Source-drain Electrodes Research Articles

Improved Performance of Pentacene Organic Field-Effect Transistors by Inserting a V2O5 Metal Oxide Layer

We fabricate pentacene-based organic field effect transistors (OFETs), inserting a transition metal oxide (V2O5) layer between the pentacene and Al source-drain (S/D) electrodes. The performance of the devices with V2O5/Al S/D electrodes is considerably improved compared to the pentacene-based OFET with only Al S/D electrodes. After the 10-nm V2O5 layer modification, the effective field-effect mobility of the devices increases from 2.7 × 10−3 cm2 /V·s to 8.93× 10−1 cm2/V·s. Owing to the change of the injection property, the effective threshold voltage (Vth) is changed from −7.5 V to −5 V and the on/off ratio shifts from 102 to 104. Moreover, the dispersion of sub-threshold current in the devices disappears. These performance improvements are ascribed to the low carrier injection barrier and the reduction of contact resistance. It is indicated that V2O5 layer modification is an effective approach to improve pentacene-based OFET performance.

Organic thin-film transistor memory with Ag floating-gate

Organic thin-film transistor memories were realized by inserting a floating-gate layer in the Nylon 6 gate dielectrics. The transistors presented significant hysteresis behaviors and memory effect. The performance of the transistor memories, such as the memory window and the retention time, was improved greatly by using the separated silver nanoparticles instead of the silver film as the floating-gate. After the ITO source-drain electrode of the transistors treated by the oxygen plasma, the performance was further improved. The operation mechanism of the presented transistor memories was also provided and discussed.

Laser printing of a semiconducting oligomer as active layer in organic thin film transistors: Impact of a protecting triazene layer

Organic thin-film transistor (OTFT) devices were achieved using the laser-induced forward transfer technique. As p-type organic semiconductor, distyryl-quaterthiophene (DS4T) was vacuum-deposited on a donor substrate and transferred with picosecond laser pulses on Si/SiO2-based receiver substrates to form an organic active layer. To avoid laser damage of the organic thin film, a UV-sensitive aryltriazene polymer as a sacrificial layer was used. The polymer layer, deposited on the donor substrate prior to the organic layer deposition, has high absorption at the laser wavelength and does not contaminate the printed pixels. The DS4T pixels printed on receiver substrates have well defined morphological properties as shown by atomic force microscopy and scanning electronic microscopy. OTFT devices were characterized in top- and bottom-contact configurations using thermally evaporated gold lines as source-drain electrodes. DS4T pixels printed as active layer for charge transport not only issue mobility values comparable to DS4T layers prepared by vacuum evaporation but also a relative electrical stability over time.

Fully Gravure Printed Half Adder on Plastic Foils

All-printed half adders will be the first step to the way of printing an arithmetic logic unit which will be further expanded to printing microprocessors directly onto flexible plastic foils. In this letter, the half-adder circuit has been constructed using an all gravure printing process on poly(ethylene terephtalate) foils. To successfully operate the printed half adder, we first simulate the half adder using the parameters extracted from gravure-printed single-walled carbon nanotube (SWNT)-based thin-film transistors (TFTs) to provide a tolerable range of fluctuations of electrical parameters of the gravure-printed SWNT-based TFTs. Based on the close comparison between simulation results and attained electrical parameters of printed TFTs, controlling waviness of printed drain-source electrodes has been found to be a key factor for successfully executing the function of a printed half adder on the plastic foils.

Aligned Circular-Type Nanowire Transistors Grown on Multilayer Graphene Film

High-performance transistors using semiconducting nanowires are very promising devices for flexible or transparent electronic applications. However, nanowire-based transistors inevitably have placement and alignment issues, which make them troublesome for real-world applications and can be limiting factors in novel applications. Here we present a novel device structure that can be easily adapted for producing high-yield nanowire transistors. We fabricated fully transparent circular tin oxide (SnO(2)) nanowire transistors employing multilayer graphene films (MGFs) as a seed electrode and aligned nanowires as a semiconductor channel. The nanowires were grown directly on MGFs without metal catalysts through a vapor-solid (VS) mechanism. On the basis of these properties, aligned SnO(2) nanowires were grown only on the exposed MGF using patterned MGF/SiO(2) structures. Regardless of the growth direction of the nanowires centered on the MGF, the as-grown nanowires can play the role of transistor channels because of the circular shape of the gate and the source-drain electrodes. Consequently, the yield rate of circular nanowire transistor structure was around two times as high as that of existing linear nanowire transistor structure.

