Large Current Ratio Research Articles

Oxygen-doped zirconium nitride based transparent resistive random access memory devices fabricated by radio frequency sputtering method

In this work, we present a feasibility of bipolar resistive switching (RS) characteristics for Oxygen-doped zirconium nitride (O-doped ZrNx) films, produced by sputtering method, which shows a high optical transmittance of approximately 78% in the visible region as well as near ultra-violet region. In addition, in a RS test, the device has a large current ratio of 5 × 103 in positive bias region and 5 × 105 in negative bias region. Then, to evaluate an ability of data storage for the proposed memory devices, we measured a retention time for 104 s at room temperature (RT) and 85 °C as well. As a result, the set and reset states were stably maintained with a current ratio of ∼102 at 85 °C to ∼103 at RT. This result means that the transparent memory by controlling the working pressure during sputtering process to deposit the ZrNx films could be a milestone for future see-through electronic devices.

Gate tunable WSe2-BP van der Waals heterojunction devices.

Due to the weak screening effect, the concentration and type of charge carriers in 2D semiconductor heterostructures can be effectively tuned by electrostatic gating, enabling us to realize different types of heterojunctions in a single device. Such 'type tunable' properties are useful for designing novel electrical or optoelectrical devices. Here, we demonstrate a 'type tunable' heterojunction device construct with two pieces of ambipolar 2D semiconductors: WSe2 and black phosphorus (BP). This heterojunction could be tuned to either the p-p junction or n-n junction by gate modulation. The p-p junction shows a large current rectification ratio while the n-n junction shows a negligible current rectification ratio, indicating a large valence band offset and a small conduction band offset at the WSe2/BP interface. In the optoelectrical measurements, we found the amplitude and even the polarity of photocurrent could be modulated by electrostatic gating. Our study could further enhance the understanding of designing devices based on these 'type tunable' van der Waals heterojunctions. Moreover, the properties of the WSe2/BP interface were also experimentally identified through the electrical and optoelectrical measurements in our study.

Water‐Induced Scandium Oxide Dielectric for Low‐Operating Voltage n‐ and p‐Type Metal‐Oxide Thin‐Film Transistors

Solution‐processed metal‐oxide thin films based on high dielectric constant (k) materials have been extensively studied for use in low‐cost and high‐performance thin‐film transistors (TFTs). Here, scandium oxide (ScOx) is fabricated as a TFT dielectric with excellent electrical properties using a novel water‐inducement method. The thin films are annealed at various temperatures and characterized by using X‐ray diffraction, atomic‐force microscopy, X‐ray photoelectron spectroscopy, optical spectroscopy, and a series of electrical measurements. The optimized ScOx thin film exhibits a low‐leakage current density of 0.2 nA cm−2 at 2 MV cm−1, a large areal capacitance of 460 nF cm−2 at 20 Hz and a permittivity of 12.1. To verify the possible applications of ScOx thin films as the gate dielectric in complementary metal oxide semiconductor (CMOS) electronics, they were integrated in both n‐type InZnO (IZO) and p‐type CuO TFTs for testing. The water‐induced full oxide IZO/ScOx TFTs exhibit an excellent performance, including a high electron mobility of 27.7 cm2 V−1 s−1, a large current ratio (Ion/Ioff) of 2.7 × 107 and high stability. Moreover, as far as we know it is the first time that solution‐processed p‐type oxide TFTs based on a high‐k dielectric are achieved. The as‐fabricated p‐type CuO/ScOx TFTs exhibit a large Ion/Ioff of around 105 and a hole mobility of 0.8 cm2 V−1 at an operating voltage of 3 V. To the best of our knowledge, these electrical parameters are among the highest performances for solution‐processed p‐type TFTs, which represents a great step towards the achievement of low‐cost, all‐oxide, and low‐power consumption CMOS logics.

