Field-effect Mobility Of Thin-film Transistors Research Articles

Low-temperature solution-processed nanoparticle-doped nickel oxide thin-film transistor

Developing p-type oxide thin-film transistors (TFTs) is an essential path for further application in complementary metal oxide semiconductor (CMOS) components. However, the inferior electrical performance of p-type MO TFTs compared to n-type TFTs remains an ongoing challenge. Herein, for the first time, a low temperature, facile material engineering approach by incorporating n-type nanoparticles (NPs) was proposed for preparing p-type transparent NiOx TFTs. The characteristics of thin films blending NPs and the electrical performances of TFTs were investigated. The field effect mobility of TFTs with doping was nearly 20 times higher than pristine TFTs without doping, which was mainly beneficial from the suitable band alignment between NPs and p-type oxide, the increasing Ni3+ oxidation state in NiOx, as well as the improved dielectric/semiconductor interface quality. Electrons from drain electrode injected into metal oxide turn accepted in NPs rather than being trapped in the dielectric/semiconductor interface due to a strong surface electron depletion effect of NPs. NPs with small particle size and appropriate concentration would promote continuous hole transport by electrons transferring and reducing the interface trap state. The facile material engineering strategy is a promising technique for preparing p-type transparent MO-TFTs at low temperature, which showed great potential to be applicable in CMOS circuits on flexible substrates.

Effects of interfacial dielectric layers on the charge transport characteristics in sol-gel based amorphous metal-oxide thin-film transistors

We report the effects of interfacial dielectric layers on the field-effect mobility of thin-film transistors (TFTs) with sol-gel based amorphous indium-gallium-zinc oxide (IGZO) films on thermally-grown silicon dioxide (SiO2) and sol-gel based zirconium dioxide (ZrO2) dielectrics. When the field-effect mobility as a function of the channel carrier concentration was extracted, the SiO2 based IGZO-TFT exhibited higher mobility than the ZrO2 based IGZO-TFT at a low carrier concentration. However, the field-effect mobility of the ZrO2 based IGZO-TFT increased as the gate charges increasing due to the gate bias applied. We also investigate the effects of interfacial dielectric layers on the charge transport characteristics in IGZO-TFTs by conducting temperature-dependent field-effect mobility measurements in the temperature range of 120 – 300 K. By analyzing an Arrhenius plot of the results, we confirm that the charge transport of IGZO-TFTs is dominant by the multiple-trapping-and-release model and discuss the effective mobility, which is significantly affected by the interfacial trap level and channel carrier concentration. Finally, we investigate the figure of merit and the activation energy of IGZO-TFTs with ZrO2 dielectrics fabricated by the optimized solution-process developed in this study through comparisons with other sol-gel based amorphous metal-oxide TFTs reported in the literature.

Atmospheric-pressure plasma treatment toward high-quality solution-processed aluminum oxide gate dielectric films in thin-film transistors

We present an atmospheric-pressure plasma (APP) treatment technique to improve the electrical performance of solution-processed dielectric films. This technique can successfully reduce leakage current and frequency dependence of solution-processed dielectric films. The APP treatment contributes to the conversion of metal hydroxide to metal oxide, and in the case of a solution-treated AlOx dielectric thin film, it effectively ascribes to the formation of high-quality AlOx dielectric thin films. The capacitance of the untreated AlOx dielectric thin film varies up to 9.9% with frequency change, but the capacitance of the APP treated AlOx dielectric thin film varies within 1.5%. When the solution-processed InOx thin-film transistors (TFTs) were fabricated using these dielectric films, the field-effect mobility of TFTs with the APP-treated AlOx dielectric film was increased significantly from 9.77 to 26.79 cm2 V−1 s−1 in comparison to that of TFTs with the untreated AlOx dielectric film. We also have confirmed that these results are similar to the properties of the sample prepared at high annealing temperature including electrical performance, conduction mechanism and chemical structure. The APP treatment technique provides a new opportunity to effectively improve the electrical performance of solution-processed dielectrics in the atmosphere at low temperature.

