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Showing papers on "Thin-film transistor published in 2009"


Journal ArticleDOI
01 Jun 2009
TL;DR: In this article, the authors presented the world largest flexible full color 6.5-inch active matrix organic light emitting diode (AMOLED) display with top emission mode on plastic film.
Abstract: The world largest flexible full color 6.5-inch active matrix organic light emitting diode (AMOLED) display with top emission mode on plastic film is demonstrated. The active matrix backplanes were fabricated using metal oxide thin film transistors (TFTs). The n-channel metal oxide TFTs on plastic film exhibited field-effect mobility of 17.8 cm2/Vs, threshold voltage of 0.4 V, on/off ratio of 1.1× 108, and subthreshold slope of 0.34 V/dec. These TFT performance characteristics made it possible to integrate scan driver circuit, demux switching and compensation circuit on the panel. Bending tests were performed with TFT backplane samples to determine critical curvature radius to which the panel can be bent without TFT performance degradation. The results were compared with the calculations that took into account thicknesses and mechanical constants of flexible substrate and of thin-film layers in AMOLED device.

997 citations


Patent
10 Sep 2009
TL;DR: In this article, a thin film transistor with a channel layer consisting of a conductive oxide semiconductor, a pair of electrodes on the channel layer, and a protective film covering an exposed surface of the channel, exposed to the gap between the pairs of electrodes.
Abstract: The present invention provides a thin film transistor including: a channel layer mainly containing a conductive oxide semiconductor; a pair of electrodes on the channel layer; and a protective film covering an exposed surface of the channel layer, exposed to the gap between the pair of electrodes. The protective film includes at least an oxygen transmission film in contact with the channel layer, and an oxygen disturbance film hardly transmitting oxygen in comparison with the oxygen transmission film, in this order from the channel layer side. A length of the oxygen disturbance film in a direction where the pair of electrodes face each other is equal to or larger than a value obtained by multiplying a width of the pair of electrodes in a direction orthogonal to the direction where the pair of electrodes face each other by 0.55.

986 citations


Journal ArticleDOI
01 Jun 2009
TL;DR: In this article, inverted-staggered amorphous In-Ga-Zn-O (a-IGZO) TFTs were fabricated and measured temperature dependence of the TFT characteristics.
Abstract: We fabricated inverted-staggered amorphous In-Ga-Zn-O (a-IGZO) TFTs and measured temperature dependence of the TFT characteristics. A Vth shift between 120°C and 180°C was as large as about 4 V. In the analysis with 2-D numerical simulation, we could reproduce the measured result by assuming two kinds of donor-like states as carrier generation sources.

962 citations


Journal ArticleDOI
01 Jun 2009
TL;DR: In this article, an indium-oxide-based transparent oxide TFT, which the active layer is prepared by DC sputtering, is presented, and the fabricated TOS TFTs show high mobility (37 cm2/V-s), high ON/OFF current ratio and large onstate current.
Abstract: Results on indium-oxide-based transparent oxide TFTs, which the active layer is prepared by DC sputtering, are presented. The fabricated TOS TFTs show high mobility (37 cm2/V-s), high ON/OFF current ratio and large on-state current. Fabricating oxide TFTs on temperature-sensitive substrates is also attainable owing to the low temperature process of the active layer preparation.

960 citations


Proceedings ArticleDOI
01 Dec 2009
TL;DR: In this paper, a self-aligned top-gate amorphous oxide TFTs for large size and high resolution displays are presented, where Ar plasma is exposed on the source/drain region of active layer to minimize the source and drain series resistances.
Abstract: We have demonstrated self-aligned top-gate amorphous oxide TFTs for large size and high resolution displays. The processes such as source/drain and channel engineering have been developed to realize the self-aligned top gate structure. Ar plasma is exposed on the source/drain region of active layer to minimize the source/drain series resistances. To prevent the conductive channel, N 2 O plasma is also treated on the channel region of active layer. We obtain a field effect mobility of 5.5 cm2/V·s, a threshold voltage of 1.1 V, and a sub-threshold swing of 0.35 V/decade at sub-micron a-GIZO TFTs with the length of 0.67#x00B5;m. Furthermore, a-IZO TFTs fabricated for gate and data driver circuits on glass substrate exhibit excellent electrical properties such as a field effect mobility of 115 cm2/V·s, a threshold voltage of 0.2 V, a sub-threshold swing of 0.2 V/decade, and low threshold voltage shift less than 1 V under bias temperature stress for 3 hr.

