Properties Of Field-effect Transistors Research Articles

Macroscopic Interfacial Property as a Determining Parameter for Reliable Prediction of Charge Mobility in Organic Transistors

Physical or chemical properties characterizing a surface of gate dielectric have a huge impact on the electrical properties of organic field-effect transistors. Here, we applied various organic interlayers between an organic semiconductor and a gate dielectric to describe field-effect mobilities being a function of a certain macroscopic parameter associated with the surface energy of gate dielectric. The organic interlayers with various chemical moieties, that is, hydroxyl, methyl, octadecyl, polystyrene, and polymethylmetacrylate, are obtained using diverse organosilane compounds and hydroxyl-end-terminated polymer brushes. Two prototypical vapor-deposited p-type organic small molecules, dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene and pentacene, are used as semiconducting layers. We separate the surface energy of the organic interlayers into two terms, that is polar and dispersive terms, and define three parameters consisting of these two terms, so-called surface energy ratio, polar ratio, and polarity. The three parameters are plotted with the field-effect mobilities and it becomes apparent that the field-effect mobility is a function of polar ratio and polarity regardless of the semiconducting material as well as its morphology and crystallinity. In particular, the polarity that is the polar energy term divided by the total surface energy showed a clear exponential relationship, allowing a reliable prediction of field-effect mobilities.

Sensor Properties of Field-Effect Transistors Based on Graphene Oxide and Nafion Films with Proton Conductivity

The proton conductivity of graphene oxide (GO) and Nafion films was studied depending on the humidity and voltage on electrodes. The electric properties of the films were similar, but the mobility of positive charges in Nafion was approximately two orders of magnitude higher than in GO. In GO films, the negative ion current with a positive voltage bias was up to ~10% of the proton current, while in Nafion films it was almost absent (<1%). The sensors based on GO and Nafion films were most effective at humidity (RH) in the range 20–80%.

Effects of Electron Beam Irradiation and Thiol Molecule Treatment on the Properties of MoS2 Field Effect Transistors

We investigated the effects of the structural defects intentionally created by electron-beam irradiation with an energy of 30 keV on the electrical properties of monolayer MoS2 field effect transistors (FETs). We observed that the created defects by electron beam irradiation on the MoS2 surface working as trap sites deteriorated the carrier mobility and carrier concentration with increasing the subthreshold swing value and shifting the threshold voltage in MoS2 FETs. The electrical properties of electron-beam irradiated MoS2 FETs were slightly improved by treating the devices with thiol-terminated molecules which presumably passivated the structural defects of MoS2. The results of this study may enhance the understanding of the electrical properties of MoS2 FETs in terms of creating and passivating defect sites.

Effects of source/drain electrode contact length on the photoresponsive properties of organic field-effect transistors

With the continual increase of the carrier mobility of organic semiconductors, there is a great need for optimizing the contact between source/drain electrode and organics in order to further improve the performance of organic field-effect transistors. The effects of Au source/drain electrode contact length on the photosensitivity in pentacene-based organic field-effect transistors were systematically investigated. The results show that at a given gate voltage and drain voltage, the drain current increases with the contact length at first and then tends to saturate at a contact length of 0.7 mm. It is observed that the effective mobility under illumination, both in the linear region and the saturation region, as well as the photoresponsivity and the external quantum efficiency, increase with contact length. All of these can be attributed to the reduction of contact resistance with the increase of contact length. Moreover, analytical expressions were derived and successfully describe the measured dependence of the drain current on the contact length.

The ambipolar evolution of a high-performance WSe2 transistor assisted by a ferroelectric polymer

In recent years, the electrical characteristics of WSe2 field-effect transistors (FETs) have been widely investigated with various dielectrics. Among them, being able to perfectly tune the polarity of WSe2 is meaningful and promising work. In this work, we systematically study the electrical properties of bilayer WSe2 FETs modulated by ferroelectric polymer poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)). Compared to traditional gate dielectric SiO2, the P(VDF-TrFE) can not only tune both electron and hole concentrations to the same high level, but also improve the hole mobility of bilayer WSe2 to 265.96 cm2 V−1 s−1 under SiO2 gating. Its drain current on/off ratio is also improved to 2 × 105 for p-type and 4 × 105 for n-type driven by P(VDF-TrFE). More importantly, the ambipolar behaviors of bilayer WSe2 are effectively achieved and maintained because of the remnant polarization field of P(VDF-TrFE). This work indicates that WSe2 FETs with P(VDF-TrFE) gating have huge potential for complementary logic transistor applications, and paves an effective way to achieve in-plane p–n junctions.

