The influences of substrate temperature on ambipolar organic heterojunction transistors

Organic heterojunction thin-film transistors are fabricated based on copper phthalocyanine (CuPc) and hexadecafluorophtholocyaninatocopper (F₁₆CuPc) as double active layers, which exhibit typical ambipolar conduction. Several substrate temperatures are utilized to tune film morphology, which results in a remarkable change on the electric characteristics of organic transistors. The highest balanced mobility value of 2.91 × 10⁻² cm²/V s for hole and 1.04 × 10⁻² cm²/V s for electron are obtained by depositing F₁₆CuPc at 150 °C and CuPc at 200 °C, respectively, which are comparable to those conventional single-layer devices. This result demonstrates that the growth conditions of organic heterojunctions play a crucial role in ambipolar devices.     

Organic field-effect transistors with thermal-cured polyacrylate gate dielectric films

Chemically and thermally stable, durable, thermal-cured polyacrylates having a hydrophobic nature, which were prepared by mixing polyacrylate having reactive sites and functional-anhydride, exhibit good insulation properties and high breakdown voltage (> 4.0 MV/cm) as a dielectric. Plastic-based organic thin-film transistors with the thermal-acryl dielectric layer showed typical current-voltage characteristics; the field-effect mobility was calculated to be 0.22 cm² V^− 1 s^− 1, while the threshold voltage was approximately − 8 V. It has been found that thin dielectric layers gave higher field-effect mobility.     

Silicon oxide synthesized using an atmospheric pressure microplasma jet from a tetraethoxysilane and oxygen mixture

Local deposition of SiO_x was studied using an atmospheric pressure very-high-frequency (VHF) inductive coupling microplasma jet (AP-MPJ) from a tetraethoxysilane ((Si(OC₂H₅)₄), TEOS) and oxygen mixture. The SiO_x obtained showed the dielectric constant of 3.8 with a low leakage current of the order of ∼ 10^− 6 A ·cm^− 2 up to 8 MV ·cm^− 1. Bottom-gated sputtered-ZnO thin-film transistors with a AP-MPJ SiO_x as a gated dielectric layer exhibited a relatively high field-effect mobility of 24 cm² V^− 1 s^− 1, a threshold voltage of 14 V and an on/off current ratio of ∼ 10⁴, a performance comparable to that of thermal silicon dioxide. The TFT performance was also obtained for the top-gated ZnO-TFTs with a field-effect mobility of 1.4 cm² ·V^− 1 s^− 1, a threshold voltage of − 1.9 V, and an on/off current ratio of ∼ 10³.     

Synthesis and field effect characteristics of Na2Ti3O7 nanowires

Na₂Ti₃O₇ nanowires with diameters of about 80-130 nm and lengths up to several tens of micrometers are synthesized via a simple hydrothermal method and characterized by the field-emission scanning electron microscopy and X-ray diffraction. Back-gate field-effect transistors based on these nanowires are fabricated on indium tin oxide glass substrates with polymethyl-methacrylate-co-glyciclyl-methacrylate as the gate insulator layers. Typical p-type semiconductor material properties are observed in our investigations. The field-effect mobility is about 0.1 cm²/Vs. The capacitance per unit area of the dielectric is 3.43 nF/cm² (dielectric constant, k = 3.9). The on/off ratio is around 10³ at the conduction of 10 V.     

Zinc oxide thin film transistors using MgO-Bi1.5Zn1.0Nb1.5O7 composite gate insulator on glass substrate

We report on high mobility ZnO thin film transistors (TFTs) (< 5 V), utilizing a room temperature grown MgO-Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) composite gate insulator on a glass substrate. 30 mol% MgO added BZN composite gate insulators exhibited greatly enhanced leakage current characteristics (~< 2 × 10^− 8 A/cm² at 0.3 MV/cm) due to the high breakdown strength of MgO, while retaining an appropriate high-k dielectric constant of 32. The ZnO-TFTs with MgO-BZN composite gate insulators showed a high field-effect mobility of 37.2 cm²/Vs, a reasonable on-off ratio of 1.54 × 10⁵, a subthreshold swing of 460 mV/dec, and a low threshold voltage of 1.7 V.     

