High On-off Ratio Research Articles

Solution-processed ITO thin-film transistors with doping of gallium oxide show high on-off ratios and work at 1 mV drain voltage

Indium tin oxide (ITO) is generally used as an electrode material but has recently been demonstrated to be a competitive candidate for use in semiconductor layers in high-performance thin-film transistors (TFTs), due to its high mobility and strong resistance to wet-etching. Here, we demonstrate TFTs using solution-processed, ultra-thin ITO films with outstanding switching performance. These devices exhibit a mobility of up to 15 cm2 V−1 s−1 and a high on-off ratio of 108. Because the device exhibits significant instability under stress tests, moderate doping with Ga as a dopant is introduced to form Ga-doped ITO TFTs. The resulting device has much enhanced stability, near-zero turn-on voltage, and a high on-off current ratio of 108. Through further involvement of an AlOx dielectric layer, the Ga-doped ITO TFTs exhibit a high apparent mobility of more than 40 cm2 V−1 s−1 and operate at small gate voltages (3 V). Remarkably, the device maintains an on-off ratio of over 104 at drain voltages as small as 1 mV.

Modeling of on-off Current and Cutoff Frequency in Organic Thin Film Transistors

Organic thin-film transistors (OTFTs) are providing incitement in various integrated circuit applications because of advantages like low cost and being flexible.In this paper we present an analytical modeling of on-off current and cutoff frequency in organic thin film transistors. The proposed model accurately describes both the parameters uniquely and is based on the standard equations of transistor . The model parameters are listed out and simulation of the final model is performed in MATLAB . Furthermore, it has been concluded that the high cutoff frequency of OTFTs can be achieved with short channel length and high gate voltage ,further high on-off current ratio can be attained by lowering semiconducting layers’ thickness and increasing the mobility.

A Flexible Carbon Nanotube Sen-Memory Device.

In a modern electronics system, charge-coupled devices and data storage devices are the two most indispensable components. Although there has been rapid and independent progress in their development during the last three decades, a cofunctionality of both sensing and memory at single-unit level is yet premature for flexible electronics. For wearable electronics that work in ultralow power conditions and involve strains, conventional sensing-and-memory systems suffer from low sensitivity and are not able to directly transform sensed information into sufficient memory. Here, a new transformative device is demonstrated, which is called "sen-memory", that exhibits the dual functionality of sensing and memory in a monolithic integrated circuit. The active channel of the device is formed by a carbon nanotube thin film and the floating gate is formed by a controllably oxidized aluminum nanoparticle array for electrical- and optical-programming. The device exhibits a high on-off current ratio of ≈106 , a long-term retention of ≈108 s, and durable flexibility at a bending strain of 0.4%. It is shown that the device senses a photogenerated pattern in seconds at zero bias and memorizes an image for a couple of years.

Direct optical-structure correlation in atomically thin dichalcogenides and heterostructures

Atomically thin transition metal dichalcogenides (TMDs) have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios, which can be further modulated by making more complex TMD heterostructures. However, resolution limits of conventional optical methods do not allow for direct optical-structural correlation measurements in these materials, particularly of buried interfaces in TMD heterostructures. Here we use, for the first time, electron beam induced cathodoluminescence in a scanning transmission electron microscope (CL-STEM) to measure optical properties of monolayer TMDs (WS2, MoS2 and WSSe alloy) encapsulated between layers of hBN. We observe dark areas resulting from localized (~ 100 nm) imperfect interfaces and monolayer folding, which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling. We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties. To overcome the limitation of small electron interaction volume in TMD monolayer (and hence low photon yield), we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important. CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.

Rational design of [e]-fusion induced high-performance DHP/CPD based photoswitches.

