Excellent Switching Characteristics Research Articles

Ultra-high aspect ratio poly-Si FinFET using an improved spacer formation technique

An improved spacer formation technique was proposed and developed to fabricate poly-Si fin field-effect transistors (FinFETs) with an ultra-high aspect ratio. The as-demonstrated FinFETs have a fin channel with a width and height of 22 nm and 230 nm, respectively, corresponding to an aspect ratio of 10.5. The electrical and temperature properties of the FinFETs are described in detail in this paper. The poly-Si FinFETs exhibit a steep subthreshold swing (196 mV/dec), a low leakage current (∼10−14 A), a high on/off current ratio (2.2 × 107 at VDS = 0.1 V), and a low drain-induced barrier lowering effect (0.28 V). The excellent switching characteristics are attributed to the ultrathin channel body and the multi-gate structure combined with high-k Al2O3 dielectric. Furthermore, the electron field-effective mobility increases as the temperature increases. An analytical fitting model was derived and was utilized to account for this phenomenon. The fitting results indicate that the positive temperature coefficient originates from the grain boundary-controlled mechanism in the low gate voltage regime.

Organic bistable memory devices based on MoO3 nanoparticle embedded Alq3 structures

Organic bistable memory devices were fabricated by embedding a thin layer of molybdenum trioxide (MoO3) between two tris-(8-hydroxyquinoline)aluminum (Alq3) layers. The device exhibited excellent switching characteristics with an ON/OFF current ratio of 1.15 × 103 at a read voltage of 1 V. The device showed repeatable write–erase capability and good stability in both the conductance states. These conductance states are non-volatile in nature and can be obtained by applying appropriate voltage pulses. The effect of MoO3 layer thickness and its location in the Alq3 matrix on characteristics of the memory device was investigated. The field emission scanning electron microscopy (FE-SEM) images of the MoO3 layer revealed the presence of isolated nanoparticles. Based on the experimental results, a mechanism has been proposed for explaining the conductance switching of fabricated devices.

High-Performance Solution-Processed Zinc–Tin-Oxide Thin-Film Transistors Employing Ferroelectric Copolymers Fabricated at Low Temperature for Transparent Flexible Displays

The use of the charge-screening method to obtain high-performance solution-processed zinc–tin-oxide (ZTO) thin-film transistors (TFTs) for transparent flexible displays is investigated. The proposed ZTO TFTs employing solution-processed ferroelectric, poly (vinylidenefluoride-co- trifluoroethylene) (PVDF-TrFE) copolymer exhibit excellent switching characteristics, including a shift in the threshold voltage toward 0 V and a decrease in the OFF-state current and the sub-threshold swing. The long-term stability of ZTO/PVDF-TrFE TFTs against electrical bias combined with visible-light illumination at 60 °C is also demonstrated. The proposed method is based on an all-solution-process that was carried out below 100°C except the metal deposition, and this allows the ZTO TFT matrix to be integrated into transparent display backplanes on flexible substrates.

MoS2 transistors with 1-nanometer gate lengths.

Scaling of silicon (Si) transistors is predicted to fail below 5-nanometer (nm) gate lengths because of severe short channel effects. As an alternative to Si, certain layered semiconductors are attractive for their atomically uniform thickness down to a monolayer, lower dielectric constants, larger band gaps, and heavier carrier effective mass. Here, we demonstrate molybdenum disulfide (MoS2) transistors with a 1-nm physical gate length using a single-walled carbon nanotube as the gate electrode. These ultrashort devices exhibit excellent switching characteristics with near ideal subthreshold swing of ~65 millivolts per decade and an On/Off current ratio of ~106 Simulations show an effective channel length of ~3.9 nm in the Off state and ~1 nm in the On state.

Solution-processed laminated ZrO2/Al2O3 dielectric for low-voltage indium zinc oxide thin-film transistors

We fabricated a solution-processed laminated dielectric and investigated its structural, optical, and electrical properties. The laminated ZrO2 (Z) and Al2O3 (A) dielectric effectively blocked the leakage current density (J leak) and showed a high breakdown voltage. In particular, the AZA laminated dielectric showed a lower J leak and a higher breakdown voltage than the ZAZ dielectric, because of the large band gap and minimal defects in the Al2O3 film. Finally, we demonstrated the low-voltage indium zinc oxide thin-film transistor (less than 3 V) on laminated dielectric, which displayed excellent switching characteristics.

