Thin-film Transistors Parameters Research Articles

Correlation between CO2 Sensitivity and Channel-Layer Thickness in In2O3 Thin-Film Transistor Gas Sensors

CO2 monitoring is important for achieving net-zero emissions. Here, we report on a CO2 gas sensor based on an In2O3 thin-film transistor (TFT), which is expected to realize both low-temperature operation and high sensitivity. The effect of channel thickness on TFT performance is well known; however, its effect on CO2 sensitivity has not been fully investigated. We fabricated In2O3 TFTs of various thicknesses to evaluate the effect of channel thickness on CO2 sensitivity. Consequently, TFT gas sensors with thinner channels exhibited higher CO2 sensitivity. This is because the surface effect is more prominent for a thinner film, suggesting that charge transfer between gas molecules and the channel surface through gas adsorption has a significant impact on changes in the TFT parameters in the subthreshold region. The results showed that the In2O3 TFT in thin channels is a promising candidate for CO2-sensitive TFT gas sensors and is useful for understanding an effect of gas adsorption in oxide TFTs with a very thin channel as well.

Strengthening Multi-Factor Authentication Through Physically Unclonable Functions in PVDF-HFP-Phase-Dependent a-IGZO Thin-Film Transistors.

For enhanced security in hardware-based security devices, it is essential to extract various independent characteristics from a single device to generate multiple keys based on specific values. Additionally, the secure destruction of authentication information is crucial for the integrity of the data. Doped amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) using poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) induce a dipole doping effect through a phase-transition process, creating physically unclonable function (PUF) devices for secure user information protection. The PUF security key, generated at VGS = 20V in a 20×10 grid, demonstrates uniformity of 42% and inter-Hamming distance (inter-HD) of 49.79% in the β-phase of PVDF-HFP. However, in the γ-phase, the uniformity drops to 22.5%, and inter-HD decreases to 35.74%, indicating potential security key destruction during the phase transition. To enhance security, a multi-factor authentication (MFA) system is integrated, utilizing five security keys extracted from various TFT parameters. The security keys from turn-on voltage (VON), VGS = 20V, VGS = 30V, mobility, and threshold voltage (Vth) exhibit near-ideal uniformities and inter-HDs, with the highest values of 58% and 51.68%, respectively. The dual security system, combining phase transition and MFA, establishes a robust protection mechanism for privacy-sensitive user information.

A 6-bit Analog-to-Digital Converter for Electromyography Implemented Using N-Type Metal-Oxide Thin-Film Transistors

Consisting of more than 2000 n-type metal-oxide thin-film transistors (TFTs), a 6-bit analog-to-digital converter (ADC) adopting the subranging architecture is designed, fabricated, characterized and applied to digitize an electromyogram (EMG). Consisting of amplifiers each with a feedback stage for gain-boosting, the comparators at the heart of the ADC incorporate a scheme for compensating the offset error arising from the inevitable non-uniformity of the TFT parameters. Compared to the specifications of reported ADCs based on TFTs, a higher sampling rate of 1000 S/s, a smaller differential non-linearity of 0.76 least-significant bit (LSB) and a smaller integral non-linearity of 0.88 LSB are achieved. Since the same low-temperature TFT technology for fabricating the EMG acquisition system on a flexible substrate is used to construct the ADC, the two could be readily monolithically integrated.

Improvement in the electrical properties of a-IGZO TFTs by using PA-PI as a modification layer

High performance display requires thin film transistors (TFTs) to have better electrical characteristics. In this study, we used photoalignment polyimide (PA-PI) as the modification layer to modify the morphology of the insulating layer and improve the contact characteristics between the insulating layer and the active layer, so as to improve the electrical properties of amorphous Indium Gallium Zinc Oxide (a-IGZO) TFTs. The modified layer was prepared on the polymethylmethacrylate (PMMA) insulating layer and then irradiated with 254 nm UV polarized light. The anisotropy and surface morphology of the modified layer were investigated, and devices based on different insulating layer structures were prepared and tested. The typical device parameters of IGZO TFTs prepared under optimal conditions showed a high saturation mobility (μsat) of 9.68 cm2/V·s, the switching ratio was 1.01 × 105, the threshold voltage was 3.7 V, and the sub-threshold swing was 2.1 V/dec. The highest saturated carrier mobility μsat increased by 145% compared with PMMA single insulation layer devices.

