WSe2 Field-effect Transistors Research Articles

Fabrication and comparison of MoS2 and WSe2 field-effect transistor biosensors

The authors present a study on the evolution behaviors of the transfer characteristics of MoS2 and WSe2 field-effect transistor biosensors when they are subjected to tumor necrosis factor-alpha and streptavidin solutions with varying analyte concentrations. Both MoS2 and WSe2 sensors exhibit very low detection limits (∼60 fM for tumor necrosis factor-alpha detection; ∼70 fM for streptavidin detection). However, WSe2 sensors exhibit the higher linear-regime sensitivities in comparison with MoS2 sensors. In particular, WSe2 sensors exhibit high linear-regime sensitivities up to ∼1.54%/fM for detecting streptavidin at a concentration of ∼70 fM. Such relatively higher sensitivities obtained from WSe2 sensors are attributed to their intrinsic ambipolar transfer characteristics, which make their ON-state carrier concentrations significantly lower than those of MoS2 sensors, and therefore, the target-molecule-induced doping effect results in more prominent channel conductance modulation in WSe2 transistor sensors than in MoS2 sensors. Furthermore, this work strongly implies that the target-molecule-induced surface scattering also plays an important role in determining the response behaviors of the sensors made from atomically layered materials. Especially, the competition between target-molecule-induced p-doping and surface-scattering effects is responsible for the sensor behavior variation observed in the p-type conduction branch of WSe2 sensors. This work advances the critical device physics highly relevant with the fabrication and implementation of reliable nanoelectronic biosensors based on emerging atomically layered semiconductors.

Quantum Mechanical Electrostatics and Transport Simulation and Performance Evaluation of Short Channel Monolayer WSe2 Field Effect Transistor

In this work a numerical electrostatics and transport simulator is developed for monolayer WSe2 channel Field Effect Transistor considering Quantum Mechanical effects. The electrostatics simulator solves 1-D Schrödinger-Poisson equation self-consistently in the direction perpendicular to the channel. Whereas, the transport simulator employs Fast Uncoupled Mode Space (FUMS) approach with Non-Equlibrium Green’s Function (NEGF) formalism. The simulator explored 20 nm long monolayer WSe2 channel FET with top and bottom oxide thickness 3 and 5 nm respectively. This device showed excellent ON state performance with maximum ON current reaching up to 467.2 A/m and maximum transconductance of 887.7 S/m. On the other hand, the OFF state and short channel performances also showed promise with 1010 on/off current ratio, 77.72 mV/dec Subthreshold Slope (SS) and 14.91 mV/V Drain Induced Barrier Lowering (DIBL). The overall performance reveals great potential of this monolayer WSe2 FET in many low power and high speed applications.

High Photoresponsivity and Short Photoresponse Times in Few-Layered WSe2 Transistors.

Here, we report the photoconducting response of field-effect transistors based on three atomic layers of chemical vapor transport grown WSe2 crystals mechanically exfoliated onto SiO2. We find that trilayered WSe2 field-effect transistors, built with the simplest possible architecture, can display high hole mobilities ranging from 350 cm(2)/(V s) at room temperature (saturating at a value of ∼500 cm(2)/(V s) below 50 K) displaying a strong photocurrent response, which leads to exceptionally high photoresponsivities up to 7 A/W under white light illumination of the entire channel for power densities p < 10(2) W/m(2). Under a fixed wavelength of λ = 532 nm and a laser spot size smaller than the conducting channel area, we extract photoresponsitivities approaching 100 mA/W with concomitantly high external quantum efficiencies up to ∼40% at room temperature. These values surpass values recently reported from more complex architectures, such as graphene and transition metal dichalcogenides based heterostructures. Also, trilayered WSe2 phototransistors display photoresponse times on the order of 10 μs. Our results indicate that the addition of a few atomic layers considerably decreases the photoresponse times, probably by minimizing the interaction with the substrates, while maintaining a very high photoresponsivity.

Gate-modulated conductance of few-layer WSe2 field-effect transistors in the subgap regime: Schottky barrier transistor and subgap impurity states

Two key subjects stand out in the pursuit of semiconductor research: material quality and contact technology. The fledging field of atomically thin transition metal dichalcogenides (TMDCs) faces a number of challenges in both efforts. This work attempts to establish a connection between the two by examining the gate-dependent conductance of few-layer (1-5L) WSe2 field effect devices. Measurements and modeling of the subgap regime reveal Schottky barrier transistor behavior. We show that transmission through the contact barrier is dominated by thermionic field emission (TFE) at room temperature, despite the lack of intentional doping. The TFE process arises due to a large number of subgap impurity states, the presence of which also leads to high mobility edge carrier densities. The density of states of such impurity states is self-consistently determined to be approximately 1–2 × 1013/cm2/eV in our devices. We demonstrate that substrate is unlikely to be a major source of the impurity states and suspect that lattice defects within the material itself are primarily responsible. Our experiments provide key information to advance the quality and understanding of TMDC materials and electrical devices.

