Subthreshold Gate Voltage Swing Research Articles

Junctionless Electric-Double-Layer TFTs on Paper Substrate

Junctionless Low-voltage electric-double-layer (EDL) thin-film transistors (TFTs) gated by solution-processed chitosan dielectric have been fabricated on paper substrate. The operating voltage is found to be as low as 1.0 V due to the large gate specific capacitance (5.4 μF cm−2 at 20 Hz) related to EDL effect induced by mobile ion. The field-effect mobility, subthreshold gate voltage swing and current on/off ratio is estimated to be 2.3 cm2 Vs−1, 110 mV decade−1 and 2 × 105, respectively. The reproducibility of pulse respond and stability of such device are also demonstrated, which indicates that the Junctionless Low-voltage EDL TFT has a potential for flexible electronics.

Fabrication of high-performance InGaZnOx thin film transistors based on control of oxidation using a low-temperature plasma

This work demonstrated the low-temperature control of the oxidation of Amorphous InGaZnOx (a-IGZO) films using inductively coupled plasma as a means of precisely tuning the properties of thin film transistors (TFTs) and as an alternative to post-deposition annealing at high temperatures. The effects of the plasma treatment of the as-deposited a-IGZO films were investigated by assessing the electrical properties of TFTs incorporating these films. A TFT fabricated using an a-IGZO film exposed to an Ar-H2-O2 plasma at substrate temperatures as low as 300 °C exhibited the best performance, with a field effect mobility as high as 42.2 cm2 V−1 s−1, a subthreshold gate voltage swing of 1.2 V decade−1, and a threshold voltage of 2.8 V. The improved transfer characteristics of TFTs fabricated with a-IGZO thin films treated using an Ar-H2-O2 plasma are attributed to the termination of oxygen vacancies around Ga and Zn atoms by OH radicals in the gas phase.

Process controllability of inductively coupled plasma-enhanced reactive sputter deposition for the fabrication of amorphous InGaZnOx channel thin-film transistors

The process controllability of inductively coupled plasma-enhanced reactive sputter deposition for the fabrication of amorphous InGaZnOx (a-IGZO) channel thin-film transistors (TFTs) was investigated. a-IGZO film deposition with the addition of H2 gas was performed using a plasma-assisted reactive sputtering system to control the oxidation process during a-IGZO film formation by balancing the oxidation and reduction reactions. Optical emission spectroscopy measurements indicate the possibility for the oxidation reaction to be inhibited by a decrease in the density of oxygen atoms and the reduction effect of hydrogen during a-IGZO film deposition due to the addition of H2 gas. The characteristics of TFTs fabricated using a-IGZO films deposited with a plasma-enhanced magnetron sputtering deposition system were investigated. The results indicate the possibility of expanding the process window by controlling the balance between oxidation and reduction with the addition of H2 gas. TFTs with a-IGZO films that were deposited with the addition of H2 gas exhibited good performance with a field-effect mobility (μFE) of 15.3 cm2 V−1 s−1 and a subthreshold gate voltage swing (S) of 0.48 V decade−1.

High-Performance Homojunction a-IGZO TFTs With Selectively Defined Low-Resistive a-IGZO Source/Drain Electrodes

We report high-performance homojunction amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with low-resistive a-IGZO source/drain (S/D) electrodes. The a-IGZO S/D electrodes are selectively treated with high-power NF3 plasma, which reduces their resistivity from ~16 to 5.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> Ω · cm. X-ray photoelectron spectroscopy indicates an increase in weakly bonded oxygen and a substantial amount of indium-fluorine and zinc-fluorine bonds at the a-IGZO top surface (extending to ~7 nm into the bulk) after plasma treatment. Temperature-dependent conductivity measurements show metallic behavior of the a-IGZO after treatment. It is concluded that fluorine atoms substitute for oxygen atoms-generating free electrons in the process and/or occupy oxygen vacancy sites-eliminating electron trap sites. As a result, the homojunction TFTs show good ON-state characteristics with typical field-effect mobility, subthreshold gate-voltage swing, and turn-ON voltage of 19 ± 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s, 178 ± 30 mV/decade, and -3.2 ± 1.5 V, respectively. Good stability at high temperature and under bias and light stress are also exhibited by the homojunction TFTs, verifying a stable doping effect by the NF3 plasma treatment.