Determination of charge carrier mobility of hole transporting polytriarylamine-based diodes

Hole transport properties of three different side chain poly(triarylamines) have been determined by means of the analysis of steady-state current-voltage characteristics using co-planar diode structures. The interpretation is based on space-charge limited models with field-dependent mobility. Mobilities between ~ 10^− 8 and 10^− 6 cm² V^− 1 s^− 1 are obtained. The highest mobility is achieved for poly(tetraphenylbenzidine) devices and the lowest for poly(triphenylamine) devices. Electron-rich methoxy substituents increase the mobility of poly(triphenylamine)s. A comparison of the mobility values with those obtained using organic field-effect transistors is also given.     

Copper phthalocyanine thin-film field-effect transistor with SiO2/Ta2O5/SiO2 multilayer insulator

Top-contact Copper phthalocyanine (CuPc) thin-film field-effect transistor (TFT) with SiO₂/Ta₂O₅/SiO₂ (STS) multilayer as the dielectric was fabricated and investigated. With the multi-layer dielectric, drive voltage was remarkably reduced. A relatively large on-current of 1.1 × 10⁻⁷ A at a V_GS of −15 V was obtained due to the strong coupling capability provided by the STS multilayer gate insulator. The device shows a moderate performance: saturation mobility of μ_sat = 6.12 × 10⁻⁴ cm²/V s, on-current to off-current ratio of I_on/I_off = 1.1 × 10³, threshold voltage of V_TH = −3.2 V and sub-threshold swing SS = 1.6 V/dec. Atomic force microscope images show that the STS multilayer has a relative smooth surface. Experiment results indicate that STS multilayer is a promising insulator for the low drive voltage CuPc-based TFTs.     

Solution-processed zinc tetrabenzoporphyrin thin-films and transistors

Thin-films and organic field-effect transistors fabricated from a solution-processable precursor of zinc tetrabenzoporphyrin (ZnTBP) are reported. Amorphous, insulating precursor films were deposited by spin-casting and thermally converted into polycrystalline, semiconducting thin-films comprising grains on the order of 5 μm in diameter. Thin-film X-ray diffraction indicates a monoclinic unit cell with molecules arranged in a herringbone pattern, which in conjunction with optical and atomic force microscopy indicate a thin-film with grains comprised of randomly oriented ZnTBP aggregates. Optical absorption measurements display broad absorption with bands characteristic of a D_4h symmetric porphyrin molecule. Organic field-effect transistors displayed field-effect mobilities on the order of 10^− 2 cm²/V s and ON-/OFF-current ratios exceeding 10².     

High-performance amorphous indium-gallium-zinc oxide thin-film transistors with polymer gate dielectric

The fabrication of amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) with a spin-coated polymer gate dielectric on a glass substrate is reported. The interface state density at the poly(4-vinylphenol)/a-IGZO interface is only around 4.05 × 10¹¹ cm^− 2. The TFTs' threshold voltage, subthreshold swing, on-off current ratio, and carrier mobility are 2.6 V, 1.3 V/decade, 1 × 10⁵, and 21.8 cm²/V s, respectively. These characteristics indicate that the TFTs are suitable for use as nonvolatile memory devices and in flexible electronic applications.     

