper critical field, Hc2(0), and coherence length, ξ(0), to be independently estimated. Our results are analyzed in terms of available theories for these phenomena.

THERMAL INVESTIGATION OF COMMON 2D FETs AND NEW GENERATION OF 3D FETs USING BOLTZMANN TRANSPORT EQUATION IN NANOSCALE

The thermal performance of two-dimensional (2D) field-effect transistors (FET) is investigated frequently by solving the Fourier heat diffusion law and the Boltzmann transport equation (BTE). With the introduction of the new generation of 3D FETs in which their thickness is less than the phonon mean-free-path it is necessary to carefully simulate the thermal performance of such devices. This paper numerically integrates the BTE in common 2D transistors including planar single layer and Silicon-On-Insulator (SOI) transistor, and the new generation of 3D transistors including FinFET and Tri-Gate devices. In order to decrease the directional dependency of results in 3D simulations; the Legendre equal-weight (PN-EW) quadrature set has been employed. It is found that if similar switching time is assumed for 3D and 2D FETs while the new generation of 3D FETs has less net energy consumption, they have higher hot-spot temperature. The results show continuous heat flux distribution normal to the silicon/oxide interface while the temperature jump is seen at the interface in double layer transistors.

Lateral Graphene–hBCN Heterostructures as a Platform for Fully Two-Dimensional Transistors

We propose that lateral heterostructures of single-atomic-layer graphene and hexagonal boron-carbon-nitrogen (hBCN) domains, can represent a powerful platform for the fabrication and the technological exploration of real two-dimensional field-effect transistors. Indeed, hBCN domains have an energy bandgap between 1 and 5 eV, and are lattice-matched with graphene; therefore they can be used in the channel of a FET to effectively inhibit charge transport when the transistor needs to be switched off. We show through ab initio and atomistic simulations that a FET with a graphene-hBCN-graphene heterostructure in the channel can exceed the requirements of the International Technology Roadmap for Semiconductors for logic transistors at the 10 and 7 nm technology nodes. Considering the main figures of merit for digital electronics, a FET with gate length of 7 nm at a supply voltage of 0.6 V exhibits I(on)/I(off) ratio larger than 10(4), intrinsic delay time of about 0.1 ps, and a power-delay-product close to 0.1 nJ/m. More complex graphene-hBCN heterostructures can allow the realization of different multifunctional devices, translating on a truly two-dimensional structure some of the device principles proposed during the first wave of nanoelectronics based on III-V heterostructures, as for example the resonant tunneling FET.

The passivation/modification of AlGaAs/GaAs surfaces by amorphous TiO 2 for the bio-sensing use in electrolytes

In order to apply two-dimensional electron-gas field effect transistors (2DEG-FETs) for bio-sensing devices operating in electrolytes, the chemical/electrical properties of TiO 2 thin film is investigated as a material for the gate oxide of FETs. TiO 2 films of thickness 13–15 nm are prepared with sol–gel technique on the gate surfaces of AlGaAs/GaAs 2DEG-FETs, followed by heat treatment at 450 °C. The TiO 2 surface has nanometer-scale roughness with a peak-to-peak height difference of approximately 10 nm. The TiO 2/2DEG-FETs exhibit a good performance for operation in electrolytes in the dark, with a transconductance of 0.20 mA/V. Large and continuous threshold shifts to the negative side of V gs are resulted when the TiO 2/2DEG-FETs are operated under light at negative gate biases ( V gs < 0 V). This reveals that photoexcited holes are accumulated at the interface of TiO 2/GaAs by the negative gate biases. The sensitivity for pH and H 2O 2 is examined for the TiO 2/2DEG-FETs, where a sensitivity of 71 mV/pH and a finite response for H 2O 2 are obtained. Cell culture on TiO 2 surfaces is successfully demonstrated, showing a high potential of the TiO 2/2DEG-FETs for cell activity sensors.