Capacitance–voltage analysis of electrical properties for WSe2 field effect transistors with high-k encapsulation layer

Doping effects in devices based on two-dimensional (2D) materials have been widely studied. However, detailed analysis and the mechanism of the doping effect caused by encapsulation layers has not been sufficiently explored. In this work, we present experimental studies on the n-doping effect in WSe2 field effect transistors (FETs) with a high-k encapsulation layer (Al2O3) grown by atomic layer deposition. In addition, we demonstrate the mechanism and origin of the doping effect. After encapsulation of the Al2O3 layer, the threshold voltage of the WSe2 FET negatively shifted with the increase of the on-current. The capacitance–voltage measurements of the metal insulator semiconductor (MIS) structure proved the presence of the positive fixed charges within the Al2O3 layer. The flat-band voltage of the MIS structure of Au/Al2O3/SiO2/Si was shifted toward the negative direction on account of the positive fixed charges in the Al2O3 layer. Our results clearly revealed that the fixed charges in the Al2O3 encapsulation layer modulated the Fermi energy level via the field effect. Moreover, these results possibly provide fundamental ideas and guidelines to design 2D materials FETs with high-performance and reliability.

Effect of substrate and temperature on the electronic properties of monolayer molybdenum disulfide field-effect transistors

The use of two-dimensional nanostructured molybdenum disulfide (MoS2) films in field-effect transistors (FETs) in place of graphene was investigated. Monolayer MoS2 films were fabricated by chemical vapor deposition. The output and transfer curves of supported and suspended MoS2 FETs were measured. The mobility of the suspended device reached 364.2 cm2 V−1 s−1 at 150 °C. The hysteresis of the supported device in transfer curves was much larger than that of the suspended device, and it increased at higher temperatures. These results indicate that the device mobility was limited by Coulomb scattering at ambient temperature, and surface/interface phonon scattering at 150 °C, and the injection of electrons, via quantum tunneling through the Schottky barrier at the contact, was enhanced at higher temperatures and led to the increase of the hysteresis. The suspended MoS2 films show potential for application as a channel material in electronic devices, and further understanding the causes of hysteresis in a material is important for its use in technologies, such as memory devices and sensing cells.

Fabrication and Characterization of Two-Dimensional Layered MoS2 Thin Films by Pulsed Laser Deposition

Direct growth of uniform wafer-scale two-dimensional (2D) layered materials using a universal method is of vital importance for utilizing 2D layers into practical applications. Here, we report on the structural and transport properties of large-scale few-layer MoS2 back-gated field effect transistors (FETs), fabricated using conventional pulsed laser deposition (PLD) technique. Raman spectroscopy and transmission electron microscopy results confirmed that the obtained MoS2 layers on SiO2/Si substrate are multilayers. The FETs devices exhibit a relative high on/off ratio of 5 × 102 and mobility of 0.124 cm2V−1S−1. Our results suggest that the PLD would be a suitable pathway to grow 2D layers for future industrial device applications.

Improving the electrical properties of InAs nanowire field effect transistors by covering them with Y2O3/HfO2 layers.

Quasi-one-dimensional semiconducting materials have attracted increasing attention due to their excellent ability to downscale the size of transistors. However, in quasi-one-dimensional nanowire (NW) transistors, their surface and interface properties play a very important role mainly due to the large surface-to-volume ratio of NWs and surface scattering, which degrade their carrier mobility. Herein, we developed a new method to cover the channel surface of InAs NW field effect transistors (FETs) with Y2O3/HfO2 layers to improve their electrical properties. We successfully fabricated nine FETs and measured their electrical properties, which improve after depositing the Y2O3/HfO2 layers, including an increase in on-state current, decrease in off-state current, increase in transconductance, increase in electron mobility and decrease in subthreshold swing. By comparing the properties of Y2O3/HfO2-covered devices with that of the FETs fabricated without the Y2O3 covering or without annealing, we prove that it is the combined Y2O3/HfO2 layers instead of only the Y2O3 or HfO2 layer that improve the electrical properties of the FETs. The Cs-corrected high-resolution scanning transmission electron microscopy study demonstrates that Y can actually diffuse through the native oxide layer (confirmed to be InOx) and reach the surface of the InAs NWs. Our results indicate that the desirable characteristics of Y2O3 and the surface passivation by HfO2 improve the electrical properties of the InAs NW FETs, in which Y2O3 plays an important role to modify and stabilize the interface between the InAs NWs and the outside dielectric layer. Furthermore, this method should also be applicable to other III-V materials.