We report an effective strategy for improving the electronic transport and switching behaviors of dimethyldihydropyrene/cyclophanediene (DHP/CPD)-based molecular devices, an intriguing photoswitch that can be triggered by ultraviolet/visible (UV-vis) light irradiation. Aiming to obtain molecular devices with high on-off ratios, we assess a series of molecular designs formed by [e]-fusing different arenes on a conjugated macrocycle to modulate the photochemical and electronic properties. Here, the switching mechanism and transport properties of [e]-fused DHP/CPD-based nanojunctions are theoretically investigated by first-principles calculations. As a result, the large diversity in electrical conductance between the closed and open forms certifies the substantial switching behavior observed in these sandwich structures. The maximum on-off ratios in all designed photoswitches are greater than 102. Further analysis confirms the improvement of switching performance caused by [e]-fusion. Notably, in the benzo-fused molecular junctions, the maximum on-off ratio is up to 103, which is 55 times larger than that of the un-fused one. We also find that the position of the switch core can remarkably affect the performance of photoswichable nanodevices.

Fully transparent field-effect transistor with high drain current and on-off ratio

We report a fully transparent thin-film transistor utilizing a La-doped BaSnO3 channel layer that provides a drain current of 0.468 mA/μm and an on-off ratio of 1.5 × 108. The La-doped BaSnO3 channel is grown on a 100–150 nm thick unintentionally doped BaSnO3 buffer layer on a (001) MgO substrate by molecular-beam epitaxy. Unpatterned channel layers show mobilities of 127–184 cm2 V−1 s−1 at carrier concentrations in the low to mid 1019 cm−3 range. The BaSnO3 is patterned by reactive ion etching under conditions preserving the high mobility and conductivity. Using this patterning method, a sub-micron-scale thin film transistor exhibiting complete depletion at room temperature is achieved.

Vertical GaN-on-GaN Schottky Barrier Diodes With Multi-Floating Metal Rings

Vertical GaN Schottky barrier diodes (SBDs) with floating metal rings (FMRs) as edge termination structures have been fabricated on bulk GaN substrates. Devices with different FMR geometries were investigated including various numbers of rings and various spacings between rings. These devices have a low Ron of 1.16~1.59 mΩ·cm 2 , a turn-on voltage of 0.96~ 0.94 V, a high on-off ratio of 10 9 , a nearly ideal ideality factor of 1.03~1.09, and a Schottky barrier height of 1.11~1.18 eV at room temperature. These devices have similar forward electrical characteristics, indicating that FMRs don't degrade the device rectifying performance. The ideality factor decreased and the Schottky barrier height increased with increasing temperature from 300 K to 420 K, where the temperature dependencies of the two parameters indicate the inhomogeneity of the metal/semiconductor Schottky interface. In addition, FMRs can improve device breakdown voltages. As the number of FMRs increased from 0 to 20, the reverse breakdown voltage increased from 223 to 289 V. As the spacing between the FMRs increased from 1.5 to 3 μm, the reverse breakdown voltage increased from 233 to 290 V, respectively. These results indicate multiple FMRs with proper spacings can effectively improve breakdown performance without degrading the device forward characteristics. This work represents a useful reference for the FMR termination design for GaN power devices.

High on-off ratio graphene switch via electrical double layer gating

This paper discusses the production and investigation of novel graphene switches by gating through an electrical double layer (EDL). Controlled voltage biases across a liquid dielectric and graphene induce electrochemical reactions within the dielectric and produce high electric fields in an EDL at the surface of graphene. As the electrochemical reactions occur within the dielectric, the EDL strength separates the electrochemically-produced ions based on their polarity, and provides the necessary molecular activation and deactivation energies to form weak, reversible molecular bonds between the produced ions and graphene. The reversible bonds between the ions and graphene are used to dynamically alter the electronic transport through graphene, which introduces an exciting assortment of device possibilities. Whereas traditional graphene devices are unuseful for electronic switches or digital logic due to an insufficient bandgap of graphene, the presented graphene electrochemical field-effect transistors (GEC-FETs) exhibit ON-OFF ratios larger than 104 with OFF-resistances as high as 10 $\text{M}\Omega $ . Channel current, gate voltage, and dielectric medium are varied and compared to show their effect on device performance. The presented device and associated techniques show potential for integration in graphene digital-logic architectures.

Schottky Barrier Variable Graphene/Multilayer-MoS2 Heterojunction Transistor Used to Overcome Short Channel Effects.