957 citations


Journal ArticleDOI
01 Jun 2009
TL;DR: In this paper, the transparent bottom gate TFTs using Al and Sn-doped zinc indium oxide (AT-ZIO) as an active layer were fabricated using RF magnetron sputtering at room temperature.
Abstract: We have fabricated the transparent bottom gate TFTs using Al and Sn-doped zinc indium oxide (AT-ZIO) as an active layer. The AT-ZIO active layer was deposited by RF magnetron sputtering at room temperature, and AT-ZIO TFT showed a field effect mobility of 15.6 cm2/Vs even before annealing. The mobility increased with increasing In2O3 content and post-annealing temperature. The AT-ZIO TFT exhibited afield effect mobility of 33 cm2/Vs, a sub-threshold swing of 0.08 V/dec, and an on/off current ratio of more than 109 after Al2O3 passivation and post-annealing. We have fabricated AMOLED panels with the bottom gate AT-ZIO TFT back-plane successfully.

933 citations


Journal ArticleDOI
TL;DR: The syntheses and comprehensive characterization of 14 organic semiconductors based on perylene bisimide (PBI) dyes that are equipped with up to four halogen substituent in the bay area of the perylene core and five different highly fluorinated imide substituents are described, making them suitable for a wide range of practical applications.
Abstract: The syntheses and comprehensive characterization of 14 organic semiconductors based on perylene bisimide (PBI) dyes that are equipped with up to four halogen substituents in the bay area of the perylene core and five different highly fluorinated imide substituents are described. The influence of the substituents on the LUMO level and the solid state packing of PBIs was examined by cyclic voltammetry and single crystal structure analyses of seven PBI derivatives, respectively. Top-contact/bottom-gate organic thin film transistor (OTFT) devices were constructed by vacuum deposition of these PBIs on SiO2 gate dielectrics that had been pretreated with n-octadecyl triethoxysilane in vapor phase (OTS-V) or solution phase (OTS-S). The electrical characterization of all devices was accomplished in a nitrogen atmosphere as well as in air, and the structural features of thin films were explored by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM). Several of those PBIs that bear only hydr...

609 citations


Journal ArticleDOI
TL;DR: In this paper, self-assembled monolayers and multilayers (SAMs) of organic molecules have been used to achieve low gate leakage currents and good chemical/thermal stability.
Abstract: Principal goals in organic thin-film transistor (OTFT) gate dielectric research include achieving: (i) low gate leakage currents and good chemical/thermal stability, (ii) minimized interface trap state densities to maximize charge transport efficiency, (iii) compatibility with both p- and n- channel organic semiconductors, (iv) enhanced capacitance to lower OTFT operating voltages, and (v) efficient fabrication via solution-phase processing methods. In this Review, we focus on a prominent class of alternative gate dielectric materials: self-assembled monolayers (SAMs) and multilayers (SAMTs) of organic molecules having good insulating properties and large capacitance values, requisite properties for addressing these challenges. We first describe the formation and properties of SAMs on various surfaces (metals and oxides), followed by a discussion of fundamental factors governing charge transport through SAMs. The last section focuses on the roles that SAMs and SAMTs play in OTFTs, such as surface treatments, gate dielectrics, and finally as the semiconductor layer in ultra-thin OTFTs.

595 citations


Journal ArticleDOI
TL;DR: The origins of the prominent features of AOS devices from the viewpoint of materials science of A OS indicate that electron transport in oxide semiconductors are insensitive to random structures and these oxides do not form high-density defects that affect electron transport and TFT operation.
Abstract: Amorphous oxide semiconductors (AOSs) are expected as new channel materials in thin-film transistors (TFTs) for large-area and/or flexible flat-panel displays and other giant-microelectronics devices. So far, many prototype displays have been demonstrated in these four years since the first report of AOS TFT. The most prominent feature of AOS TFTs is that they operate with good performances even if they are fabricated at low temperatures without a defect passivation treatment. The TFT mobilities exceed 10 cm2/(Vmiddots), which are more than ten times larger than those of conventional amorphous semiconductor devices. In addition, they operate at low voltages, e.g., < 5 V owing to their small subthreshold voltage swings. These features indicate that electron transport in oxide semiconductors are insensitive to random structures and these oxides do not form high-density defects that affect electron transport and TFT operation. In this paper, we discuss the origins of the prominent features of AOS devices from the viewpoint of materials science of AOS.