Scaling Limit of Bilayer Phosphorene FETs

We investigate bilayer phosphorene field-effect transistors (FETs) by self-consistent atomistic quantum transport simulations. Despite a penalty in electrostatic control for multiple layers, 10-nm-channel bilayer phosphorene FETs can exhibit excellent device characteristics, such as I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> > 3 mA/μm, large current ratio (>10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> ), and small subthreshold swing (SS) of 66 mV/dec, with a double-gate device structure. While the scaling of gate dielectric monotonically enhances the overall performance of this device, channel length can only be scaled down to ~8 nm due to significant short-channel effects. We benchmark bilayer phosphorene FETs against bilayer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FETs along with a monolayer phosphorene device, which reveals that bilayer phosphorene FETs have favorable switching characteristics over other similar 2-D bilayer semiconductor devices, making both monolayer and bilayer phosphorene attractive for future switching applications. Our simulation results not only provide the performance and scaling limit of bilayer phosphorene FETs but also create irreplaceable insights into proper device design and parameter optimizations.

Sub-0.5 V Highly Stable Aqueous Salt Gated Metal Oxide Electronics.

Recently, growing interest in implantable bionics and biochemical sensors spurred the research for developing non-conventional electronics with excellent device characteristics at low operation voltages and prolonged device stability under physiological conditions. Herein, we report high-performance aqueous electrolyte-gated thin-film transistors using a sol-gel amorphous metal oxide semiconductor and aqueous electrolyte dielectrics based on small ionic salts. The proper selection of channel material (i.e., indium-gallium-zinc-oxide) and precautious passivation of non-channel areas enabled the development of simple but highly stable metal oxide transistors manifested by low operation voltages within 0.5 V, high transconductance of ~1.0 mS, large current on-off ratios over 107, and fast inverter responses up to several hundred hertz without device degradation even in physiologically-relevant ionic solutions. In conjunction with excellent transistor characteristics, investigation of the electrochemical nature of the metal oxide-electrolyte interface may contribute to the development of a viable bio-electronic platform directly interfacing with biological entities in vivo.

Electric field modulation of Schottky barrier height in graphene/MoSe2 van der Waals heterointerface

We demonstrate a vertical field-effect transistor based on a graphene/MoSe2 van der Waals (vdW) heterostructure. The vdW interface between the graphene and MoSe2 exhibits a Schottky barrier with an ideality factor of around 1.3, suggesting a high-quality interface. Owing to the low density of states in graphene, the position of the Fermi level in the graphene can be strongly modulated by an external electric field. Therefore, the Schottky barrier height at the graphene/MoSe2 vdW interface is also modulated. We demonstrate a large current ON-OFF ratio of 105. These results point to the potential high performance of the graphene/MoSe2 vdW heterostructure for electronics applications.

(Invited) Vertical Field Effect Transistor Based on Graphene/Transition Metal Dichalcogenide Van Der Waals Heterostructure

Owing to the absence of a band gap, the direct applicability of graphene for transistors and logic devices is still limited. Recent studies have shown that the van der Waals heterostructure of graphene and other two-dimensional (2D) materials can be a great candidate to overcome these issues. The layered structure of these materials exhibits no direct chemical bonding between the different layers. Thus the vdW junction between these materials is, in principle, free from lattice mismatch issues and interface reactions. With these advantages, these vdW heterostructures has been demonstrated vertical field effect transistor (FET) operations as well as large current ON-OFF ratio [1]. However, realization of a large ON-OFF ratio simultaneously with a large ON current density has been still challenging. Here we fabricated exfoliated-graphene/MoS2/Ti vertical heterostructure including vdW interface built from freshly cleaved surface of graphene and MoS2 by using mechanical exfoliation and dry transfer method (Fig. 1(a)). The vertical transport of the device measured under the modulation of back gate voltage V G exhibits large modulation of its conductance (Fig. 1(b)). We obtained large current ON-OFF ratio of 105 as well as large ON current density of 104 A/cm2 [2]. Such large current modulation is attributed to the modulation of the Schottky barrier at graphene/MoS2 interface as schematically depicted in the right panel of Fig. 1(b). At the V G = +50 V, the Fermi level of the graphene is highest; therefore the Schottky barrier height at graphene/MoS2 junction is lowest. The reduction of Schottky barrier height makes device to be low resistance ON state. On the contrary, at the V G= -50 V, the Fermi level of the graphene is lowest; therefore the Schottky barrier height is highest. Here current flow in the device is greatly suppressed and works as OFF state. The importance of Schottky-barrier modulation on the performance of the transistor could be confirmed by the plot between the current ON-OFF ratio ION/IOFF vs. modulation of barrier height Δφ=φ max-φ min; these data are obtained from the series of devices with different thickness of MoS2 (Fig. 1(c)); results reveal correlation between two values [3]. These results suggest that even larger current modulation can be possible if one could have larger modulation of Schottky barrier height. For this purpose, we compared MoS2, MoSe2, and α-MoTe2 as transition metal dichalcogenide (TMD) materials in graphene/TMD/Ti heterostructure; these materials have different electron affinity and thus provide systematic change of band alignment to graphene (Fig. 1(d)). We show that the transport in the graphene/TMD junction is sensitive to the band alignment [4]. The V G dependence of the current revealed systematic change from MoS2 to MoTe2 (Fig. 1(e)). We demonstrated large current ON-OFF ratio and ON current density on MoSe2-based heterostructure; the value is comparable to that of MoS2-based device. These comparisons suggest that further improvement may be possible by optimizing device structure. We believe that our fabricated exfoliated-graphene/TMD/metal vertical heterostructure reveal superior performance to other existing graphene-based vertical transistors and present an important advance toward electronics applications.