Molecular doping effect in bottom-gate, bottom-contact pentacene thin-film transistors

A bottom-gate, bottom-contact (BGBC) organic thin-film transistor (OTFT) with carrier-doped regions over source-drain electrodes was investigated. Device simulation with our originally developed device simulator demonstrates that heavily doped layers (p+ layers) on top of the source-drain contact region can compensate the deficiency of charge carriers at the source-channel interface during transistor operation, leading to the increase of the drain current and the apparent field-effect mobility. The phenomena expected with the device simulation were experimentally confirmed in typical BGBC pentacene thin-film transistors. The 5-nm-thick p+ layers, located 10 nm (or 20 nm) over the source-drain electrodes, were prepared by coevaporation of pentacene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an acceptor dopant. Since the molecular doping in this study can increase the drain current without positive shift of threshold voltage, p+ layers were formed precisely on top of the source-drain regions. This study shows that common inferior characteristics of bottom-contact OTFT devices mainly derive from the supply shortage of charge carriers to the channel region. The importance of reliable molecular doping techniques or heavily doped semiconductor materials for improving OTFT device performance is clearly suggested.

Interface diffusion characteristics of Al–2 at.%Nd/n + a-Si:H and Al–2 at.%Nd/n + poly-Si bilayers

In order to use Al–2 at.%Nd as the source-drain electrode of hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs) without diffusion barrier, the diffusion characteristics of Al–2 at.%Nd into phosphorus-doped (n+) a-Si:H were studied, and diffusion between Al–2 at.%Nd and phosphorous-doped poly-Si (n + poly-Si) was also investigated for comparison. The electric resistance variation of Al–2 at.%Nd, n + a-Si:H, and n + poly-Si was measured by four-point probe method at every annealing step, and each surface was inspected by optical microscope. Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy were used to analyze atomic diffusion progress with the variation of annealing temperature. Through these analyses, we ascertain that the Nd element of Al–2 at.%Nd hinders the diffusion between Al and Si, and n + a-Si:H is stable up to 300 °C and n + poly-Si is stable up to 400 °C against the diffusion of Al–2 at.%Nd. Thus Al–2 at.%Nd can be utilized as the source-drain electrode of a-Si:H TFTs below 300 °C and poly-Si TFTs below 400 °C without diffusion barrier.

High Performance Organic Thin Film Transistors with Solution Processed TTF-TCNQ Charge Transfer Salt as Electrodes

Fabrication of high-performance organic thin film transistors (OTFTs) with solution processed organic charge transfer complex (TTF-TCNQ) film as bottom contact source-drain electrodes is reported. A novel capillary based method was used to deposit the source-drain electrodes from solution and to create the channel between the electrodes. Both p- and n-type OTFTs have been fabricated with solution deposited organic charge transfer film as contact electrodes. Comparison of the device performances between OTFTs with TTF-TCNQ as source-drain electrodes and those with Au electrodes (both top and bottom contact) indicate that better results have been obtained in organic complex film contacted OTFT. The high mobility, low threshold voltage, and efficient carrier injection in both types of OTFTs implies the potential use of the TTF-TCNQ based complex material as low-cost contact electrodes. The lower work function of the TTF-TCNQ electrode and better contact of the complex film with the organic thin film owing to the organic-organic interface results in efficient charge transfer into the semiconductor yielding high device performance. The present method having organic metal as contact materials promises great potential for the fabrication of all-organics and plastic electronics devices with high throughput and low-cost processing.

Selective Contact Anneal Effects on Indium Oxide Nanowire Transistors using Femtosecond Laser

Nanowire materials have gained great interest as promising candidates for high-performance logic devices to sustain the progress in device scaling. However, little research has been conducted to investigate the role of contacts on the device performance accompanied by an appropriate physical model in nanodevices, although effects of the contacts will prevail as the channel scales. In this study, we investigate the effect of annealing using a femtosecond-laser focused at the contact region between the source-drain electrodes and the nanowire. On the basis of the direct comparison of device characteristics before and after annealing, a contact model is introduced, which could be generally applicable to nanowire transistors with overlap between gate region and source-drain regions. Low-frequency noise measurements in the devices reveal that the Id2 normalized noise spectrum and Hooge’s constant are reduced following laser annealing.