576 citations


Journal ArticleDOI
Robert A. Street1
TL;DR: In this paper, an intense search has developed for new materials and fabrication techniques that can improve the performance, lower manufacturing cost, and enable new functionality of TFTs, including organic semiconductor, metal oxides, nanowires, printing technology as well as thin-film silicon materials with new properties.
Abstract: Thin-film transistors (TFTs) matured later than silicon integrated circuits, but in the past 15 years the technology has grown into a huge industry based on display applications, with amorphous and polycrystalline silicon as the incumbent technology. Recently, an intense search has developed for new materials and new fabrication techniques that can improve the performance, lower manufacturing cost, and enable new functionality. There are now many new options – organic semiconductor (OSCs), metal oxides, nanowires, printing technology as well as thin-film silicon materials with new properties. All of the new materials have something to offer but none is entirely without technical problems.

530 citations


Journal ArticleDOI
TL;DR: In this paper, the authors have fabricated 6.5 in. flexible full-color top-emission active matrix organic light-emitting diode display on a polyimide (PI) substrate driven amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs).
Abstract: We have fabricated 6.5 in. flexible full-color top-emission active matrix organic light-emitting diode display on a polyimide (PI) substrate driven amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs). The a-IGZO TFTs exhibited field-effect mobility (μFE) of 15.1 cm2/V s, subthreshold slope of 0.25 V/dec, threshold voltage (VTH) of 0.9 V. The electrical characteristics of TFTs on PI substrate, including a bias-stress instability after 1 h long gate bias at 15 V, were indistinguishable from those on glass substrate and showed high degree of spatial uniformity. TFT samples on 10 μm thick PI substrate withstood bending down to R=3 mm under tension and compression without any performance degradation.

Journal ArticleDOI
TL;DR: This work has achieved solution-based assembly of separated nanotube thin films on complete 3 in.
Abstract: Preseparated, semiconductive enriched carbon nanotubes hold great potential for thin-film transistors and display applications due to their high mobility, high percentage of semiconductive nanotubes, and room-temperature processing compatibility. Here in this paper, we report our progress on wafer-scale processing of separated nanotube thin-film transistors (SN-TFTs) for display applications, including key technology components such as wafer-scale assembly of high-density, uniform separated nanotube networks, high-yield fabrication of devices with superior performance, and demonstration of organic light-emitting diode (OLED) switching controlled by a SN-TFT. On the basis of separated nanotubes with 95% semiconductive nanotubes, we have achieved solution-based assembly of separated nanotube thin films on complete 3 in. Si/SiO2 wafers, and further carried out wafer-scale fabrication to produce transistors with high yield (>98%), small sheet resistance (∼25 kΩ/sq), high current density (∼10 μA/μm), and super...

Patent
22 Jun 2009
TL;DR: In this paper, a gate-insulated thin-film transistor with a halogen block in between the blocking layer and a gate insulator is described. But the block is not used to prevent the transistor from being contaminated with impurities such as alkali ions.
Abstract: A gate-insulated thin film transistor is disclosed. One improvement is that the thin film transistor is formed on a substrate through a blocking layer in between so that it is possible to prevent the transistor from being contaminated with impurities such as alkali ions which exist in the substrate. Also, a halogen is added to either or both of the blocking layer and a gate insulator of the transistor in order that impurities such as alkaline ions, dangling bonds and the like can be neutralized, therefore, the reliability of the device is improved.