In brief

PAGE 1259 Researchers in China have proposed an unconventional dual-band antenna with two transverse slots etched onto a substrate integrated waveguide (SIW) cavity. The design exhibits good stopband characteristics including significantly improved non-radiating gain and band-edge selectivity. Experimental results verify the effectiveness of the antenna and promote its use in wireless communication applications. PAGE 1302 i-vector based speech recognition frameworks require effective channel compensation methods for enlarging class differences while reducing class variations. Using a discriminatively learned network (DLN) a team of Chinese researchers used both speaker classification and supervising verification signals for identification. The DLN performs better than linear discriminant analysis (LDA) methods. Electric field distributions at different frequencies. PAGE 1298 A large collaboration of researchers from the USA and Qatar report the enhancement of electrical characteristics in thin film transistors (TFTs) by stacking layers with alternating precursor solution concentrations. The mobility of the fabricated alternating-layer TFT was higher than those with only single concentrations. Future work will study the underlying reason for this effect. Similar DNN parameter architecture. PAGE 1292 A novel analytical model of polarimetric dispersion (PD) based on frequency-domain range pulse compression has been analysed and proposed by researchers in China. The simulation results indicate that spaceborne P-band ultra-wideband synthetic aperture radar systems are heavily influenced by PD effects from ionospheric Faraday rotation. Field effect mobility of TFTs at different concentrations. PAGE 1260 A simple internal antenna combining a planar inverted F antenna and a loop antenna to achieve wideband ultra-high frequency operation for digital television applications has been presented by researchers in Korea. The antenna is implementable within T models with sufficiently wide bandwidth and high peak gain to provide good performance. Polarimetric distortion coefficient function with bandwidth. 3D structure of proposed loop antenna.

Morphological Influence of Solution-Processed Zinc Oxide Films on Electrical Characteristics of Thin-Film Transistors.

We report on the morphological influence of solution-processed zinc oxide (ZnO) semiconductor films on the electrical characteristics of ZnO thin-film transistors (TFTs). Different film morphologies were produced by controlling the spin-coating condition of a precursor solution, and the ZnO films were analyzed using atomic force microscopy, X-ray diffraction, X-ray photoemission spectroscopy, and Hall measurement. It is shown that ZnO TFTs have a superior performance in terms of the threshold voltage and field-effect mobility, when ZnO crystallites are more densely packed in the film. This is attributed to lower electrical resistivity and higher Hall mobility in a densely packed ZnO film. In the results of consecutive TFT operations, a positive shift in the threshold voltage occurred irrespective of the film morphology, but the morphological influence on the variation in the field-effect mobility was evident. The field-effect mobility in TFTs having a densely packed ZnO film increased continuously during consecutive TFT operations, which is in contrast to the mobility decrease observed in the less packed case. An analysis of the field-effect conductivities ascribes these results to the difference in energetic traps, which originate from structural defects in the ZnO films. Consequently, the morphological influence of solution-processed ZnO films on the TFT performance can be understood through the packing property of ZnO crystallites.

Effects of Pb Doping on Hole Transport Properties and Thin-Film Transistor Characteristics of SnO Thin Films

Effects of Pb doping on the microstructural, optical, and hole transport properties of Sn1−xfPbxfO films fabricated by pulsed laser deposition were investigated. It was found that Pb was not solved in bulk ceramic samples, while solved up to xf = 0.035 in the thin films. The Pb doping enlarged the optical bandgap, decreased the hole concentration, and reduced the Hall mobility (μHall, from 1.95 to 0.68 cm2 V−1 s−1 up to xf = 0.035) as xf increased. The field-effect mobility in thin-film transistors also decreased with xf (μlin, from 0.34 to 0.0024 cm2 V−1 s−1), but the deterioration of μlin was significantly larger than that of μHall. It is speculated that the deterioration of μHall would be due to the increase in grain boundary potential barriers. The deterioration of μlin, low on-state current, and high off-state current are explained semiquantitatively by a simple hole trap model. Similar explanations would be applied also to previous reports on Sb / Y doped-SnO. Location of the highest occupied states of Pb2+ relative to that of Sn2+ (i.e., Pb 6s2 and Sn 5s2, respectively) clarified by DFTcalculations provides an explantion for the observed result.