Effects of protons within SiO2 solid-state electrolyte on performances of oxide electric-double-layer thin film transistor

SiO2-based solid state electrolyte films are deposited at room temperature by using the plasma-enhanced chemical vapor deposition (PECVD) technique. An electric-double-layer (EDL) effect has been observed. Then, indium-zinc-oxide thin-film transistors (IZO TFTs) are fabricated by using such SiO2 films as dielectrics in a self-assembling process through a shadow mask. The IZO films for source/drain electrodes and channel are deposited on the nanogranular SiO2 film by RF sputtering the IZO target in an Ar ambient. Such TFTs exhibit a good performance at an ultralow operation voltage of 1.5 V, with a high field-effect mobility of 11.9 cm2/Vs, a small subthreshold swing of 94.5 mV/decade, and a large current on-off ratio of 7.14×106. Effects of protons in the SiO2-based solid state electrolyte films on the electrical performances of the IZO TFTs are also studied. It is observed that a big EDL capacitance can be obtained for SiO2 films dipped in pure water, as a result of the fact that there are more protons in such SiO2 films. Because of the migration of protons in SiO2 electrolytes, an anti-clockwise hysteresis is observed on the transfer curve. Moreover, a bigger hysteresis is observed at a higher gate voltage scan rate. Gate bias stressing stabilities are also studied the shifts in threshold voltage are observed to obey a stretched exponential function.

Novel field-effect Schottky barrier transistors based on graphene-MoS2 heterojunctions.

Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5–20 cm2/V·s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 105) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm2/V·s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides an opportunity to overcome the limitation of 60 mV/decade for conventional CMOS devices. The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.

Formation of SiGe nanocrystals embedded in Al2O3 for the application of write-once-read-many-times memory

The structure of SiGe nanocrystals embedded in Al2O3 formed by sequential deposition of Al2O3/Si/Ge/Al2O3 and a subsequent annealing was confirmed by transmission electron microscopy and energy dispersive spectroscopy (EDS), and its application for write-once-read-many-times (WORM) memory devices was explored in this study. By applying a −10 V pulse for 1 s, a large amount of holes injected from Si substrate are stored in the nanocrystals and consequently, the current at +1.5 V increases by a factor of 104 as compared to that of the initial state. Even with a smaller −5 V pulse for 1 μs, a sufficiently large current ratio of 36 can still be obtained, verifying the low power operation. Since holes are stored in nanocrystals which are isolated from Si substrate by Al2O3 with good integrity and correspond to a large valence band offset with respect to Al2O3, desirable read endurance up to 105 cycles and excellent retention over 100 yr are achieved. Combining these promising characteristics, WORM memory devices are appropriate for high-performance archival storage applications.

Intrinsic and extrinsic resistive switching in a planar diode based on silver oxide nanoparticles

Resistive switching is investigated in thin-film planar diodes using silver oxide nanoparticles capped in a polymer. The conduction channel is directly exposed to the ambient atmosphere. Two types of switching are observed. In air, the hysteresis loop in the current–voltage characteristics is S-shaped. The high conductance state is volatile and unreliable. The switching is mediated by moisture and electrochemistry. In vacuum, the hysteresis loops are symmetric, N-shaped and exhibit a negative differential resistance region. The conductance states are non-volatile with good data retention, programming cycling endurance and large current modulation ratio. The switching is attributed to electroforming of silver oxide clusters.