Chemical Doping Effects of Gas Molecules on Black Phosphorus Field-Effect Transistors

Black phosphorus (BP) is a recently rediscovered layered material with outstanding optical and electrical properties. Its large adsorption energy and high surface-to-volume ratio have motivated chemical sensing and doping applications. We analyzed the charge transport properties as BP-based field-effect transistors (FETs) were chemically doped by adsorption of gas molecules. The electron-withdrawing NO2 and the electron-donating NH3 molecules were applied to observe p-doping and n-doping behaviors, respectively, in BP channel layer. Y-function method was used to extract the device parameters, allowing us to elucidate the chemical doping effects of different types of gas molecules in BP FETs. Excellent sensing ability of BP-based devices to gas molecules was also demonstrated. Our results provide a facile method to control the device properties of BP-based FET via the chemical doping.

Field effect transistor photodetector based on graphene and perovskite quantum dots

Graphene is an attractive optoelectronic material for various optoelectronic devices, especially in the field of photoelectric detection due to its high carrier mobility and fast response time. However, the relatively low light absorption cross-section and fast electron-hole recombination rate can lead to rapid exciton annihilation and small light gain, which restrict the commercial applications of pure graphene-based photodetector. The perovskite has attracted much attention because of its high photoelectric conversion efficiency in the field of solar cells. The perovskite has the advantages of long carrier diffusion distance and high optical absorption coefficient, which can effectively make up for the shortcomings of pure graphene-based field-effect transistor. In this work, a field-effect transistor photodetector is demonstrated with the combination of graphene and halide perovskite quantum dots (CsPbI3) serving as conductive channel materials. The graphene is prepared by plasma enhanced chemical vapor deposition, and the quantum dots are CsPbI3 perovskite. The electrical properties of graphene and pure graphene-based field-effect transistor are detected and analyzed by using the Raman spectrum. The results show that the graphene has good intrinsic electrical properties. Unlike previous report in which bulk perovskite was used, the perovskite quantum dot field-effect transistor photodetector has an obvious light response to 400 nm signal light, and shows the excellent photoelectrical performance. Under the illumination of 400 nm light, the signal light could be detected steadily and repeatedly by the graphene-perovskite quantum dot photodetector and converted into photocurrent. The photocurrent of the photodetector has a rapid rise, and the maximum value can reach 64 A at a light power of 12 W. The corresponding responsivity is 6.4 AW-1, which is two orders of magnitude higher than that of the general single graphene photodetector (10-2 AW-1), and it is also higher than that of perovskite-based photodetector (0.4 AW-1). In addition, the photoconductive gain and detectivity arrive at 3.7104 and 6107 Jones (1 Jones=1 cmHz1/2W-1), respectively. The results of this study demonstrate that the graphene-perovskite quantum dot photodetector can be a promising candidate for commercial UV light detectors.

Semiconductor:Insulator Blends for Speed Enhancement in Organic Photodiodes

AbstractOrganic photodiodes (OPDs) have been studied intensively for a number of years, as they enable low‐cost fabrication of sensor systems for imaging, medical, or industrial applications. While the device performance in terms of responsivity and detectivity is continuously improving, detection speed still lags behind. One of the major limiting factors is the large transit times of the charges. In this work, an approach of blending insulating polymers into a poly(3‐hexylthiophene‐2,5‐diyl) and [6,6]‐phenyl C61 butyric acid methyl ester bulk heterojunction system, which has already shown beneficial effects on the transport properties in organic field effect transistors or organic solar cells, is followed. Starting from a basic morphological study using poly(methyl methacrylate) (PMMA) with different molecular weights, both the steady‐state and dynamic characteristics of the OPDs are examined. Transient photocurrent measurements show that the inclusion of PMMA results in an increase of the −3 dB cut‐off frequency without adversely affecting the device steady‐state performance under illumination or in the dark. This increase is attributed to a reduced transit time in the active layer when PMMA is present.