Fabrication of high-performance ultra-thin-body SnO2 thin-film transistors using microwave-irradiation post-deposition annealing

We report on the fabrication of high-performance ultra-thin-body (UTB) SnO2 thin-film transistors (TFTs) using microwave-irradiation post-deposition annealing (PDA) at a low process temperature (&amp;lt;100 °C). We confirm that the electrical characteristics of SnO2 TFTs become drastically enhanced below a body thickness of 10 nm. The microwave-annealed UTB SnO2 TFTs with a thickness of 5 nm exhibited increased optical transmittance, as well as remarkable transfer characteristics: a high mobility of 35.4 cm2 V−1 s−1, a drain current on/off ratio of 4.5 × 107, a steep subthreshold gate voltage swing of 623 mV/dec, and a clear enhancement-mode behavior. Additionally, the microwave-annealed SnO2 TFTs exhibited a better positive gate-bias stress/negative gate-bias stress immunity than thermally annealed SnO2 TFTs. Therefore, the thickness of the UTB SnO2 TFTs, as well as the microwave-annealing process, are both shown to be essential for transparent and flexible display technology.

Effects of controlling the interface trap densities in InGaZnO thin-film transistors on their threshold voltage shifts

In this paper, the threshold voltage stability characteristics of indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFT) are discussed. The IGZO TFTs were found to induce a parallel threshold voltage (Vth) shift with changing field effect mobility (μFE) or a sub-threshold gate voltage swing (SS) due to various thermal annealing conditions. The IGZO TFT that was post-annealed in an O2 ambient was found to be more stable for use in oxide-based TFT devices and to have better performance characteristics, such as the on/off current ratio (Ion/off ), SS, and Vth, than other TFTs did. The mechanism for improving the Vth stability in the post-annealed IGZO TFT is a decrease in the number of trap sites for the electrons and the weak oxygen bonding in the IGZO thin films. The device’s performance could be significantly affected by adjusting the annealing conditions. This mechanism is closely related to that of modulation annealing, where the number of localized trapped carriers and defect centers at the interface or in the channel layer are reduced.

Nanocrystalline cellulose applied simultaneously as the gate dielectric and the substrate in flexible field effect transistors

Cotton-based nanocrystalline cellulose (NCC), also known as nanopaper, one of the major sources of renewable materials, is a promising substrate and component for producing low cost fully recyclable flexible paper electronic devices and systems due to its properties (lightweight, stiffness, non-toxicity, transparency, low thermal expansion, gas impermeability and improved mechanical properties). Here, we have demonstrated for the first time a thin transparent nanopaper-based field effect transistor (FET) where NCC is simultaneously used as the substrate and as the gate dielectric layer in an ‘interstrate’ structure, since the device is built on both sides of the NCC films; while the active channel layer is based on oxide amorphous semiconductors, the gate electrode is based on a transparent conductive oxide. Such hybrid FETs present excellent operating characteristics such as high channel saturation mobility (>7 cm2 V −1 s−1), drain–source current on/off modulation ratio higher than 105, enhancement n-type operation and subthreshold gate voltage swing of 2.11 V/decade. The NCC film FET characteristics have been measured in air ambient conditions and present good stability, after two weeks of being processed, without any type of encapsulation or passivation layer. The results obtained are comparable to ones produced for conventional cellulose paper, marking this out as a promising approach for attaining high-performance disposable electronics such as paper displays, smart labels, smart packaging, RFID (radio-frequency identification) and point-of-care systems for self-analysis in bioscience applications, among others.

The electrical, optical, and structural properties of amorphous indium gallium zinc oxide films and channel thin-film transistors

The electrical, optical, and structural properties of amorphous indium gallium zinc oxide (a-IGZO) films deposited at room temperature (RT) examined before and after annealing using a radio frequency (RF) magnetron sputtering system with different hydrogen and oxygen gas flow ratios. The carrier concentration and resistivity of the a-IGZO films fabricated under O2/Ar+O2 and O2/Ar–4%H2+O2 atmospheres were greatly dependent on the addition of hydrogen and heat treatment. Thin-film transistors (TFTs) with an a-IGZO channel layer deposited under O2/Ar–4%H2+O2=1.6% exhibited good subthreshold gate voltage swing (S), on/off ratio, threshold voltage and μFE of 0.4Vdecade−1, 108, 0.3V and 4.8cm2V−1s−1, respectively. From analysis of the interfacial structure in TFTs before and after annealing, the electrical conductivity of the a-IGZO channel layer was greatly affected in regard to TFT performance due to the amorphous a-IGZO channel layer and SiO2 gate insulator.