Design and simulation of double-lightly doped MOSCNT using non-equilibrium Green’s function

In this paper, we propose a new ohmic-structure of ballistic carbon nanotube field-effect transistors (CNTFETs) in which the source and drain regions are doped stepwise and the device acts as MOSFET like CNTFET (MOSCNT). The number of lightly doped regions and their doping concentrations are optimized to obtain the lowest OFF current. To study the device characteristics, the Poisson–Schrodinger equations are solved self-consistently using the Nonequilibrium Green’s Function (NEGF) formalism in the mode space approach. To find the Hamiltonian matrix, the tight-binding approximation with only pz orbital is used. The obtained results show that the stepwise regions lead to barrier widening due to the reduction in potential gradient. Therefore, the band-to-band tunneling (BTBT) and the ambipolar behavior of the device decrease due to band engineering. This causes to the superior reduction of OFF current and dissipative power. In addition, the device performance shows lower subthreshold swing (SS), smaller drain induced barrier lowering (DIBL), and larger current ratio than that of the previous structures.

Effect of non-lattice oxygen on ZrO2-based resistive switching memory

ZrO2-based resistive switching memory has attracted much attention according to its possible application in the next-generation nonvolatile memory. The Al/ZrO2/Pt resistive switching memory with bipolar resistive switching behavior is revealed in this work. The thickness of the ZrO2 film is only 20 nm. The device yield improved by the non-lattice oxygen existing in the ZrO2 film deposited at room temperature is firstly proposed. The stable resistive switching behavior and the long retention time with a large current ratio are also observed. Furthermore, it is demonstrated that the resistive switching mechanism agrees with the formation and rupture of a conductive filament in the ZrO2 film. In addition, the Al/ZrO2/Pt resistive switching memory is also possible for application in flexible electronic equipment because it can be fully fabricated at room temperature.

Pentacene-Based Thin Film Transistor with Inkjet-Printed Nanocomposite High-K Dielectrics

The nanocomposite gate insulating film of a pentacene-based thin film transistor was deposited by inkjet printing. In this study, utilizing the pearl miller to crumble the agglomerations and the dispersant to well stabilize the dispersion of nano-TiO2particles in the polymer matrix of the ink increases the dose concentration for pico-jetting, which could be as the gate dielectric film made by inkjet printing without the photography process. Finally, we realized top contact pentacene-TFTs and successfully accomplished the purpose of directly patternability and increase the performance of the device based on the nanocomposite by inkjet printing. These devices exhibited p-channel TFT characteristics with a high field-effect mobility (a saturation mobility of ̃0.58 cm2 V−1 s−1), a large current ratio (&gt;103) and a low operation voltage (&lt;6 V). Furthermore, we accorded the deposited mechanisms which caused the interface difference between of inkjet printing and spin coating. And we used XRD, SEM, Raman spectroscopy to help us analyze the transfer characteristics of pentacene films and the performance of OTFTs.

Fluctuation theorems for capacitively coupled electronic currents

The counting statistics of electron transport is theoretically studied in a system with two capacitively coupled parallel transport channels. Each channel is composed of a quantum dot connected by tunneling to two reservoirs. The nonequilibrium steady state of the system is controlled by two affinities or thermodynamic forces, each one determined by the two reservoirs of each channel. The status of a single-current fluctuation theorem is investigated starting from the fundamental two-current fluctuation theorem, which is a consequence of microreversibility. We show that the single-current fluctuation theorem holds in the limit of a large Coulomb repulsion between the two parallel quantum dots, as well as in the limit of a large current ratio between the parallel channels. In this latter limit, the symmetry relation of the single-current fluctuation theorem is satisfied with respect to an effective affinity that is much lower than the affinity determined by the reservoirs. This back-action effect is quantitatively characterized.

Fully depleted double-gate MSDRAM cell with additional nonvolatile functionality

We demonstrate a new fully depleted (FD) double-gate (DG) MSDRAM cell, which features SONOS type storage node at the back-gate (control-gate). This single-transistor cell, based on the meta-stable dip (MSD) hysteresis effect, can also be operated in non-volatile memory (NVM) mode. The NVM functionality is achieved by Fowler–Nordheim tunneling hole injection into the nitride storage node; the injected holes induce a permanent inversion layer in silicon body. The proposed device shows a large current ratio between ‘1’ and ‘0’ states (∼10 3) and a wide memory window (∼3 V). The effect of the NVM functionality on the MSD hysteresis was investigated and combined with the effect of the control-gate bias. The SONOS charging can be used for replacing the second gate (i.e. enabling single-gate MSDRAM) or for achieving ‘unified’ memory operation.