A single-layer MoS2 achieves excellent gate controllability within the nanoscale channel length of a field-effect transistor (FET) owing to an ultra-short screening length. However, multilayer MoS2 (ML-MoS2) is more vulnerable to short channel effects (SCEs) owing to its thickness and long screening length. We eliminated the SCEs in an ML-MoS2 FET (thickness of 4-13 nm) at a channel length of sub-30 nm using a Schottky barrier (SB) variable graphene/ML-MoS2 heterojunction. Although the band modulation in the ML-MoS2 channel worsens with a decrease in the channel length, which is similar to the SCEs occurring in conventional FETs, the variable Fermi level (EF) of a graphene electrode along the gate voltage allows control of the SB at the graphene/MoS2 junction and backs up the current modulation through a variable SB. Electrical measurements and a theoretical band simulation demonstrate the efficient SB modulation of our graphene nanogap (GrNG) ML-MoS2 FET with three distinct carrier transports along Vgs: a thermionic emission at a low SB, Fowler-Nordheim tunneling at a moderate SB, and direct tunneling at a high SB. Our GrNG FET shows an extremely high on-off current ratio of ∼108, which is approximately three-orders of magnitude better than a previously reported metal nanogap (MeNG) FET and a self-aligned metal/graphene nanogap FET with a similar MoS2 thickness. Our GrNG FET also exhibits a 100,000-times higher on-off ratio, 100-times lower subthreshold swing, and 10-times lower drain induced barrier.

Magnetic and transport properties of a ferromagnetic layered semiconductor MnIn2Se4

Magnetic two-dimensional (2D) materials hold considerable promise for the next generation of spintronic devices. In this study, a ternary 2D layered semiconductor compound, MnIn2Se4, with good photoelectric properties and low-temperature ferromagnetism is introduced. Field-effect transistors based on few-layer MnIn2Se4 exhibit n-type characteristics, with a high on-off ratio of up to 450 times, and exhibit a good photoresponse with an on-off ratio of 25 times. Magnetic measurements show that few-layer MnIn2Se4 exhibits ferromagnetic behavior with a perpendicular anisotropy at 2 K and a Curie temperature of ∼7 K. This study suggests that MnIn2Se4 has potential in applications involving magnetic and optoelectronic devices.

Improving Performance of Tin-Doped-Zinc-Oxide Thin-Film Transistors by Optimizing Channel Structure

In this paper, we investigated the performance of thin-film transistors (TFTs) with different channel configurations including single-active-layer (SAL) Sn-Zn-O (TZO), dual-active-layers (DAL) In-Sn-O (ITO)/TZO, and triple-active-layers (TAL) TZO/ITO/TZO. The TAL TFTs were found to combine the advantages of SAL TFTs (a low off-state current) and DAL TFTs (a high mobility and a low threshold voltage). The proposed TAL TFTs exhibit superior electrical performance, e.g. a high on-off state current ratio of 2 × 108, a low threshold voltage of 0.63 V, a high field effect mobility of 128.6 cm2/Vs, and a low off-state current of 3.3 pA. The surface morphology and characteristics of the ITO and TZO films were investigated and the TZO film was found to be C-axis-aligned crystalline (CAAC). A simplified resistance model was deduced to explain the channel resistance of the proposed TFTs. At last, TAL TFTs with different channel lengths were also discussed to show the stability and the uniformity of our fabrication process. Owing to its low-processing temperature, superior electrical performance, and low cost, TFTs with the proposed TAL channel configuration are highly promising for flexible displays where the polymeric substrates are heat-sensitive and a low processing temperature is desirable.

Repair of Oxygen Vacancies and Improvement of HfO2/MoS2 Interface by NH3-Plasma Treatment

The effects of treating the HfO2 gate dielectric in different plasmas (O2, N2, and NH3) on the electrical characteristics of a back-gated MoS2 transistor are investigated. It is found that the electrical properties of the device improved after HfO2 is treated by these plasmas, in which NH3-plasma treatment results in the best electrical characteristics: high ON–OFF ratio of $1.26\times 10^{{7}}$ , high extrinsic carrier mobility of 61.5 cm2/ $\text{V}\cdot \text{s}$ , small subthreshold swing (SS) of 111 mV/dec, low interface-state density of $2.79\times 10^{12}$ eV $^{-{1}}\,\,\cdot $ cm−2, and small hysteresis of 43 mV. These should be attributed to the incorporation of more N into the HfO2 film by the NH3 plasma to repair the oxygen vacancies and increase the $k$ value of the HfO2 film. Moreover, H from the NH3 plasma effectively passivates the dangling bonds at/near the HfO2/MoS2 interface, thus creating an excellent MoS2/HfO2 interface.