Enhanced mobility of Li-doped ZnO thin film transistors fabricated by mist chemical vapor deposition

Mist chemical vapor deposition (mist-CVD)-processed, lithium (Li)-doped ZnO thin film transistors (TFTs) are investigated. Li doping significantly increases the field-effect mobility in TFTs up to ∼100 times greater than that of undoped ZnO. The addition of Li into mist-CVD-grown ZnO semiconductors leads to improved film quality, which results from the enhanced crystallinity and reduced defect states, including oxygen vacancies. Our results suggest that Li doping of ZnO-based oxide semiconductors could serve as an effective strategy for high-performance, mist-CVD-processed oxide TFTs with low-cost and low-temperature fabrication.

Vapor-Induced Improvements in Field Effect Mobility of Transparent a-IGZO TFTs

We found that high pressure assisted annealing with water vapor (HPV) is effective for improving the field effect mobility of amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors (TFTs). The HPV-annealed TFTs exhibited better performance in field effect mobility compared with dry annealing under atmospheric pressure. Secondary ion mass spectroscopy (SIMS) analysis revealed that the hydrogen content increased through the whole a-IGZO film, despite the oxygen content increasing only within 20 nm depth from the a-IGZO surface. Consequently, hydrogen can affect front channel components and improve the field effect mobility of TFTs.

Composite Semiconductor Material of Carbon Nanotubes and Poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] for High-Performance Organic Thin-Film Transistors

A nonpercolating network of non-covalently functionalized single-walled carbon nanotubes was embedded within air-stable poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] (PQT-12) thin films for the purpose of enhancing the field-effect mobility in thin-film transistors. The host polymer was used to stabilize the nanotubes in suspension through π-orbital overlap caused by simple application of ultrasonication. The stable nanotube suspension was cast into two different device architectures, both of which exhibited excellent on/off ratios ranging from 105 to 106 and dramatically improved mobilities compared with pristine PQT-12 semiconductor. A single-layer film with nanotubes embedded throughout was easy to fabricate and had mobility up to 0.34 cm2/Vs, an enhancement of over 3× compared with PQT-12. Placing the nanotubes closer to the dielectric surface in a dual-layer approach resulted in a mobility improvement of up to six times (0.58 cm2/Vs). The effects of the nanotube content on the polymer interaction within the suspension, film morphology, and electrical properties were investigated as well.

Characterization of Intrinsic Field-Effect Mobility in TFTs by De-Embedding the Effect of Parasitic Source and Drain Resistances

Due to voltage drops across parasitic resistances in semiconductor devices, extracted performance parameters can be strongly dependent on the geometrical structure. In this letter, we report a characterization technique for the intrinsic field-effect mobility μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FEo</sub> in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) by de-embedding the parasitic source and drain resistances <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> , respectively. We obtained the channel-length ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> ) -independent intrinsic field-effect mobility μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FEo</sub> from TFTs with various channel lengths on the same wafer. We expect that this characterization technique for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> -independent intrinsic field-effect mobility is useful for accurate characterization, consistent modeling, and robust simulation of a-IGZO TFT circuits.