Low-voltage antimony-doped SnO2 nanowire transparent transistors gated by microporous SiO2-based proton conductors

A battery drivable low-voltage transparent lightly antimony(Sb)-doped SnO2 nanowire electric-double-layer (EDL) field-effect transistor (FET) is fabricated on an ITO glass substrate at room temperature. An ultralow operation voltage of 1 V is obtained on account of an untralarge specific gate capacitance (∼ 2.14 μF/cm2) directly bound up with mobile ions—induced EDL (sandwiched between the top and bottom electrodes) effect. The transparent FET shows excellent electric characteristics with a field-effect mobility of 54.43 cm2/V·s, current on/off ration of 2 × 104, and subthreshold gate voltage swing (S = dVgs/d(logIds)) of 140 mV/decade. The threshold voltage Vth (0.1 V) is estimated which indicates that the SnO2 namowire transistor operates in an n-type enhanced mode. Such a low-voltage transparent nanowire transistor gated by a microporous SiO2-based solid electrolyte is very promising for battery-powered portable nanoscale sensors.

Flexible Electronics Based on InGaZnO Transparent Thin Film Transistors

There is increasing interest in use of conducting oxide materials in new forms of transparent, flexible or wearable electronics on cheap substrates, including paper. While Si-based thin film transistors (TFTs) are widely used in displays, there are some drawbacks such as light sensitivity and light degradation and low field effect mobility (&lt;1 cm2/Vs). For example, virtually all liquid crystal displays (LCDs) use TFTs imbedded in the panel itself. One of the promising alternatives to use of Si TFTs involves amorphous or nanocrystalline n-type oxide semiconductors. There have been promising results with zinc oxide, indium gallium oxide and zinc tin oxide channels. In this paper, recent progress in these new materials for TFTs on substrates such as paper is reviewed. In addition, InGaZnO transistor arrays show promise for driving laminar electroluminescent, organic light-emitting diode (OLED) and LCD displays. These transistors may potentially operate at up to an order of magnitude faster than Si TFTs. We have fabricated bottomgate amorphous (α-) indium-gallium-zinc-oxide (InGaZnO4) thin film transistors (TFTs) on both paper and glass substrates at low processing temperature (≤100°C). As a water and solvent barrier layer, cyclotene (BCB 3022-35 from Dow Chemical) was spin-coated on the entire paper substrate. TFTs on the paper substrates exhibited saturation mobility (μsat) of 1.2 cm2.V-1.s-1, threshold voltage (VTH) of 1.9V, subthreshold gate-voltage swing (S) of 0.65V.decade-1, and drain current onto- off ratio (ION/IOFFSubscript text) of ~104. These values were only slightly inferior to those obtained from devices on glass substrates (μsat~2.1 cm2.V-1.s-1, VTH ~0 V, S~0.74 V.decade-1, and ION/IOFF=105- 106). The uneven surface of the paper sheet led to relatively poor contact resistance between source-drain electrodes and channel layer. Future areas for development are identified.

Low-Voltage Organic/Inorganic Hybrid Transparent Thin-Film Transistors Gated by Chitosan-Based Proton Conductors

Solution-processed solid chitosan-based proton conductor film shows a large specific gate capacitance of ~7.47 μF/cm2 due to the interfacial electric-double-layer effect. Low-voltage (1.5 V) organic/inorganic hybrid transparent thin-film transistors with patterned indium-tin-oxide channels gated by chitosan films are self-assembled on glass substrates by only one metal shadow mask. The subthreshold gate voltage swing, current on/off ratio, and field-effect mobility are estimated to be 93 mV/dec, ~106, and 5.74 cm2·V-1·s-1, respectively. The reproducibility of pulse respond of such device is also demonstrated, which indicates the absence of electrochemical doping of the channel at the interface.