Recent Progress of Ferroelectric-Gate Field-Effect Transistors and Applications to Nonvolatile Logic and FeNAND Flash Memory

We have investigated ferroelectric-gate field-effect transistors (FeFETs) with Pt/SrBi2Ta2O9/(HfO2)x(Al2O3)1−x (Hf-Al-O) and Pt/SrBi2Ta2O9/HfO2 gate stacks. The fabricated FeFETs have excellent data retention characteristics: The drain current ratio between the on- and off-states of a FeFET was more than 2 × 106 after 12 days, and the decreasing rate of this ratio was so small that the extrapolated drain current ratio after 10 years is larger than 1 × 105. A fabricated self-aligned gate Pt/SrBi2Ta2O9/Hf-Al-O/Si FET revealed a sufficiently large drain current ratio of 2.4 × 105 after 33.5 day, which is 6.5 × 104 after 10 years by extrapolation. The developed FeFETs also revealed stable retention characteristics at an elevated temperature up to 120 °C and had small transistor threshold voltage (Vth) distribution. The Vth can be adjusted by controlling channel impurity densities for both n-channel and p-channel FeFETs. These performances are now suitable to integrated circuit application with nonvolatile functions. Fundamental properties for the applications to ferroelectric-CMOS nonvolatile logic-circuits and to ferroelectric-NAND flash memories are demonstrated.

The I-V Characteristics of the 3,3',5',5-Tetra-Tert-Butyl-Azobenzene Optical Molecular Switch: A First-Principles Study

By applying nonequilibrium Green’s function formalism combined first-principles density functional theory, we investigate the electronic transport properties of 3,3′,5,5′-Tetra-tert-butyl-azobenzene(meta-TBA) optical molecular switch. This molecular switch comprises a meta-TBA molecule with the trans and cis forms, which can be reversed from one structure to another one upon photoexcitation. The influence of HOMO-LUMO gaps and the spatial distributions of molecular orbitals on the electronic transport through the molecular device are discussed in detail. Theoretical results show that there is a large current ratio in bias window, which suggests that this system can be one of good candidates for optical switches due to this unique advantage, and have real applications in the molecular circuit.

Investigation on −c-InN and a-InN:Mg field effect transistors under electrolyte gate bias

The electrical properties of N-polar undoped InN and nonpolar a-InN:Mg ion sensitive field effect transistors (ISFETs) have been investigated by electrolyte-gate-biased current-voltage (IDS-VGS) measurements. IDS-VGS characteristics reveal that the a-InN:Mg ISFETs have a large (∼52%) current variation ratio at a gate bias of 0.3 V with respect to the unbiased one, which is higher than that from the undoped InN ISFETs (∼18% and &amp;lt;0.1% for 10-nm and 1-μm-thick −c-InN epilayers, respectively). The a-InN:Mg ISFETs can also function as a pH sensor with a sensitivity of 56.5 mV/pH and a response time less than 10 s.

Advanced concepts for electromagnetic launcher power supplies incorporating magnetic flux compression

Advanced concepts of high-energy power supplies for coil launchers are presented. These concepts are designed to produce high inductive compressive ratios and large current and magnetic field multiplication ratios in the range of megamperes of current and gigawatts of active power. As a consequence of the flexibility of multiwinding rotating generators, such designs provide an extensive range of output pulse shaping in single or multiple pulses, enabling optimum operation of the coil launcher. The interaction of different stationary and rotating electrical windings in strong magnetic fields with feedback generated amplification and nonuniform compensation of the armature reaction is the key to providing a large and flexible spectrum of tailored output pulses, eliminating the need for switching and other large external electromagnetic pulse-forming components. Dynamic interactions between the internal impedance of these generators and the induced electromotive forces in various windings, as well as the role of the external passive circuit components introduced in the launcher circuit (such as capacitors and inductors), are discussed and numerically evaluated. Finally, an adaptive finite-element method numerical code is described. This code takes into account the relative motion and is designed to evaluate machines incorporating flux compression. >