Effect of Series Resistance on Field-Effect Mobility at Varying Channel Lengths and Investigation into the Enhancement of Source/Drain Metallized Thin-Film Transistor Characteristics

The degradation in device performance due to parasitic resistance along the source and drain electrodes is a serious problem in thin-film transistor fabrication. The effect of this series resistance on the field-effect mobility has been discussed and solutions for the reduction of this unexpected resistance were studied in this work. From the derivation of the drain current, it was shown that the additional resistance had a greater influence on short-channel devices. The ratio of the series resistance to the channel resistance determined the amount of degradation to the field-effect mobility. A thin-film transistor using an aluminum-metallized source/drain was suggested; it showed an improvement in comparison with the conventional doped source/drain device, with a maximum mobility of 105 cm2 V-1 s-1. However, significant degradation of the mobility in the short-channel cases exposed the limitations of the structure. By employing doping for the source/drain metallized structure the maximum mobility reached a value of 150 cm2 V-1 s-1. The decrease in the mobility caused by the channel length decrease was also improved. This study clearly explained the problem of additional resistance on the field-effect mobility of thin-film transistors and the achievements of a device using a self-aligned fabrication process with metallized electrodes.

In Situ Analysis of Hole Injection Barrier of O[sub 2] Plasma-Treated Au with Pentacene Using Photoemission Spectroscopy

Pentacene was in situ deposited on both bare-Au and O 2 plasma-treated Au (O 2 -Au). The band structure at the interface of Au with pentacene was quantitatively determined using ultraviolet photoelectron spectroscopy. The work function of Au increased from 4.65 to 5.28 eV as the Au surface was treated with O 2 plasma. The corresponding interface dipoles were -0.30 eV for bare Au and -0.71 eV for O 2 -Au, respectively. Thus the hole injection barrier at the interface reduced from 0.45 to 0.15 eV by the O 2 plasma before the deposition of pentacene, leading to an increase of the linear field-effect mobility of thin-film transistors.

Dielectric Properties of Ultra Dense (3 g/cm3) Silicon Nitride Deposited by Hot Wire CVD at Industrially Relevant High Deposition Rates

AbstractFor silicon nitride (SiNx) deposited at 3 nm/s using hot wire chemical vapor deposition (HWCVD), the mass-density reached an ultra high value of 3.0 g/cm3. Etch rates in a 16BHF solution show that the lowest etch rate occurs for films with a N/Si ratio of 1.2, the ratio where also the maximum in mass density occurs. The thus found etch rate of 7 nm/min is much better than that for PECVD layers, even when made at a much lower deposition rate. The root-mean-square (rms) roughness measured on 300 nm thick SiN1.2 layers is only about 1 nm, which is advantageous for obtaining high field-effect mobility in thin-film transistors. SiN1.2 films have succesfully been tested in “all hot wire” thin film transistors (TFTs). SiNx films with various x values in the range 1.0 &lt; × &lt;1.5 have been incorporated in metal-insulator-semiconductor structures with n-type c-Si wafers to determine their electrical properties from C-V and I-V measurements. We analyzed the behavior of the static dielectric constant, fixed nitride charges and trapped nitride charges as function of N/Si ratio. I-V measurements show that the HW SiNx films with N/Si ≥ 1.33 have high dielectric breakdown fields that exceed 5.9 MV/cm. For these films we deduce a low positive fixed nitride charge density of 6.2-7.8 × 1016 cm-3 from the flat band voltage and from the small hysteresis in the backward sweep we deduce a low fast trapped charge density of 1.3-1.7 × 1011 cm-2. The dielectric constant ε for different compositions is seen not to change appreciably over the whole range and amounts to 6.3 ± 0.1. These high-density SiNx films possess very low tensile stress (down to 16 MPa), which will be helpful in for instance, plastic electronics applications. HWCVD provides high quality a-SiNx materials with good dielectric properties at a high deposition rate.

Improvement of the electrical performance in metal-induced laterally crystallized polycrystalline silicon thin-film transistors by crystal filtering

The electrical performance of the polycrystalline silicon (poly-Si) thin-film transistors (TFTs) is greatly affected by the microstructure of the poly-Si. The crystal filtering method is employed to improve the electrical properties of poly-Si TFTs using metal-induced lateral crystallization by the ordering of Si grains. Experimental results indicated that the field-effect mobility of TFTs having crystal filtered poly-Si is more than 2.5 times higher than that of conventional poly-Si TFTs. It is found that the ordered microstructure is one of the key parameters to improve the electrical performance in poly-Si TFTs.