Investigation on the Doping Dependence of Solution-Processed Zinc Tin Oxide Thin Film and Thin-Film Transistors

The effect of the tin (Sn) concentration in zinc tin oxide (ZTO) films fabricated using a spin-coating process and its effect on ZTO channel thin film transistors (TFTs) with various Sn concentrations were examined. Spin-coated ZTO films with various Sn concentrations were nanocrystalline and had high transmittance (>85%) in the visible region. As the Sn concentration increased, the carrier concentration and resistivity of the nanocrystalline ZTO (nc-ZTO) films ranged from 9.6 × 1014 cm−3 to 2.2 × 1016 cm−3 and from 1.5 × 103 Ω-cm to 1.6 × 102 Ω-cm, respectively. The nc-ZTO channel TFTs that were deposited as a function of Sn and Zn concentrations exhibited a subthreshold gate voltage swing (S) of 1.1–1.2 V decade−1, an on/off ratio of 105–106, a threshold voltage (Vth) of -0.8–1.7 V, and a μFE value of 2.4–2.6 cm2 V−1 s−1. The threshold voltage shift toward the negative gate bias as the Sn concentration in the nc-ZTO TFTs increased indicates the existence of sufficient charge carriers needed to form conductive channels.

High-performance ZnO thin-film transistor fabricated by atomic layer deposition

We report the fabrication and characteristics of a ZnO thin-film transistor (TFT) using a 50 nm thick ZnO film as an active layer on an Al2O3 gate dielectric film deposited by atomic layer deposition. Lowering the deposition temperature allowed the control of the carrier concentration of the active channel layer (ZnO film) in the TFT device. The ZnO TFT fabricated at 110 °C exhibited high-performance TFT characteristics including a saturation field-effect mobility of 11.86 cm2 V−1 s−1, an on-to-off current ratio of 3.09 × 107 and a sub-threshold gate-voltage swing of 0.72 V decade−1.

Enhanced performance of a-IGZO thin-film transistors by forming AZO/IGZO heterojunction source/drain contacts

A low-cost Al-doped ZnO (AZO) thin film was deposited by radio-frequency magnetron sputtering with different Ar/O2 flow ratios. The optical and electrical properties of an AZO film were investigated. A highly conductive AZO film was inserted between the amorphous InGaZnO (a-IGZO) channel and the metal Al electrode to form a heterojunction source/drain contact, and bottom-gate amorphous a-IGZO thin-film transistors (TFTs) with a high κ HfON gate dielectric were fabricated. The AZO film reduced the source/drain contact resistivity down to 79 Ω cm. Enhanced device performance of a-IGZO TFT with Al/AZO bi-layer S/D electrodes (W/L = 500/40 µm) was achieved with a saturation mobility of 13.7 cm2 V−1 s−1, a threshold voltage of 0.6 V, an on-off current ratio of 4.7 × 106, and a subthreshold gate voltage swing of 0.25 V dec−1. It demonstrated the potential application of the AZO film as a promising S/D contact material for the fabrication of the high performance TFTs.

High-Performance a-IGZO Thin-Film Transistor Using $ \hbox{Ta}_{2}\hbox{O}_{5}$ Gate Dielectric

In this letter, we report the fabrication of an amorphous indium gallium zinc oxide (a-IGZO) thin-film transistor with a high-k dielectric layer on a glass substrate. The room-temperature-deposited a-IGZO channel with Ta2O5 exhibits the following operating characteristics: a threshold voltage of 0.25 V, a drain-source current on/off ratio of 105, a subthreshold gate voltage swing of 0.61 V/decade, and a high field-effect mobility of 61.5 cm2/V·s; these characteristics make it suitable for use as a switching transistor and in low-power applications.