Patent
13 Jan 2009
TL;DR: In this paper, a thin film transistor (TFT) using an oxide semiconductor as an active layer, a method of manufacturing the TFT, and a flat panel display device has been presented.
Abstract: A thin film transistor (TFT) using an oxide semiconductor as an active layer, a method of manufacturing the TFT, and a flat panel display device having the TFT include source and drain electrodes formed on a substrate; an active layer formed of an oxide semiconductor disposed on the source and drain electrodes; a gate electrode; and an interfacial stability layer formed on at least one of top and bottom surfaces of the active layer. In the TFT, the interfacial stability layer is formed of an oxide having a band gap of 3.0 to 8.0 eV. Since the interfacial stability layer has the same characteristics as a gate insulating layer and a passivation layer, chemically high interface stability is maintained. Since the interfacial stability layer has a band gap equal to or greater than that of the active layer, charge trapping is physically prevented.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the threshold voltage stability for a-In-Ga-Zn-O thin film transistors (TFTs) deposited at room temperature and annealed at 400°C in dry or wet O2 atmospheres.
Abstract: Threshold voltage (Vth) stability was examined under constant current stress for a-In–Ga–Zn–O thin film transistors (TFTs) deposited at room temperature and annealed at 400 °C in dry or wet O2 atmospheres. All the TFTs exhibited positive Vth shifts (ΔVth) and the ΔVth value was reduced by the thermal annealing to <2 V for 50 h. TFT simulations revealed that the ΔVth for the annealed TFTs is explained by increase in deep charged defects. Large ΔVth over 10 V and deterioration in subthreshold voltage swing were observed in the unannealed TFTs, which are attributed to the increase in shallow trap states.


Patent
07 Oct 2009
TL;DR: In this paper, a thin-film transistor capable of improving reliability in the thin-layer transistor including an oxide semiconductor layer is presented, provided that a gate electrode, a gate insulating film, and a source/drain electrode are available.
Abstract: Provided is a thin film transistor capable of improving reliability in the thin film transistor including an oxide semiconductor layer. A thin film transistor including: a gate electrode; a gate insulating film formed on the gate electrode; an oxide semiconductor layer forming a channel region corresponding to the gate electrode on the gate insulating film; a channel protective film formed at least in a region corresponding to the channel region on the oxide semiconductor layer; and a source/drain electrode. A top face and a side face of the oxide semiconductor layer are covered with the source/drain electrode and the channel protective layer on the gate insulating film.

Journal ArticleDOI
TL;DR: Here, sol-gel-beta-alumina films are described as transistor gate dielectrics with solution-deposited zinc-oxide-based semiconductors and indium tin oxide (ITO) gate electrodes and an all-solution-processed, low-voltage transparent oxide transistor on an ITO glass substrate.
Abstract: Sodium beta-alumina (SBA) has high two-dimensional conductivity, owing to mobile sodium ions in lattice planes, between which are insulating AlO(x) layers. SBA can provide high capacitance perpendicular to the planes, while causing negligible leakage current owing to the lack of electron carriers and limited mobility of sodium ions through the aluminium oxide layers. Here, we describe sol-gel-beta-alumina films as transistor gate dielectrics with solution-deposited zinc-oxide-based semiconductors and indium tin oxide (ITO) gate electrodes. The transistors operate in air with a few volts input. The highest electron mobility, 28.0 cm2 V(-1) s(-1), was from zinc tin oxide (ZTO), with an on/off ratio of 2 x 10(4). ZTO over a lower-temperature, amorphous dielectric, had a mobility of 10 cm2 V(-1) s(-1). We also used silicon wafer and flexible polyimide-aluminium foil substrates for solution-processed n-type oxide and organic transistors. Using poly(3,4-ethylenedioxythiophene) poly(styrenesulphonate) conducting polymer electrodes, we prepared an all-solution-processed, low-voltage transparent oxide transistor on an ITO glass substrate.

Journal ArticleDOI
TL;DR: In this paper, a protrusion electrode structure is proposed to dramatically lower the operation voltage of the emerging blue-phase liquid crystal displays (BP-LCDs), which enables the BP-LCD to be addressed by amorphous silicon thin-film transistors (TFTs).
Abstract: A protrusion electrode structure is proposed to dramatically lower the operation voltage of the emerging blue-phase liquid crystal displays (BP-LCDs). Simulation results indicate that the generated horizontal electric field is not only strong but also penetrates deeply into the bulk LC layer. As a result, a low voltage (∼10 Vrms) and reasonably high transmittance (∼70%) BP-LCD can be achieved. This approach enables the BP-LCDs to be addressed by amorphous silicon thin-film transistors (TFTs). Widespread application of TFT BP-LCDs is foreseeable.