Anisotropic field-effect mobility of pentacene thin-film transistor: Effect of rubbed self-assembled monolayer

Enhanced and anisotropic field-effect mobility of thin-film transistors based on pentacene was obtained on a substrate covered with a prerubbed, self-assembled monolayer (SAM) of n-alkyltrichlorosilane. The field-effect mobility along the rubbing direction increased by a factor of ∼15 on rubbed n-nonyltrichlorosilane-modified surface, whereas on n-octadecyltrichlorosilane-modified surface, an increase by a factor of 2–4 was observed. The structures of pentacene films deposited on the SAM/rubbed SAM were characterized by powder x-ray diffraction, atomic force microscopy, and carbon K-edge near edge x-ray fine structure spectroscopy. An in-plane alignment effect on the pentacene packing and film morphology is proposed.

Effects of inductively coupled plasma oxidation on the properties of polycrystalline silicon films and thin film transistors

We investigated the effects of inductively coupled plasma (ICP) oxidation on the properties of polycrystalline silicon (poly-Si) films and thin film transistors (TFTs). The ICP oxidation in oxygen plasma passivated the dangling bonds in the poly-Si films, not by oxygen incorporation but by hydrogen incorporation; but the incorporated hydrogen diffused out during the TFT fabrication, so that the effect of the dangling bond passivation was not obtained in the TFT. The ICP oxidation did not remove the intragranular defects such as microtwins and stacking faults, but it reduced the interface trap density and also improved the performance of the poly-Si TFT. The field effect mobility of TFT with an ICP oxide and low-pressure chemical vapor deposited (LPCVD) oxide double layer was 30.6 cm2/V s, while that of TFT with only a LPCVD oxide was 17.2 cm2/V s.

Structural, spectroscopic and device characteristics of octithiophene

As a step further in the design of efficient organic semiconductors, we report here on the structural, spectroscopic and device characterization of octithiophene (8T), the linear conjugated octamer of thiophene. The X-ray structure and optical spectra under polarized light of the 8T single crystal are described for the first time. Vapor deposited 8T behaves as a p-type semiconductor whose transport properties are investigated in thin film transistors (TFTs). In particular, we show that the use of high purity 8T enhances by two orders of magnitude its field-effect mobility in TFTs to reach μ FET = 1.8×10 −2 cm 2/V.s, a value similar to that of non-substituted sexithiophene (6T).

Intrinsic Transport Properties and Performance Limits of Organic Field-Effect Transistors

The field-effect mobility in thin-film transistors based on alpha-sexithiophene (alpha-6T) and related materials displays a temperature dependence that is remarkably nonmonotonic. Above a transition temperature TT (specific to a given material) the transport is thermally activated, whereas below TT there is a very steep enhancement of the mobility. In the activated regime, the results are well described by the theoretical predictions for small polaron motion made by Holstein in 1959. An analysis of the transistor characteristics shows that the hopping transport in these devices is intrinsic. Performance limits for devices based on alpha-6T and related materials were established; these limits point to the strong possibility that better molecular materials for transistor applications may be designed from first principles.

A Thin-Film Transistor Using a Reactive-Ion-Beam-Deposited Polycrystalline Silicon Film

A thin-film transistor (TFT) has been fabricated at a maximum temperature of 600degC using a reactive ion-beam-deposited polycrystalline silicon (polysilicon) film. To improve TFT characteristics, the deposition conditions of polysilicon film have been investigated. It is found that polysilicon film crystallinity can be improved by controlling the energy of ionized species. X-ray diffraction spectra and reflective high-energy electron diffraction (RHEED) patterns indicate that (110)-preferred orientation dominates in the film. The deviation of the (110) textured axis becomes very small as ion energy increases. The field effect mobility of TFTs increases as ion energy increases.