Volatile Resistive Switching Memory Based on Ag Ion Drift/Diffusion—Part II: Compact Modeling

Resistive-switching random access memory (RRAM) devices based on filamentary switching are attracting widespread interest for their unique properties, such as high ON–OFF ratio, ultrasteep slope, good endurance, and low-current operation. Recently, volatile RRAMs based on Ag drift and diffusion were also demonstrated for possible applications such as crosspoint array selectors and neuromorphic computing. However, the mechanism of the volatile switching, namely, the spontaneous dissolution of the Ag filament, is still not clear. A deep understanding of the metallic filament formation and spontaneous disruption would strongly help the engineering of the device for optimized performances. Here, we present a numerical physics-based drift/diffusion modeling framework to describe the threshold switching, ${I}$ – ${V}$ characteristics, and morphological evolution of the metallic filament. The model can support TCAD-type device simulations for scaling, reliability, and variability studies.

Solution-Processed Organic Photodetector Using Hafnium Oxide as an Electron Blocking Layer.

Organic photodetector (OPD) performance is affected significantly by leakage current. In this study, to decrease OPD leakage current, we introduced hafnium oxide as an electron blocking material, using a solution fabrication process. We fabricated an OPD consisting of ITO/HfO₂/PCHT:PC60BM/Yb/Al, and measured its J-V characteristics, external quantum efficiency, and transient photocurrents. We found that the thickness of the hafnium oxide layer affected the detectivity of the prepared OPD. In particular, a device having an ultrathin hafnium oxide film (5.5 nm thick) exhibited a high on-off current ratio of up to 2.26 × 105 at -1 V, which is two times higher than that of a device having a PEDOT:PSS electron blocking layer.

High-Performance Flexible Ultraviolet Photodetectors with Ni/Cu-Codoped ZnO Nanorods Grown on PET Substrates.

As a developing technology for flexible electronic device fabrication, ultra-violet (UV) photodetectors (PDs) based on a ZnO nanostructure are an effective approach for large-area integration of sensors on nonconventional substrates, such as plastic or paper. However, photoconductive ZnO nanorods grown on flexible substrates have slow responses or recovery as well as low spectral responsivity R because of the native defects and inferior crystallinity of hydrothermally grown ZnO nanorods at low temperatures. In this study, ZnO nanorod crystallites are doped with Cu or Ni/Cu when grown on polyethylene terephthalate (PET) substrates in an attempt to improve the performance of flexible PDs. The doping with Ni/Cu or Cu not only improves the crystalline quality but also significantly suppresses the density of deep-level emission defects in as-grown ZnO nanorods, as demonstrated by X-ray diffraction and photoluminescence. Furthermore, the X-ray photoelectron spectroscopy analysis shows that doping with the transition metals significantly increases the oxygen bonding with metal ions with enhanced O/Zn stoichiometry in as-grown nanorods. The fabricated flexible PD devices based on an interdigitated electrode structure demonstrates a very high R of ~123 A/W, a high on-off current ratio of ~130, and a significant improvement in transient response speed exhibiting rise and fall time of ~8 and ~3 s, respectively, by using the ZnO nanorods codoped by Ni/Cu.

Gate Insulator Engineering in Top-Gated Indium-Tin-Oxide-Stabilized ZnO Thin-Film Transistors

This letter proposes a high-quality plasma-enhanced chemical vapor deposition (PECVD) SiO2 stack for thin-film transistors (TFTs), which contains a layer of SiH4-sourced PECVD SiO2 (SiH4-SiO2) with a layer of tetraethyl-orthosilicate (TEOS)-sourced PECVD SiO2 (TEOS-SiO2) underneath. The issues of high hydrogen content in the SiH4-SiO2 and rich hydroxyl groups in the TEOS-SiO2 were found to be well addressed by capping the TEOS-SiO2 with SiH4-SiO2. Moreover, the electrical properties of the SiO2 stacks were shown to be strengthened in comparison with their single-layer counterparts. This is owing to the mutual interactions between hydrogen groups in the SiH4-SiO2 and hydroxyl groups in the TEOS-SiO2. Top-gated indium–tin-oxide-stabilized ZnO TFTs with stacked gate insulators (GIs) were demonstrated after the optimizations of the GI layer thickness and deposition order and were shown to be free of hysteresis with a high ON–OFF ratio of over 1010 and a sharp subthreshold swing of 96.9 mV/decade. The combination of TEOS-SiO2 and SiH4-SiO2 initiates an economical strategy to achieve reliable GIs for high-performance top-gated metal-oxide TFTs.