Journal ArticleDOI
TL;DR: In this article, the gate bias stability of the ZTO TFT was investigated, showing that positive gate bias results in a positive shift of the threshold voltage due to the charge trapping in the channel/dielectric interface.
Abstract: Thin film transistors (TFTs) with amorphous zinc tin oxide (ZTO) channel layer were fabricated by a simple and low-cost solution process. The ZTO thin films are highly transparent (>90% transmittance) in the visible region. The ZTO TFTs fabricated at 400 and 500 °C are operated in enhancement mode. The TFT annealed at 500 °C shows a mobility of 14.11 cm2 V−1 s−1, a threshold voltage of 1.71 V, a subthreshold slope of 0.4 V dec−1 and an on–off current ratio greater than 108. In addition, we investigated the gate bias stability of the TFT. Positive gate bias results in a positive shift of the threshold voltage due to the charge trapping in the channel/dielectric interface.

Journal ArticleDOI
TL;DR: In this paper, a droplet of the solution is sustained at an edge of a structure on an inclined substrate, so that the crystalline domain grows in the direction of inclination.
Abstract: Field-effect mobility as high as 5 cm2/(V s) is achieved in solution-processed organic thin-film transistors with the development of a method for growing highly-oriented crystalline films of [1]benzothieno[3,2-b]benzothiophene derivatives. A droplet of the solution is sustained at an edge of a structure on an inclined substrate, so that the crystalline domain grows in the direction of inclination. The oriented growth realizes excellent molecular ordering that manifests itself in micrometer-scale molecular terraces on the surface as a result of the self-organizing function of the material. The unprecedented performance achieved using an easy fabrication process has increased attractiveness of organic thin-film transistors for industrial applications.

Journal ArticleDOI
TL;DR: In this article, a two-dimensional simulation of electrical properties of the radio frequency (RF) sputter amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) is presented.
Abstract: We reported on a two-dimensional simulation of electrical properties of the radio frequency (rf) sputter amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). The a-IGZO TFT used in this work has the following performance: field-effect mobility (μeff) of ∼12 cm2/V s, threshold voltage (Vth) of ∼1.15 V, subthreshold swing (S) of ∼0.13 V/dec, and on/off ratio over 1010. To accurately simulate the measured transistor electrical properties, the density-of-states model is developed. The donorlike states are also proposed to be associated with the oxygen vacancy in a-IGZO. The experimental and calculated results show that the rf sputter a-IGZO TFT has a very sharp conduction band-tail slope distribution (Ea=13 meV) and Ti ohmic-like source/drain contacts with a specific contact resistance lower than 2.7×10−3 Ω cm2.

Journal ArticleDOI
TL;DR: Pentacene organic thin-film transistors (OTFTs) with a high-kappa HfLaO dielectric were integrated onto flexible polyimide substrates as discussed by the authors.
Abstract: Pentacene organic thin-film transistors (OTFTs) with a high-kappa HfLaO dielectric were integrated onto flexible polyimide substrates. The pentacene OTFTs exhibited good performance, such as a low subthreshold swing of 0.13 V/decade and a threshold voltage of -1.25 V. The field-effect mobility was 0.13 cm2/Vmiddots at an operating voltage as low as only 2.5 V. These characteristics are attractive for high-switching-speed and low-power applications.

Journal ArticleDOI
TL;DR: In this article, a crystalline, dense octadecylsilane (OTS) surface modification layer was found that promotes two-dimensional semiconductor growth, and higher mobility was consistently achieved for films deposited on crystalline OTS compared to disordered OTS, with mobilities as high as 5.3 and 2.3cm 2 V � 1 s � 1 for C60 and pentacene, respectively.
Abstract: In organic thin film transistors (OTFTs), charge transport occurs in the first few monolayers of the semiconductor near the semiconductor/dielectric interface. Previous work has investigated the roles of dielectric surface energy, roughness, and chemical functionality on performance. However, large discrepancies in performance, even with apparently identical surface treatments,indicate that additionalsurface parametersmust beidentified and controlled in order to optimize OTFTs. Here, a crystalline, dense octadecylsilane (OTS) surface modification layer is found that promotes twodimensional semiconductor growth. Higher mobility is consistently achieved for films deposited on crystalline OTS compared to on disordered OTS, with mobilities as high as 5.3 and 2.3cm 2 V � 1 s � 1 for C60 and pentacene, respectively. This is a significant step toward morphological control of organic semiconductors which is directly linked to their thin film charge carrier transport.