Graphene Oxide and ZnO/Al:ZnO Based Transparent Resistive Switching Devices for Memristor Applications

Graphene Oxide (GO) based low cost flexible electronics and memory cell have recently attracted more attention for the fabrication of emerging electronic devices. As a suitable candidate for resistive random access memory technology, reduced GO (RGO) can be widely used for non-volatile switching memory applications because of its large surface area, excellent scalability, retention, and endurance properties. We demonstrated that the fabricated metal/RGO/metal memory device exhibited excellent switching characteristics, with on/off ratio of two orders of magnitude and operated threshold switching voltage of less than 1 V. We have also developed a high-temperature precursor vapor mask technique using Al2O3 to pattern the atomic layer deposition of ZnO and Al:ZnO layers on ZnO-based substrates. This technique was used to create a transparent memristor device based on Al:ZnO thin films having metallic and semiconducting and insulating transport properties ZnO. We demonstrated that adding combination of Al2O3 and TiO2 barrier layers improved the resistive switching behavior. The change in the resistance between the high and low-resistivity states of the memristor with a combination of Al2O3 and TiO2 was approximately 157 %.

High-gain complementary metal-oxide-semiconductor inverter based on multi-layer WSe2 field effect transistors without doping

A high-gain complementary metal-oxide-semiconductor (CMOS) logic inverter was implemented by fabricating p- and n-type field effect transistors (FETs) based on multi-layer WSe2 on the same wafer. Au as a high work-function metal is contacted to WSe2 for the source/drain of the p-type FET. The n-type FET has an Al electrode contacted to WSe2 for the source/drain. Both FETs were designed to have similar on-current densities (>10−7 A μm−1) and high on/off current ratios (>106). The inverter shows excellent switching characteristics including relatively high voltage gains (>25) and high noise margins (>0.9) in the range of supply voltage from 2 V to 8 V. This work has a great significance in the realization of a CMOS logic gate based on WSe2 without an additional doping scheme.

Thickness Considerations of Two-Dimensional Layered Semiconductors for Transistor Applications.

Layered two-dimensional semiconductors have attracted tremendous attention owing to their demonstrated excellent transistor switching characteristics with a large ratio of on-state to off-state current, Ion/Ioff. However, the depletion-mode nature of the transistors sets a limit on the thickness of the layered semiconductor films primarily determined by a given Ion/Ioff as an acceptable specification. Identifying the optimum thickness range is of significance for material synthesis and device fabrication. Here, we systematically investigate the thickness-dependent switching behavior of transistors with a wide thickness range of multilayer-MoS2 films. A difference in Ion/Ioff by several orders of magnitude is observed when the film thickness, t, approaches a critical depletion width. The decrease in Ion/Ioff is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm. For t larger than 100 nm, Ion/Ioff approaches unity. Simulation using technical computer-aided tools established for silicon technology faithfully reproduces the experimentally determined scaling behavior of Ion/Ioff with t. This excellent agreement confirms that multilayer-MoS2 films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate Ion/Ioff. Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors.

Fabricating and investigating properties of memristors on flexible substrate (PET)

In this study, we have fabricated resistive switching memory device based on TiO2 thin films on ITO/PET commercial substrate. The study on the transmittance spectra showed that with the 100 nm-TiO2/ITO/PET device, the transmission is more than 80 % in the visible region and approximately of 85 % at wavelength of 550 nm. Ag/TiO2/ITO devices exhibit excellent stable bipolar resistive switching characteristics under electrical field bias of -2 V -> 2 V range on the flat state and even after reproductive physical stresses of 500 cycles. Our study on TiO2 based memristors suggests that resistive switching memories are suitable for flexible transparent application in the future.

Resistive switching behavior of reduced graphene oxide memory cells for low power nonvolatile device application.

Graphene Oxide (GO) based low cost flexible electronics and memory cell have recently attracted more attention for the fabrication of emerging electronic devices. As a suitable candidate for resistive random access memory technology, reduced graphene oxide (RGO) can be widely used for non-volatile switching memory applications because of its large surface area, excellent scalability, retention, and endurance properties. We demonstrated that the fabricated metal/RGO/metal memory device exhibited excellent switching characteristics, with on/off ratio of two orders of magnitude and operated threshold switching voltage of less than 1 V. The studies on different cell diameter, thickness, scan voltages and period of time corroborate the reliability of the device as resistive random access memory. The microscopic origin of switching operation is governed by the establishment of conducting filaments due to the interface amorphous layer rupturing and the movement of oxygen in the GO layer. This interesting experimental finding indicates that device made up of thermally reduced GO shows more reliability for its use in next generation electronics devices.

Normally-Off Operation of AlGaN/GaN Heterojunction Field-Effect Transistor with Clamping Diode

This paper reports a new method to enable the normally-off operation of AlGaN/GaN heterojunction field-effect transistors (HFETs). A capacitor was connected to the gate input node of a normally-on AlGaN/GaN HFET with a Schottky gate where the Schottky gate acted as a clamping diode. The combination of the capacitor and Schottky gate functioned as a clamp circuit to downshift the input signal to enable the normally-off operation. The normally-off operation with a virtual threshold voltage of 5.3 V was successfully demonstrated with excellent dynamic switching characteristics.