All-Inkjet-Printed Vertical Heterostructure for Wafer-Scale Electronics.

In this study, we fabricated an array of all-inkjet-printed vertical Schottky barrier (SB) transistors and various logic gates on a large-area substrate. All of the electronic components, including the indium-gallium-zinc-oxide (IGZO) semiconductor, reduced graphene oxide (rGO), and indium-tin-oxide (ITO) electrodes, and the ion-gel gate dielectric, were directly and uniformly printed onto a 4 in. wafer. The vertical SB transistors had a vertically stacked structure, with the inkjet-printed IGZO semiconductor layer placed between the rGO source electrode and the ITO drain electrode. The ion-gel gate dielectric was also inkjet-printed in a coplanar gate geometry. The channel current was controlled by adjusting the SB height at the rGO/IGZO heterojunction under application of an external gate voltage. The high intrinsic capacitance of the ion-gel gate dielectric facilitated modulation of the SB height at the source/channel heterojunction to around 0.5 eV at a gate voltage lower than 2 V. The resulting vertical SB transistors exhibited a high current density of 2.0 A·cm-2, a high on-off current ratio of 106, and excellent operational and environmental stabilities. The simple device structure of the vertical SB transistors was beneficial for the fabrication of all-inkjet-printed low-power logic circuits such as the NOT, NAND, and NOR gates on a large-area substrate.

Ultraviolet to Long-Wave Infrared Photodetectors Based on a Three-Dimensional Dirac Semimetal/Organic Thin Film Heterojunction.

In this work, high-performance ultraviolet to long-wave infrared (UV-LIR) devices based on an N-type three-dimensional (3D) Dirac semimetal Cd3As2 and P-type organic (small molecules and polymers) heterojunction are prepared. Primarily, the photodetector shows a broadband photoresponse from 365 to 10600 nm. The optimized device responsivity is 729 mA/W, along with a fast response time of 282 μs and a high on-off ratio of 6268, which are 2 orders of magnitude higher than those previously reported for a 3D Dirac semimetal-based device. In the LIR region (10600 nm), the responsivity and on-off ratio can reach 81.3 mA/W and 100, respectively. In addition, the time-resolved femtosecond pump detection technology is used to reveal the relaxation time of Cd3As2/organic thin films (4.30 ps), indicating that Cd3As2/organic thin films have great potential for the manufacture of fast IR devices. These results demonstrate that the 3D Dirac semimetal/organic thin film heterojunction photodetectors will be a feasible solution for high-speed and broadband photodetectors in large-array imaging.

Multicast silicon photonic MEMS switches with gap-adjustable directional couplers.

We report on 4x20 silicon photonic MEMS switch that is capable of multicasting. The switch is built on passive optical crossbar network with gap-adjustable directional couplers. The switch has high on-off extinction ratio (59 dB), low insertion loss (< 4.0 dB), small footprint (1.2x4.5 mm2), and fast response (9.8 µs). The switching voltage is 9.6 V and 20 dB bandwidth is 31.5 nm. One-to-two and one-to-four multicast operations are demonstrated.

Single-photon quantum router in the microwave regime utilizing double superconducting resonators with tunable coupling

We provide a proposal to achieve single-photon router in the microwave domain based on a waveguide quantum electrodynamics structure, which consists of a double-resonator side coupled to a superconducting transmission line. A superconducting quantum interference device is used to realize tunable and switchable coupling between the resonators; thereby the transport of a single photon in the transmission line can be effectively controlled. Our calculations show that the device has high on-off ratio and is experimentally feasible in currently available superconducting quantum circuits.