Tailoring Phase Transition in Poly(3-hexylselenophene) Thin Films and Correlating Their Crystalline Polymorphs with Charge Transport Properties for Organic Field-Effect Transistors

Poly(3-hexylselenophene) (P3HS) carries attractive advantages over their close analogue poly(3-hexylthiophene) (P3HT), including a stronger intermolecular interaction, a better interchain charge hopping, and a narrower bandgap. However, P3HS is much less studied compared to P3HT. Herein, we report on intriguing reversible phase transition between two different crystalline polymorphs (i.e., form I and II) in P3HS thin films with different molecular weights enabled by alternating thermal and solvent vapor annealing. More importantly, the phase transition kinetics and mechanism as well as the associated changes on molecular packing structures were also scrutinized. The correlation between different P3HS crystalline polymorphs and the resulting field-effect mobilities was explored for the first time. Our study provides an insight into P3HS crystallization and phase transition, thus entailing the use of polyselenophene-based materials for a wide range of optoelectronic applications.

Improving the Stability of High-Performance Multilayer MoS2 Field-Effect Transistors.

In this study, we propose a method for improving the stability of multilayer MoS2 field-effect transistors (FETs) by O2 plasma treatment and Al2O3 passivation while sustaining the high performance of bulk MoS2 FET. The MoS2 FETs were exposed to O2 plasma for 30 s before Al2O3 encapsulation to achieve a relatively small hysteresis and high electrical performance. A MoOx layer formed during the plasma treatment was found between MoS2 and the top passivation layer. The MoOx interlayer prevents the generation of excess electron carriers in the channel, owing to Al2O3 passivation, thereby minimizing the shift in the threshold voltage (Vth) and increase of the off-current leakage. However, prolonged exposure of the MoS2 surface to O2 plasma (90 and 120 s) was found to introduce excess oxygen into the MoOx interlayer, leading to more pronounced hysteresis and a high off-current. The stable MoS2 FETs were also subjected to gate-bias stress tests under different conditions. The MoS2 transistors exhibited negligible decline in performance under positive bias stress, positive bias illumination stress, and negative bias stress, but large negative shifts in Vth were observed under negative bias illumination stress, which is attributed to the presence of sulfur vacancies. This simple approach can be applied to other transition metal dichalcogenide materials to understand their FET properties and reliability, and the resulting high-performance hysteresis-free MoS2 transistors are expected to open up new opportunities for the development of sophisticated electronic applications.

Fabrication of hybrid self-assembled monolayer/hafnium oxide gate dielectric by radical oxidation for molybdenum disulfide field-effect transistors

In this study, radical oxidation is applied to the fabrication of a hybrid self-assembled monolayer (SAM)/hafnium oxide (HfOx) gate dielectric in molybdenum disulfide (MoS2) field-effect transistors. The fabrication process involves radical oxidation to form HfOx at the surface of metallic HfN, SAM formation by immersion, and the deterministic transfer of MoS2 flakes. A subthreshold slope of 75 mV/dec and small hysteresis were demonstrated, indicating superior interfacial properties. Cross-sectional transmission electron microscopy revealed the uniform formation of the HfOx layer at the surface of HfN. The SAM is indispensable for the superior interfacial properties in MoS2 field-effect transistors. The radical oxidation is not restricted to the oxidation of silicon and germanium substrates and was also found to be applicable to the fabrication of a high-k gate dielectric. This study opens up interesting possibilities of radical oxidation for research on functional electronic devices.

Sensing properties of separative paper-based extended-gate ion-sensitive field-effect transistor for cost effective pH sensor applications

In this study, we developed a cost-effective ion-sensing field-effect transistor (FET) with an extended gate (EG) fabricated on a separative paper substrate. The pH sensing characteristics of the paper EG was compared with those of other EGs fabricated on silicon, glass, or polyimide substrates. The fabricated paper-based EGFET exhibited excellent sensitivity close to the Nernst response limit as well as to that of the other substrate-based EGFETs. In addition, we found that all EGFETs, regardless of the substrate, have similar non-ideal behavior, i.e., drift phenomenon and hysteresis width. To investigate the degradation and durability of the paper EG after prolonged use, aging-effect tests were carried out in terms of the hysteresis width and sensitivity over a course of 30 days. As a result, the paper EG maintained stable pH sensing characteristics after 30 days. Therefore, we expect that paper EGFETs can provide a cost-effective sensor platform.