Epitaxial Brownmillerite Oxide Thin Films for Reliable Switching Memory.

Resistive switching memory, which is mostly based on polycrystalline thin films, suffers from wide distributions in switching parameters-including set voltage, reset voltage, and resistance-in their low- and high-resistance states. One of the most commonly used methods to overcome this limitation is to introduce inhomogeneity. By contrast, in this paper, we obtained uniform resistive switching parameters and sufficiently low forming voltage by maximizing the uniformity of an epitaxial thin film. To achieve this result, we deposited an SrFeOx/SrRuO3 heteroepitaxial structure onto an SrTiO3 (001) substrate by pulsed laser deposition, and then we deposited an Au top electrode by electron-beam evaporation. This device exhibited excellent bipolar resistance switching characteristics, including a high on/off ratio, narrow distribution of key switching parameters, and long data retention time. We interpret these phenomena in terms of a local, reversible phase transformation in the SrFeOx film between brownmillerite and perovskite structures. Using the brownmillerite structure and atomically uniform thickness of the heteroepitaxial SrFeOx thin film, we overcame two major hurdles in the development of resistive random-access memory devices: high forming voltage and broad distributions of switching parameters.

Photocatalytic and photo electrochemical properties of cadmium zinc sulfide solid solution in the presence of Pt and RuS2 dual co-catalysts

A new combination of co-catalysts and photocatalyst, Pt-RuS2-Cd0.5Zn0.5S, is found to exhibit enhanced photocatalytic activity for hydrogen generation from water. The presence of dual co-catalysts not only improves the photocatalytic activity but also increases the stability of the CdZnS photocatalyst. Increased visible light absorption and a decrease in bandgap occur for Cd0.5Zn0.5S in the presence of the co-catalysts. All samples exist as nanocrystalline materials with a particle size in the range of 20–50nm. In this system, Ru is in 4+ oxidation state and Pt is in metallic state. Photoluminescence studies suggest the presence of a number of defect levels/surface states in these catalysts, whose energy vary with the composition of the catalyst. The enhanced photocatalytic activity of Pt-RuS2-Cd0.5Zn0.5S is attributed to the better separation of photogenerated charge carriers assisted by the dual co-catalysts. Another remarkable property exhibited by this system is the increased stability for repeated use, which can be attributed to the effective transfer of photogenerated holes from Cd0.5Zn0.5S to RuS2, which minimizes the photo corrosion of the catalyst. Photoelectrochemical results show a significant increase in the photocurrent with applied voltage and excellent switching characteristics for both Pt-RuS2-Cd0.5Zn0.5S and Cd0.5Zn0.5S. No discernible improvement in photocurrent is seen for Pt-RuS2-Cd0.5Zn0.5S compared to Cd0.5Zn0.5S. The difference in the photocatalytic and photo electrochemical property is explained based on the inherent differences of the two phenomena.

Graphene quantum dots as a highly efficient solution-processed charge trapping medium for organic nano-floating gate memory

A highly efficient solution-processible charge trapping medium is a prerequisite to developing high-performance organic nano-floating gate memory (NFGM) devices. Although several candidates for the charge trapping layer have been proposed for organic memory, a method for significantly increasing the density of stored charges in nanoscale layers remains a considerable challenge. Here, solution-processible graphene quantum dots (GQDs) were prepared by a modified thermal plasma jet method; the GQDs were mostly composed of carbon without any serious oxidation, which was confirmed by x-ray photoelectron spectroscopy. These GQDs have multiple energy levels because of their size distribution, and they can be effectively utilized as charge trapping media for organic NFGM applications. The NFGM device exhibited excellent reversible switching characteristics, with an on/off current ratio greater than 106, a stable retention time of 104 s and reliable cycling endurance over 100 cycles. In particular, we estimated that the GQDs layer trapped ∼7.2 × 1012 cm−2 charges per unit area, which is a much higher density than those of other solution-processible nanomaterials, suggesting that the GQDs layer holds promise as a highly efficient nanoscale charge trapping material.

Multilevel characteristics for bipolar resistive random access memory based on hafnium doped SiO2 switching layer

We fabricated TiN/Hf:SiO2/Pt memory cell with the small size of 1×1um2 by lithography and sputtering technology, which demonstrated excellent bipolar resistive switching (RS) characteristics. The device presents good endurance and outstanding uniformity. The coefficient of variation of Vset, Vreset, Ron and Roff were found to be 5.05%, 4.78%, 4.18%, and 15.78%, respectively. For the device with hafnium doped SiO2 switching layer, multilevel storage capability can be successfully obtained by varying either the stop voltage or the compliance current in the SET process. In addition, the impact of forming current on the RS properties was studied. We found that the ratio of On/Off current for the device increased with the decrease of the forming current, which would be beneficial for the design of low power device. Possible RS mechanisms aiming to explain the impact of forming current on the RS characteristics and multilevel storage were also deduced.