Low frequency noise characteristics in multilayer WSe2 field effect transistor

This paper investigates the low-frequency noise properties of multilayer WSe2 field effect transistors (FETs) in subthreshold, linear, and saturation regime. The measured noise power spectral density of drain current (SID) shows that the low-frequency noise in multilayer WSe2 FET fits well to a 1/fγ power law with γ ∼ 1 in the frequency range of 10 Hz–200 Hz. From the dependence of SID on the drain current, carrier mobility fluctuation is considered as a dominant low frequency noise mechanism from all operation regimes in multilayer WSe2 FET. Extracted Hooge's parameter in this study is within the value of 0.12, comparable to those of the transition metal dichalcogenide FETs in recent reports.

Hole mobility enhancement and p -doping in monolayer WSe2 by gold decoration

Tungsten diselenide (WSe2) is an attractive transition metal dichalcogenide material, since its Fermi energy close to the mid gap makes it an excellent candidate for realizing p–n junction devices and complementary digital logic applications. Doping is one of the most important technologies for controlling the Fermi energy in semiconductors, including 2D materials. Here we present a simple, stable and controllable p-doping technique on a WSe2 monolayer, where a more p-typed WSe2 field effect transistor is realized by electron transfer from the WSe2 to the gold (Au) decorated on the WSe2 surfaces. Related changes in Raman spectroscopy are also reported. The p-doping caused by Au on WSe2 monolayers lowers the channel resistance by orders of magnitude. The effective hole mobility is ∼100 (cm2/Vs) and the near ideal subthreshold swing of ∼60 mV/decade and high on/off current ratio of >106 are observed. The Au deposited on the WSe2 also serves as a protection layer to prevent a reaction between the WSe2 and the environment, making the doping stable and promising for future scalable fabrication.

Electric control of spin in monolayer WSe2 field effect transistors

We report first-principles theoretical investigations of quantum transport in a monolayer WSe2 field effect transistor (FET). Due to strong spin–orbit interaction (SOI) and the atomic structure of the two-dimensional lattice, monolayer WSe2 has an electronic structure that exhibits Zeeman-like up–down spin texture near the K and points of the Brillouin zone. In a FET, the gate electric field induces an extra, externally tunable SOI that re-orients the spins into a Rashba-like texture thereby realizing electric control of the spin. The conductance of FET is modulated by the spin texture, namely by if the spin orientation of the carrier after the gated channel region, matches or miss-matches that of the FET drain electrode. The carrier current in the FET is labelled by both the valley index and spin index, realizing valleytronics and spintronics in the same device.

High gain, low noise, fully complementary logic inverter based on bi-layer WSe2 field effect transistors

In this article, first, we show that by contact work function engineering, electrostatic doping and proper scaling of both the oxide thickness and the flake thickness, high performance p- and n-type WSe2 field effect transistors (FETs) can be realized. We report record high drive current of 98 μA/μm for the electron conduction and 110 μA/μm for the hole conduction in Schottky barrier WSe2 FETs. Then, we combine high performance WSe2 PFET with WSe2 NFET in double gated transistor geometry to demonstrate a fully complementary logic inverter. We also show that by adjusting the threshold voltages for the NFET and the PFET, the gain and the noise margin of the inverter can be significantly enhanced. The maximum gain of our chemical doping free WSe2 inverter was found to be ∼25 and the noise margin was close to its ideal value of ∼2.5 V for a supply voltage of VDD = 5.0 V.

High mobility WSe2 p- and n-type field-effect transistors contacted by highly doped graphene for low-resistance contacts.

We report the fabrication of both n-type and p-type WSe2 field-effect transistors with hexagonal boron nitride passivated channels and ionic-liquid (IL)-gated graphene contacts. Our transport measurements reveal intrinsic channel properties including a metal-insulator transition at a characteristic conductivity close to the quantum conductance e(2)/h, a high ON/OFF ratio of >10(7) at 170 K, and large electron and hole mobility of μ ≈ 200 cm(2) V(-1)s(-1) at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ∼330 cm(2) V(-1) s(-1) and that of holes to ∼270 cm(2) V(-1) s(-1). We attribute our ability to observe the intrinsic, phonon-limited conduction in both the electron and hole channels to the drastic reduction of the Schottky barriers between the channel and the graphene contact electrodes using IL gating. We elucidate this process by studying a Schottky diode consisting of a single graphene/WSe2 Schottky junction. Our results indicate the possibility to utilize chemically or electrostatically highly doped graphene for versatile, flexible, and transparent low-resistance ohmic contacts to a wide range of quasi-2D semiconductors.