Indium Oxide Thin-Film Transistors Fabricated by RF Sputtering at Room Temperature

Thin-film transistors (TFTs) were fabricated using an indium oxide (In2O3) thin film as the n-channel active layer by RF sputtering at room temperature. The TFTs showed a thickness-dependent performance in the range of 48-8 nm, which is ascribed to the total carrier number in the active layer. Optimum device performance at 8-nm-thick In2O3 TFTs had a field-effect mobility of 15.3 cm2 · V-1 · s-1, a threshold voltage of 3.1 V, an ON-OFF current ratio of 2.2 × 108, a subthreshold gate voltage swing of 0.25 V · decade-1, and, most importantly, a normally OFF characteristic. These results suggest that sputter-deposited In2O3 is a promising candidate for high-performance TFTs for transparent and flexible electronics.

Low-voltage indium gallium zinc oxide thin film transistors on paper substrates

We have fabricated bottom-gate amorphous (α-) indium-gallium-zinc-oxide (InGaZnO4) thin film transistors (TFTs) on both paper and glass substrates at low processing temperature (≤100 °C). As a water and solvent barrier layer, cyclotene (BCB 3022–35 from Dow Chemical) was spin-coated on the entire paper substrate. TFTs on the paper substrates exhibited saturation mobility (μsat) of 1.2 cm2 V−1 s−1, threshold voltage (VTH) of 1.9 V, subthreshold gate-voltage swing (S) of 0.65 V decade−1, and drain current on-to-off ratio (ION/IOFF) of ∼104. These values were only slightly inferior to those obtained from devices on glass substrates (μsat∼2.1 cm2 V−1 s−1, VTH∼0 V, S∼0.74 V decade−1, and ION/IOFF=105–106). The uneven surface of the paper sheet led to relatively poor contact resistance between source-drain electrodes and channel layer. The ability to achieve InGaZnO TFTs on cyclotene-coated paper substrates demonstrates the enormous potential for applications such as low-cost and large area electronics.

Low-voltage transparent electric-double-layer ZnO-based thin-film transistors for portable transparent electronics

Room-temperature deposited 8.0 μm-thick mesoporous SiO2 dielectric shows a huge electric double layer (EDL) gate specific capacitance (4.16 μF/cm2). Battery drivable low-voltage (1.5 V) transparent EDL thin-film transistors (TFTs) with Al-doped ZnO nanocrystal channel layer gated by such dielectric are fabricated at room-temperature. The TFTs exhibit high-performance n-type transistor characteristics with a high field-effect mobility of 14.9 cm2/V s. The current on/off ratio and subthreshold gate voltage swing are estimated to be 2×106 and 82 mV/decade, respectively. Our results demonstrate that mesoporous SiO2 dielectrics with EDL effect are very promising for battery-powered portable transparent macroelectronics on temperature-sensitive substrates.

Improvement in bias stability of amorphous-InGaZnO4 thin film transistors with SiOx passivation layers

The authors investigated the effect of SiOx passivation layers on the bias stability of bottom gate amorphous (α-) InGaZnO4 thin film transistors (TFTs) fabricated on glass substrates. The use of rapid thermal annealing for unpassivated TFTs in air improved the device performance, showing larger drain current and field effect mobility compared to the as-fabricated TFTs. Threshold voltage (VTH) and subthreshold gate-voltage swing (S) for both unpassivated and passivated devices were found to be nearly independent of the low-gate-voltage stress (5V), but both were strongly affected under a relatively high-voltage stress (&amp;gt;10V). The positive VTH and S shifts after constant gate voltage stress (+20V) of 1000s were 1.8V and 0.72Vdecade−1 for the unpassivated devices and 1V and 0.42Vdecade−1 for the passivated devices, respectively. These results demonstrate that the SiOx passivation layer significantly reduced the shift in TFT’s characteristics.

High Mobility a-IGO Films Produced at Room Temperature and Their Application in TFTs

The effect of oxygen partial pressure on the properties of In 2 O 3 Ga 2 O 3 thin films produced by sputtering at room temperature aimed at thin film transistor (TFT) application is reported in this work. When produced in the absence of oxygen, the films are polycrystalline, while in the presence of oxygen, the films are amorphous. The films' resistivity is tuned between 10 -3 and 10 4 Ω cm. Moreover, the films present a high transmittance (>80%) and a smooth surface (r rms = 1.2 nm). The high performance as-produced transistors present high saturation mobility (μ sat ≈ 43 cm 2 /V s) and a subthreshold gate-voltage swing of 0.51 V/dec, which is reduced to 0.27 V/dec after 150°C annealing.