Effect of gallium content on bias stress stability of solution-deposited Ga-Sn-Zn-O semiconductor transistors

We generated solution-processed thin film transistor (TFTs) using gallium tin zinc oxide (GTZO, Ga-Sn-Zn-O) layers as the channel that exhibit improved bias-stress stability during device operation under ambient conditions. The cause of the bias-stress stability was investigated through comparisons with zinc tin oxide (ZTO, Zn-Sn-O)-based TFTs, which suffer red from bias stress instability. Based on in-depth analysis of the electrical characteristics and chemical structure of both GTZO and ZTO layers, it was discovered that the GTZO layers had a significantly lower oxygen vacancy concentration than did the ZTO layer, which influenced the electrical performance of the GTZO transistors as well as their bias-stress stability. When 5 mol% gallium was added, a bias stress-stable transistor was obtained, exhibiting typical semiconductor behavior with a field-effect mobility of 1.2 cm² V^− 1 s^− 1, on/off ratio of 10⁶, off-current of 1 × 10^− 10 A, and threshold voltage of 19.6 V. Further doping of Ga deteriorated the device performance, which was found to be associated with decreased carrier concentration and segregation of an insulating secondary phase.     

Amorphous In-Ga-Zn-O thin-film transistor with coplanar homojunction structure

Amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) with a coplanar homojunction structure are demonstrated. The coplanar source and drain regions made of a-IGZO were formed by depositing a hydrogenated silicon nitride (SiN_X:H) layer onto the a-IGZO layer. The a-IGZO regions on which the SiN_X:H layer was directly deposited showed the low resistivity of 4.7 × 10⁻³  Ω cm and degenerated conduction. The fabricated TFT showed excellent transfer and output characteristics with a field-effect mobility of 11 cm² V^− 1 s^− 1, a subthreshold swing of 0.17 V decade^− 1, and an on-to-off current ratio larger than 1 × 10⁹. The width-normalized source-to-drain resistance (R_sdW) calculated using a channel resistance method was 51 Ω cm. This TFT also showed good stability over environment change and under electrical stress.     

Electrical and chemical analysis of zinc oxide interfaces with high dielectric constant barium tantalate and aluminum oxide in metal-insulator-semiconductor structures fabricated at Low temperatures

Zinc oxide (ZnO) was incorporated into metal-insulator-semiconductor (MIS) structures featuring high dielectric constant (high-κ) barium tantalate (BaTa₂O₆)or alumina (Al₂O₃)as the insulator, and the structures were electrically evaluated for potential applications in transparent thin film transistors. The ZnO films were deposited by radio-frequency magnetron sputtering at 100 °C whereas the dielectric films were deposited by the same method at room temperature. The leakage currents of both the BaTa₂O₆ and Al₂O₃ structures were on the order of 10⁻⁷A/cm². The trap density and trapped charge concentration at the BaTa₂O₆/ZnO interface were determined to be 6.18 × 10¹¹ eV⁻¹ cm⁻²and 5.82 × 10¹¹ cm⁻² from conductance-voltage and capacitance-voltage measurements. At the Al₂O₃/ZnO interface the trap density and trapped charge were more than an order of magnitude smaller at 1.09 × 10¹⁰ eV⁻¹ cm⁻²and 1.04 × 10¹⁰ cm⁻² respectively. The BaTa₂O₆ structures had significantly larger frequency dispersions due to the larger number of interface traps. Chemical analysis using X-ray photoelectron spectroscopy with depth profiling indicates that acceptor type defects associated with a deficiency of oxygen are related to the observed electron trapping in the BaTa₂O₆MIS structure. Overall, the results indicate that Al₂O₃ would be better suited for transparent thin film transistors deposited at low temperature or without substrate heating.     

Flexible high capacitance nanocomposite gate insulator for printed organic field-effect transistors

Ceramic-polymer nanocomposite dielectric consisting of an epoxy solution with propylene glycol methyl ether acetate as the solvent and barium titanate nanoparticles with capacitance in excess of 60 pF/mm² was developed and utilized as the gate insulator for organic field-effect transistors (OFETs). The high relative permittivity (κ = 35), bimodal nanocomposite utilized had two different filler particle sizes 200 nm and 1000 nm diameter particles. Bottom gate organic filed-effect transistors were demonstrated using a commercially available printing technology for material deposition. A metal coated plastic film was used as the flexible gate substrate. Solution processable, p-type arylamine-based amorphous organic semiconductor was utilized as the active layer. Fabricated OFETs with the solution processed nanocomposite dielectric had a high field-induced current and a low threshold voltage; these results suggest that the low operating voltage was due to the high capacitance gate insulator. In this paper, we review the characteristics of the nanocomposite dielectric material and discuss the processing and performance of the printed organic devices.     