Effect of ammonia (NH 3) plasma treatment on silicon nitride (SiN x) gate dielectric for organic thin film transistor with soluble organic semiconductor

The effect of an ammonia plasma treatment on silicon nitride (SiN x) gate dielectrics for organic thin film transistors (OTFTs) with soluble organic semiconductors (OSCs) was studied. Our OTFTs were fabricated by using the soluble derivatives of Poly(bi-thiophene) as the p-type polymer semiconductor material and SiN x thin films with nitrogen oxygen (N 2O) and ammonia (NH 3) plasma treatments as the gate dielectric. To improve the performance and stability of the OTFT devices, we studied the basic mechanism of the plasma treatment on the SiN x gate dielectric and the gold (Au) electrode surface. The SiN x-OTFTs with a NH 3 plasma treatment yielded the improved results in terms of a higher field-effect mobility ( μ fe) of 0.06 cm 2 V −1 S −1 with a lower interface charge trap density of 4.45 × 10 −11 (cm 2 eV) −1 and a lower contact resistance of 0.384 MΩ-cm. The repulsive force of the soluble OSC solvent was reduced at the edge of the source-drain (S-D) electrode due to the difference between the surface energies of the channel region and Au S-D electrodes.

Top-Gated Chemical Vapor Deposition Grown Graphene Transistors with Current Saturation

Graphene transistors are of considerable interest for radio frequency (rf) applications. In general, transistors with large transconductance and drain current saturation are desirable for rf performance, which is however nontrivial to achieve in graphene transistors. Here we report high-performance top-gated graphene transistors based on chemical vapor deposition (CVD) grown graphene with large transconductance and drain current saturation. The graphene transistors were fabricated with evaporated high dielectric constant material (HfO(2)) as the top-gate dielectrics. Length scaling studies of the transistors with channel length from 5.6 μm to 100 nm show that complete current saturation can be achieved in 5.6 μm devices and the saturation characteristics degrade as the channel length shrinks down to the 100-300 nm regime. The drain current saturation was primarily attributed to drain bias induced shift of the Dirac points. With the selective deposition of HfO(2) gate dielectrics, we have further demonstrated a simple scheme to realize a 300 nm channel length graphene transistors with self-aligned source-drain electrodes to achieve the highest transconductance of 250 μS/μm reported in CVD graphene to date.

Organic Electrodes Consisting of Dianthratetrathiafulvalene and Fullerene and Their Application in Organic Field Effect Transistors

A double layer of dianthratetrathiafulvalene (DATTF) and fullerene (C60) on an n++-Si wafer pretreated with n-octyltrichlorosilane exhibited a high electrical conductivity of σ= 0.12 S/cm and was used as source–drain electrodes in organic field effect transistors (OFETs). A simplified OFET device architecture composed of an organic semiconducting active layer and an organic electrode layer was easily fabricated by successive vacuum deposition of organic donor and acceptor layers. It was confirmed that this device configuration is applicable for both p- and n-type FET operation.

Multicomponent oxide thin-film transistors fabricated by a double-layer inkjet printing process

In this work we report on the fabrication and characterization of multicomponent metal oxide thin-film transistors with a double-layer inkjet printing process. Both the active area and source–drain electrodes of the devices are printed with inks based on metal salt precursors to form Ga 2O 3–In 2O 3–ZnO and In 2O 3–SnO respectively. Electrical characterization has shown that the devices' performance, apart from the active area composition, can also be affected by the printing drop spacing. In general, devices printed with Ga:In:Zn 2:4:1 composition present the highest field effect mobility (~ 1.75–3 cm 2 V −1 s −1). More stable devices with improved switching, but with a compromise over field effect mobility (~ 0.5–0.9 cm 2 V −1 s −1) were obtained for the 2:4:2 composition.

Organic nanofibers integrated by transfer technique in field-effect transistor devices

The electrical properties of self-assembled organic crystalline nanofibers are studied by integrating these on field-effect transistor platforms using both top and bottom contact configurations. In the staggered geometries, where the nanofibers are sandwiched between the gate and the source-drain electrodes, a better electrical conduction is observed when compared to the coplanar geometry where the nanofibers are placed over the gate and the source-drain electrodes. Qualitatively different output characteristics were observed for top and bottom contact devices reflecting the significantly different contact resistances. Bottom contact devices are dominated by contact effects, while the top contact device characteristics are determined by the nanofiber bulk properties. It is found that the contact resistance is lower for crystalline nanofibers when compared to amorphous thin films. These results shed light on the charge injection and transport properties for such organic nanostructures and thus constitute a significant step forward toward a nanofiber-based light-emitting device.