CdSe quantum dot/AlOx based non-volatile resistive memory

We present an all-solution processed bipolar non-volatile resistive memory device with CdSe quantum dot/metal–metal oxide/quantum dot structure. The two terminal device exhibits excellent switching characteristics with ON/OFF ratio >104. The device maintained its state even after removal of the bias voltage. The switching time is around 14 ns. Device did not show degradation after 4000 s retention test. The memory functionality was consistent even after multiple cycles of operation (100,000) and the device is reproducible. The switching mechanism is discussed on the basis of charge trapping in quantum dots with metal oxide serving as the barrier. The mechanism is supported by observation of variation in capacitance–frequency measurements.

Organic Bistable Switching Memory Devices with MeH-PPV and Graphene Oxide Composite

We have reported about bipolar resistive switching effect on Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]:Graphene oxide composite films, which are sandwiched between aluminum and indium tin oxide electrodes. In this case, I-V sweep curve showed a hysteretic behavior, which varied according to the polarity of the applied voltage bias. The device exhibited excellent switching characteristics, with the ON/OFF ratio being approximately two orders in magnitude. The device had good endurance (105 cycles without degradation) and long retention time (5 × 103 s) at room temperature. The bistable switching behavior varied according to the trapping and de-trapping of charges on GO sites; the carrier transport was described using the space-charge-limited current (SCLC) model.

Metal-Semiconductor Field-Effect Transistors With In–Ga–Zn–O Channel Grown by Nonvacuum-Processed Mist Chemical Vapor Deposition

In-Ga-Zn-O (IGZO) thin films (TFs) were grown by cost-effective nonvacuum solution-processed mist chemical vapor deposition. High quality AgO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Schottky contacts (SCs) were fabricated on these IGZO TFs with rectification ratios and barrier heights as high as 7.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> and 1 eV, respectively, combined with ideality factors as low as 1.32. These SCs were subsequently used as gate contacts in the production of metal-semiconductor field-effect transistors (MESFETs) with excellent switching and stability characteristics. For example, typical (W/L 785 μm/5 μm) MESFETs were capable of providing ON-currents up to 245 μA, combined with a large ON/OFF ratio of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> . A mobility of 3.2 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /(V.s) and a low subthreshold swing of 356 mV/decade were achieved in the W/L 524 μm/10 μm transistors. Under positive bias stress, these MESFETs were highly stable, demonstrating the feasibility of using a combination of mist chemical vapor deposition grown IGZO and AgO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> SCs to produce stable, low power consumption, and low-cost switching devices.

Metal–insulator transition properties of sputtered silicon-doped and un-doped vanadium dioxide films at terahertz range

Silicon-doped and un-doped vanadium dioxide (VO2) films were synthesized on high-purity single-crystal silicon substrates by means of reactive direct current magnetron sputtering followed by thermal annealing. The structure, morphology and metal–insulator transition properties of silicon-doped VO2 films at terahertz range were measured and compared to those of un-doped VO2 films. X-ray diffraction and scanning electron microscopy indicated that doping the films with silicon significantly affects the preferred crystallographic orientation and surface morphologies (grain size, pores and characteristics of grain boundaries). The temperature dependence of terahertz transmission shows that the transition temperature, hysteresis width and transition sharpness greatly depend on the silicon contents while the transition amplitude was relatively insensitive to the silicon contents. Interestingly, the VO2 film doped with a silicon content of 4.6at.% shows excellent terahertz switching characteristics, namely a small hysteresis width of 4.5°C, a giant transmission modulation ratio of about 82% and a relatively low transition temperature of 56.1°C upon heating. This work experimentally indicates that silicon doping can effectively control not only the surface morphology but also the metal–insulator transition characteristics of VO2 films at terahertz range.

Flexible organic phototransistors based on a combination of printing methods

Highly photosensitive organic phototransistors (OPTs) are successfully demonstrated on a flexible substrate using all-solution process as well as a combination of printing methods which consist of roll-to-plate reverse offset printing (ROP), inkjet printing and bar coating. Excellent electrical switching characteristics are obtained from heterogeneous interfacial properties of the reverse-offset-printed silver nanoparticle electrode and the inkjet-printed p-channel polymeric semiconductor. In particular, the OPTs exhibit remarkably photosensitivity with a photo-to-dark current ratio exceeding 5 orders. This optoelectronic properties of the combinational printed OPTs are theoretically and experimentally studied, and found the comparable tendency. In addition, excellent mechanical stability is observed with up to 0.5% of strain applied to the OPTs. Hence, by manufactured with a combination of various graphic art printing methods such as roll-to-plate ROP, inkjet printing, and bar coating, these devices are very promising candidates for large-area and low-cost printed and flexible optoelectronics applications.