Unraveling the Solution-State Supramolecular Structures of Donor-Acceptor Polymers and their Influence on Solid-State Morphology and Charge-Transport Properties.

Polymer self-assembly in solution prior to film fabrication makes solution-state structures critical for their solid-state packing and optoelectronic properties. However, unraveling the solution-state supramolecular structures is challenging, not to mention establishing a clear relationship between the solution-state structure and the charge-transport properties in field-effect transistors. Here, for the first time, it is revealed that the thin-film morphology of a conjugated polymer inherits the features of its solution-state supramolecular structures. A "solution-state supramolecular structure control" strategy is proposed to increase the electron mobility of a benzodifurandione-based oligo(p-phenylene vinylene) (BDOPV)-based polymer. It is shown that the solution-state structures of the BDOPV-based conjugated polymer can be tuned such that it forms a 1D rod-like structure in good solvent and a 2D lamellar structure in poor solvent. By tuning the solution-state structure, films with high crystallinity and good interdomain connectivity are obtained. The electron mobility significantly increases from the original value of 1.8 to 3.2 cm2 V-1 s-1 . This work demonstrates that "solution-state supramolecular structure" control is critical for understanding and optimization of the thin-film morphology and charge-transport properties of conjugated polymers.

Cyclometalated Platinum-Containing Diketopyrrolopyrrole Complexes and Polymers: Photophysics and Photovoltaic Applications

A series of organometallic complexes and polymers has been synthesized with an objective of studying their fundamental photophysical properties together with their organic photovoltaic and organic field-effect transistor properties. The metal chromophores consist of a diketopyrrolopyrrole (DPP) core, end functionalized with cyclometalated platinum “auxochrome”. The photophysical properties of the metal complex and polymers are compared with the unmetalated chromophore DPP-C8-Th-Py. The polymers Poly-DPP-Th-Pt and Poly-DPP-Ph-Pt differ structurally in their cyclometallating ligands, where they consist of 2-thienylpyridine and 2-phenylpyridine, respectively. Efficient solar spectrum coverage was observed for all chromophores; specifically, the polymer Poly-DPP-Th-Pt has an onset of absorption at ∼900 nm with an optical band gap of 1.4 eV. The triplet excited state was detected for all chromophores and probed by both nanosecond and picosecond transient absorption spectroscopy. Both polymers were employed as ...

Short-channel field-effect transistors with 9-atom and 13-atom wide graphene nanoribbons

Bottom-up synthesized graphene nanoribbons and graphene nanoribbon heterostructures have promising electronic properties for high-performance field-effect transistors and ultra-low power devices such as tunneling field-effect transistors. However, the short length and wide band gap of these graphene nanoribbons have prevented the fabrication of devices with the desired performance and switching behavior. Here, by fabricating short channel (Lch ~ 20 nm) devices with a thin, high-κ gate dielectric and a 9-atom wide (0.95 nm) armchair graphene nanoribbon as the channel material, we demonstrate field-effect transistors with high on-current (Ion > 1 μA at Vd = −1 V) and high Ion/Ioff ~ 105 at room temperature. We find that the performance of these devices is limited by tunneling through the Schottky barrier at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high-performance short-channel field-effect transistors with bottom-up synthesized armchair graphene nanoribbons.

B-doped diamond field-effect transistor with ferroelectric vinylidene fluoride–trifluoroethylene gate insulator

A B-doped diamond field-effect transistor (FET) with a ferroelectric vinylidene fluoride–trifluoroethylene (VDF–TrFE) copolymer gate insulator was fabricated. The VDF–TrFE film deposited on the B-doped diamond showed good insulating and ferroelectric properties. Also, a Pt/VDF–TrFE/B-doped diamond layered structure showed ideal behavior as a metal–ferroelectric–semiconductor (MFS) capacitor, and the memory window width was 11 V, when the gate voltage was swept from 20 to −20 V. The fabricated MFS-type FET structure showed the typical properties of a depletion-type p-channel FET and a maximum drain current density of 0.87 mA/mm at room temperature. The drain current versus gate voltage curves of the proposed FET showed a clockwise hysteresis loop owing to the ferroelectricity of the VDF–TrFE gate insulator. In addition, we demonstrated the logic inverter with the MFS-type diamond FET coupled with a load resistor, and obtained the inversion behavior of the input signal and a maximum gain of 18.4 for the present circuit.