An ambipolar to n-type transformation in pentacene-based organic field-effect transistors

We report an ambipolar to n-type transformation in pentacene-based organic field-effect transistors (OFETs) of aluminum (Al) source–drain electrodes. The hole currents of the ambipolar OFETs were decreased by over two orders of magnitude within 24h after the device fabrication, but the electron currents remained unchanged, which makes the devices present n-type only characteristics. The transformation is correlated with the interfacial interactions between Al electrodes and pentacene, as characterized by analyzing near-edge X-ray absorption fine structure spectra and Micro-Raman spectra. This study highlights the importance of metal/organic interfaces in the performance of OFETs, and demonstrates decent n-type pentacene-based OFETs with electron mobility of 0.1cm2/V/s with proper annealing treatment.

Design and Fabrication of All Organic Field Effect Transistor

All organic field effect transistor consist of poly-3-hexyloxythiophene, undoped poly-3,3”-didecyl-2,2’,5’,2”- terthiophene and polypyrrole has been successfully developed. Poly-3-hexyloxythiophene was applied as gate material. Undoped poly-3,3”-didecyl-2,2’,5’,2”-terthiophene was used as insulating layer and polypyrrole was applied as source-drain material. The multilayer polymers were deposited onto gold source-drain and gate electrodes by electropolymerization method. The spaces between the gold electrodes were 50 μm. The transistor shows a current amplification upon increasing gate voltages. Good conductivity stability upon increasing gate voltages was observed. Overall the field effect transistor has properties that similar to inorganic field effect transistor.

Frequency Characteristics of Polymer Field-Effect Transistors with Self-Aligned Electrodes Investigated by Impedance Spectroscopy

Solution-based organic field-effect transistors (OFETs) with low parasitic capacitance have been fabricated using a self-aligned method. The self-aligned processes using a cross-linking polymer gate insulator allow fabricating electrically stable polymer OFETs with small overlap area between the source-drain electrodes and the gate electrode, whose frequency characteristics have been investigated by impedance spectroscopy (IS). The IS of polymer OFETs with self-aligned electrodes reveals frequency-dependent channel formation process and the frequency response in FET structure.

Organic field-effect transistor with low-cost CuI/Al bilayer electrode

An organic field-effect transistor based on pentacene semiconductor with CuI/Al bilayer electrode is investigated. The CuI layer, directly contacting the organic semiconductor layer, serves as the hole-injection layer. The overcoated metal layer is responsible for the reduction in contact barrier. Compared with the device with a single metal (Al, Au) layer used as the source-drain electrode, the device with CuI/Al electrodes considerably improves the hole mobility and the on/off current ratio and greatly reduces the threshold voltage. Results of X-ray photoelectron and ultraviolet/visible absorption studies reveal that the reduction in the contact barrier can be attributed to an electron transfer from pentacene and Al to CuI.

Performance improvement in pentacene organic thin film transistors by inserting a C60 ultrathin layer

The contact effect on the performances of organic thin film transistors is studied here. A C60 ultrathin layer is inserted between Al source—drain electrode and pentacene to reduce the contact resistance. By a 3 nm C60 modification, the injection barrier is lowered and the contact resistance is reduced. Thus, the field-effect mobility increases from 0.12 to 0.52 cm2/(V·s). It means that inserting a C60 ultra thin layer is a good method to improve the organic thin film transistor (OTFT) performance. The output curve is simulated by using a charge drift model. Considering the contact effect, the field effect mobility is improved to 1.15 cm2/(V·s). It indicates that further reducing the contact resistance of OTFTs should be carried out.

Transparent organic thin-film transistors and nonvolatile memory devices fabricated on flexible plastic substrates

Transparent and flexible organic thin-film transistors (OTFTs) and nonvolatile memory devices were developed. The memory device is based on OTFTs by incorporating self-assembled gold nanoparticles in the organic dielectric layers as the charge storage elements. The organic memory devices exhibited good programmable memory properties with respect to the program/erase operations, resulting in controllable and reliable threshold voltage shifts by hole trapping/detrapping in the gold nanoparticles. The organic memory devices could be electrically programmed and erased, leading to a large memory window of around 15 V. Additionally, the long term data retention (>105 s) and the stable cyclic bending tests (>2000 cycles) confirmed that the flexible memory devices exhibited good electrical reliability as well as mechanical stability. The organic memory devices were composed of a solution-processed organic dielectric layer/metallic nanoparticles and a low-temperature processed organic semiconductor layer. Additionally, transparent indium-tin oxide (ITO) deposited at room temperature was used for the gate and source–drain electrodes to fabricate transparent memory devices. Therefore, this approach could potentially be applied to advanced transparent and flexible electronic devices as well as integrated organic device circuits.