Electron transport in InGaO3(ZnO)m (m=integer) studied using single-crystalline thin films and transparent MISFETs

We have investigated the characteristics of transparent metal-insulator-semiconductor field-effect transistors (MISFETs) fabricated using InGaO₃(ZnO)_m (m=integer) single-crystalline thin films as n-channel layers and amorphous alumina as gate insulator films. The MISFETs exhibit good characteristics such as insensitivity to visible light illumination, off-current as low as ∼1 nA with a positive threshold voltage of ∼3 V and on/off current ratio of 10⁵. The field-effect mobility increased from ∼1 to ∼10 cm² (V s)⁻¹ as the m-value increased. Room temperature Hall mobility also increased. However, unexpectedly these values were lower than the field-effect mobility. It is explained by existence of shallow localized state in the homologous compounds.     

Study on properties of CaO-SiO2-B2O3 system glass-ceramic

Low temperature co-fired ceramic (LTCC) is prepared by sintering a glass selected from CaO-SiO₂-B₂O₃ system, and its sintered bodies are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that the optimal sintering temperature for this glass-ceramic is 820 °C for 15 min, and the major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. The glass-ceramic possesses excellent dielectric properties: ?_r = 6.5, tan δ < 2 × 10⁻³ at 10 MHz, temperature coefficient of dielectric constant about −51 × 10⁻⁶ °C⁻¹ and coefficient of thermal expansion about 8 × 10⁻⁶ °C⁻¹ at 20-400 °C. Thus, this material is supposed to be suitable for the tape casting process and be compatible with Ag electrode, which could be used as the LTCC materials for the application in wireless communications.     

Double-gate organic thin-film transistor using a photosensitive polymer as the dielectric layer

Dichromated poly (vinyl alcohol) (DCPVA) has been proposed for protecting the pentacene active layer from organic solvents such as PGMEA. DCPVA is water-soluble, and the damage to the pentacene active layer can be minimized compared to other materials dissolved in organic solvents. In this study, the dielectric properties and leakage current density of DCPVA gate dielectric samples with different ammonium dichromate (ADC) concentrations and exposure times were obtained from 6.73 to 10.5 and 6.37 × 10^− 5 to 4.87 × 10^− 8 (A/cm²) respectively. Also the FTIR spectroscopy was used to determine the cross-linked DCPVA films based on the relative intensity of the vibration bands of chromium(VI) and the OH group. According to FTIR, the performance of double-gate OTFTs based on blending ADC:PVA ratio of 0.25:1 film as dielectric layers of double-gate OTFT devices to exclude the acute influence of large gate leakage current caused by excess hydroxyl groups and residual Cr⁶⁺ in dielectric layer on device performance.By analyzing the electrical performance, it was found that the fabricating method of the top-dielectric layer has a great effect on the performance of double-gate OTFTs. SEM images of pentacene microstructure show significant cracks on the top of the pentacene layer due to the swelling effect of the DCPVA polymer after post-baking. Therefore, we used vacuum drying to form photosensitive DCPVA as the top-dielectric layer of a double-gate OTFT circuit array. The use of top-dielectric and electrode layers on the top of pentacene significantly increased the performance of double-gate OTFTs. In summary, we eventually demonstrate that double-gate OTFTs using a low-temperature solution process exhibited an equivalent mobility of 0.53 cm²/Vs and on-off ratio of 8 × 10³.     