Journal ArticleDOI
TL;DR: In this paper, electrical properties, electronic structures, and thin-film transistors (TFTs) of Tin Monoxide (SnO) were reported, which indicated that the closed-shell 5s 2 orbitals of Sn 2+ ions heavily contribute to the hole conduction path in SnO.
Abstract: Tin monoxide (SnO) is a stable p-type oxide semiconductor. This paper reports electrical properties, electronic structures, and thin-film transistors (TFTs) of SnO. Epitaxial films were fabricated by pulsed laser deposition. The Hall mobility and the hole density of the epitaxial films were 2.4 cm 2 V -1 S -1 and 2.5 x 10 17 , respectively. X-ray photoelectron spectroscopy (PES) indicated that the closed-shell 5s 2 orbitals of Sn 2+ ions heavily contribute to the hole conduction path in SnO. Top gate type TFTs (W/L=300/50 μm) employing 20 nm thick SnO channels exhibited field-effect mobilities μ sat = 0.7 cm 2 V -1 s -1 and μ lin = 1.3 cm 2 V -1 s -1 , which are larger by two orders of magnitude than those reported for p-channel oxide TFrs to date. On/off current ratios were ~10 2 and subthreshold voltage swings (S) ~7 V/decade. The parameters required for TFT simulations were estimated by ultraviolet PES and first-principles calculations. The TFT simulations indicated that subgap hole trap density in the SnO channel is > 10 19 cm -3 , which limits the TFT mobilities and the S values.

Journal ArticleDOI
TL;DR: In this paper, the authors developed amorphous hafnium-indium-zinc oxide (HIZO) thin films as oxide semiconductors and investigated the films electrically and physically.
Abstract: We developed amorphous hafnium-indium-zinc oxide (HIZO) thin films as oxide semiconductors and investigated the films electrically and physically. Adding of hafnium (Hf) element can suppress growing the columnar structure and drastically decrease the carrier concentration and hall mobility in HIZO films. The thin film transistors (TFTs) with amorphous HIZO active channel exhibit good electrical properties with field effect mobility of around 10 cm2/Vs, S of 0.23 V/decade, and high Ion/off ratio of over 108, enough to operate the next electronic devices. In particular, under bias-temperature stress test, the HIZO TFTs with 0.3 mol % (Hf content) showed only 0.46 V shift in threshold voltage, compared with 3.25 V shift in HIZO TFT (0.1 mol %). The Hf ions may play a key role to improve the instability of TFTs due to high oxygen bonding ability. Therefore, the amorphous HIZO semiconductor will be a prominent candidate as an operation device for large area electronic applications.

Journal ArticleDOI
TL;DR: In this article, the buried interface composition of polymer-fullerene blends is found by near-edge x-ray absorption fine structure spectroscopy to depend on the surface energy of the substrate upon which they are cast.
Abstract: The buried interface composition of polymer-fullerene blends is found by near-edge x-ray absorption fine structure spectroscopy to depend on the surface energy of the substrate upon which they are cast. The interface composition determines the type of charge transport measured with thin film transistors. These results have implications for organic photovoltaics device design and the use of transistors to evaluate bulk mobility in blends.


Journal ArticleDOI
TL;DR: A quantitative study of the dynamics of threshold-voltage shifts with time in gallium-indium zinc oxide amorphous thin-film transistors using standard analysis based on the stretched exponential relaxation is presented in this paper.
Abstract: A quantitative study of the dynamics of threshold-voltage shifts with time in gallium-indium zinc oxide amorphous thin-film transistors is presented using standard analysis based on the stretched exponential relaxation. For devices using thermal silicon oxide as gate dielectric, the relaxation time is 3×105 s at room temperature with activation energy of 0.68 eV. These transistors approach the stability of the amorphous silicon transistors. The threshold voltage shift is faster after water vapor exposure suggesting that the origin of this instability is charge trapping at residual-water-related trap sites.