Metal-Semiconductor Barrier Modulation for High Photoresponse in Transition Metal Dichalcogenide Field Effect Transistors

A gate-controlled metal-semiconductor barrier modulation and its effect on carrier transport were investigated in two-dimensional (2D) transition metal dichalcogenide (TMDC) field effect transistors (FETs). A strong photoresponse was observed in both unipolar MoS2 and ambipolar WSe2 FETs (i) at the high drain voltage due to a high electric field along the channel for separating photo-excited charge carriers and (ii) at the certain gate voltage due to the optimized barriers for the collection of photo-excited charge carriers at metal contacts. The effective barrier height between Ti/Au and TMDCs was estimated by a low temperature measurement. An ohmic contact behavior and drain-induced barrier lowering (DIBL) were clearly observed in MoS2 FET. In contrast, a Schottky-to-ohmic contact transition was observed in WSe2 FET as the gate voltage increases, due to the change of majority carrier transport from holes to electrons. The gate-dependent barrier modulation effectively controls the carrier transport, demonstrating its great potential in 2D TMDCs for electronic and optoelectronic applications.

Toward Low-Power Electronics: Tunneling Phenomena in Transition Metal Dichalcogenides

In this article, we explore, experimentally, the impact of band-to-band tunneling on the electronic transport of double-gated WSe2 field-effect transistors (FETs) and Schottky barrier tunneling of holes in back-gated MoS2 FETs. We show that by scaling the flake thickness and the thickness of the gate oxide, the tunneling current can be increased by several orders of magnitude. We also perform numerical calculations based on Landauer formalism and WKB approximation to explain our experimental findings. Based on our simple model, we discuss the impact of band gap and effective mass on the band-to-band tunneling current and evaluate the performance limits for a set of dichalcogenides in the context of tunneling transistors for low-power applications. Our findings suggest that WTe2 is an excellent choice for tunneling field-effect transistors.

WSe2 field effect transistors with enhanced ambipolar characteristics

One of the most relevant features that a semiconducting channel material can offer when used in a field-effect transistor (FET) layout is its capability to enable both electron transport in the conduction band and hole transport in the valence band. In this way, complementary metal-oxide-semiconductor type applications become feasible once similar electron and hole drive current densities are achieved, and the threshold voltages are properly adjusted. In this article, we demonstrate pronounced ambipolar device characteristics of multilayer WSe2 FETs using different contact electrodes. Our study reveals that nickel electrodes facilitate electron injection while palladium electrodes are more efficient for hole injection. We also show, as an interesting demonstration, that by using nickel as the source contact electrode and palladium as the drain contact electrode, ambipolar device characteristics with similar on-state performance for both the electron and the hole branch can be achieved in WSe2 FETs. Finally, we discuss a unique technique based on the asymmetry in the ambipolar device characteristics to extract the Schottky barrier heights for such metal to WSe2 contacts.

(Invited) High-Performance Field-Effect-Transistors on Monolayer-WSe2

Monolayer Tungsten Diselenide (WSe2) exhibits tremendous advantages as a channel material for next-generation field-effect-transistors (FETs). This paper reviews the relevant physics and properties of WSe2 and highlights the excellent scalability of monolayer WSe2 for ultra-short channel (sub-5 nm) FETs. The crucial role of metal-WSe2 contacts in determining the performance of monolayer WSe2 FETs is also emphasized using experiments guided by ab-initio density functional theory (DFT). With a suitably chosen contact, a back-gated monolayer WSe2 FET on Al2O3 substrate is shown to exhibit both high mobility and high ON-current.

Role of Metal Contacts in Designing High-Performance Monolayer n-Type WSe2 Field Effect Transistors

This work presents a systematic study toward the design and first demonstration of high-performance n-type monolayer tungsten diselenide (WSe2) field effect transistors (FET) by selecting the contact metal based on understanding the physics of contact between metal and monolayer WSe2. Device measurements supported by ab initio density functional theory (DFT) calculations indicate that the d-orbitals of the contact metal play a key role in forming low resistance ohmic contacts with monolayer WSe2. On the basis of this understanding, indium (In) leads to small ohmic contact resistance with WSe2 and consequently, back-gated In-WSe2 FETs attained a record ON-current of 210 μA/μm, which is the highest value achieved in any monolayer transition-metal dichalcogenide- (TMD) based FET to date. An electron mobility of 142 cm(2)/V·s (with an ON/OFF current ratio exceeding 10(6)) is also achieved with In-WSe2 FETs at room temperature. This is the highest electron mobility reported for any back gated monolayer TMD material till date. The performance of n-type monolayer WSe2 FET was further improved by Al2O3 deposition on top of WSe2 to suppress the Coulomb scattering. Under the high-κ dielectric environment, electron mobility of Ag-WSe2 FET reached ~202 cm(2)/V·s with an ON/OFF ratio of over 10(6) and a high ON-current of 205 μA/μm. In tandem with a recent report of p-type monolayer WSe2 FET ( Fang , H . et al. Nano Lett. 2012 , 12 , ( 7 ), 3788 - 3792 ), this demonstration of a high-performance n-type monolayer WSe2 FET corroborates the superb potential of WSe2 for complementary digital logic applications.