Annealing effect on dielectric and leakage current characteristics of Mn-doped Ba0.6Sr0.4TiO3 thin films as gate insulators for low voltage ZnO thin film transistor

We report on the dielectric properties and leakage current characteristics of 3 mol% Mn-doped Ba_0.6Sr_0.4TiO₃ (BST) thin films post-annealed up to 600 °C following room temperature deposition. The suitability of 3 mol% Mn-doped BST films as gate insulators for low voltage ZnO thin film transistors (TFTs) is investigated. The dielectric constant of 3 mol% Mn-doped BST films increased from 24 at in-situ deposition up to 260 at an annealing temperature of 600 °C due to increased crystallinity and the formation of perovskite phase. The measured leakage current density of 3 mol% Mn-doped BST films remained on the order of 5 × 10^− 9 to 10^− 8 A/cm² without further reduction as the annealing temperature increased, thereby demonstrating significant improvement in the leakage current characteristics of in-situ grown Mn-doped BST films as compared to that (5 × 10^− 4 A/cm² at 5 V) of pure BST films. All room temperature processed ZnO-TFTs using a 3 mol% Mn-doped BST gate insulator exhibited a field effect mobility of 1.0 cm²/Vs and low voltage device performance of less than 7 V.     

Improved performance of fluorinated copper phthalocyanine thin film transistors using an organic pn junction: Effect of copper phthalocyanine film thickness

We have investigated the effect of film thickness of copper phthalocyanine (CuPc) on improving fluorinated copper phthalocyanine (F₁₆CuPc) thin film transistor (TFT) performance with an organic pn junction. Electron field-effect mobility is exponentially enhanced up to 2.0 × 10^− 2 cm² V^− 1 s^− 1 with increasing of CuPc film thickness, and then unchanged when the CuPc thickness is over the saturation thickness (3 monolayers). The charge carrier density at the interface of F₁₆CuPc/CuPc decreases the total TFT resistance, which leads to the increase of mobility. Threshold voltage is suppressed with increasing CuPc films. On the other hand, larger current on/off ratio is obtained when islanded CuPc films are formed on the surface of F₁₆CuPc films. Therefore, employing an organic pn junction is an effective and simple method to fabricate high performance of n-channel transistors for practical applications.     

Flexible organic thin-film transistors using single-walled carbon nanotubes as an activated channel

We report on the fabrication of organic thin-film transistors (OTFTs) with a spun cross linked poly-4-vinylphenol (PVP) dielectric on a polyethersulphone (PES) flexible substrate. To improve the electrical performance of OTFTs, we employed a random single-walled carbon nanotubes (SWNTs) network as a carrier transfer underlay without sacrificing the flexibility of the TFTs. The random SWNTs showed that they can act as a semiconducting channel and conduction path to shorten the channel length in our TFTs. The flexible thin-film transistors (TFTs) with a random SWNTs/pentacene bilayer as an active channel exhibited an improved saturation field effect mobility (µ_sat) of 2.6 × 10^− 1 cm²/Vs compared to that of TFTs without the SWNTs underlay, while creating only a minor reduction of the current on/off ratio.     

Amorphous oxide channel TFTs

Thin film transistors (TFTs) using amorphous oxides of post-transition metals: indium, gallium, and zinc for the channel materials are fabricated with radio-frequency magnetron sputtering methods for the deposition of the channel and the gate insulator layers, at room temperature with no high-temperature post-deposition annealing process. The TFTs operate as n-channel field-effect transistors with various structures of top/bottom gate and top/bottom source-and-drain contact including the inverse-stagger types, and with various materials for the gate insulators, the electrodes, and the substrates. The TFTs having smoother channel interfaces show the better performance at the saturation mobility beyond 10 cm² V^− 1 s^− 1 and the on-to-off current ratio over 10⁸ than the rough channel interfaces. The ring oscillator circuits operate with five-stage inverters of the top-gate TFTs or the inverse-stagger TFTs. Organic light-emission diode cells are driven by a simple circuit of the TFTs. It is also found by a combinatorial approach to the material exploration that the TFT characteristics can be controlled by the composition ratio of the metals in the channel layers. The amorphous oxide channel TFTs fabricated with sputtering deposition at low temperature could be a candidate for key devices of large-area flexible electronics.