Micron channel length ZnO thin film transistors using bilayer electrodes

Micro light-emitting diodes (Micro-LEDs) are currently attracting more and more attention. Thin film transistors (TFTs) with micron channel lengths can be used to drive Micro-LEDs. The key parameters of TFTs, such as mobility, ION/IOFF and threshold voltage, still need to be improved. In this study, we propose and experimentally demonstrate ZnO TFTs using bilayer electrodes to overcome the short channel effects when the channel length is scaled down to 3 μm. Ti, Mo and Sn interlayers not only serve as diffusion barriers to prohibit migration of Cu atoms from the top electrodes, but also enhance adhesive energy of the metal electrodes on ZnO channel layers. ZnO TFTs using Cu/Ti bilayer electrodes exhibit the best performance, e.g., a high mobility of 45.3 cm2V-1s−1, a high ION/IOFF ratio of 4.28 × 109, a low subthreshold of 0.24 V/dec and a proper threshold voltage of 1.13 V. The high mobility can be attributed to a significant decrease of the barrier height and a slight narrowing of the space charge layer, and the high ratio of ION/IOFF is concerned with the high electron concentration under an ON-state condition. Thus, ZnO TFTs using Cu/Ti bilayer electrodes can be used in next-generation displays.

Frequency doubler based on unipolar thin-film-transistor technologies

In this study, a new simulation-based design of thin-film-transistor (TFT) frequency-doubling circuits is proposed. Considering the increasingly high technological relevance of novel TFT channel materials showing strongly unipolar charge transport characteristics, the proposed frequency doubler adopts a unipolar circuit topology composed of organic p-type TFTs as a model system. By comparing different circuit structures and optimizing key TFT parameters, an apparent symmetry at the specific, non-traditional voltage-transfer curve of the simple two-transistor logic inverter is obtained, which is directly exploited to produce an ideal frequency-multiplying behavior over a wide operational frequency range.

A novel extraction method of device parameters for thin-film transistors (TFTs)

Accurately extracting device parameters plays an import role in the application and development of TFTs. Although several methods have been proposed to extract the device parameters of TFT devices with amorphous channel materials, these current methods are unsuitable for AOS TFT devices, due to the different charge transport mechanisms. Here, a novel method of extracting device parameters has been presented for IGZO TFTs. The proposed method can accurately and quickly extract device parameters of TFTs through the simple mathematical process for transfer curves. Based on the proposed method, the critical parameters in IGZO TFT devices, such as γ, Vth and μ0, were obtained. And the temperature dependence of device parameters has been discussed in detail.

Fabrication of Amorphous ZnO TFT with Tunable Channel Length

The paper discusses the fabrication of amorphous zinc oxide (a:ZnO)-top gate top contact (the gate is situated on the top and the contacts are taken from the top) thin-film transistor (TFT) using spin-spray pyrolysis (SSP) unit. The solutions of ZnO and poly-vinyl-alcohol (PVA) are synthesized separately by the sol–gel method. The ZnO and PVA solution are coated over glass substrate using the SSP unit and drop-cast technique, respectively. These films are characterized using an optical profilometer, X-ray diffractometer, energy dispersive X-ray analysis, ultraviolet–visible spectrophotometry and Keithley measurement system. The XRD and EDAX measurement confirm the amorphous nature and composition respectively. The thickness of the ZnO and PVA film are 460 nm and 600 nm, respectively. The resistance of ZnO film is 100 KΩ. Using the absorption spectrum, we calculate the bandgap of ZnO and PVA thin film samples which are 3.05 eV and 3.9 eV respectively. The metal–insulator–metal (MIM) structure is fabricated using fluorine-doped tin oxide, PVA and silver paste. The MIM structure is characterized for the electrical parameters such as capacitance and dielectric constant $$(\upvarepsilon r)$$ which is found to be 0.4 μF and 8 respectively at 40 Hz frequency. The TFT is fabricated by depositing the aluminium as electrode, ZnO as channel layer and and PVA as insulator layer of TFT with different W/L ratios. The TFT parameters such as threshold voltage (Vth), on/off current ratio (Ion/off) and mobility are studied and presented. The results confirm that Vth decreases with the shrinking of channel length. Current ratio Ion/off increases with the reduction of the channel length due to the increase in on-current value.

InZnON-based Thin Film Transistor for the Development of New Type Photo Detec-tors with a Colloidal Quantum Dots Sensitive Layer

Some applications of thin film transistors (TFTs) need the bottom-gate architecture and unpassivated channel backside. In present work, we propose rather simple way to fabricate nitrogen doped InZnO-based TFT by DC reactive magnetron sputtering technique. The moderate nitrogen doping of the channel's semiconductor layer is studied in terms of the reproducibility and stability device's electrical characteristics. When nitrogen concentration in gas mix reaches the upper level of 71 % the best TFT parameters are achieved such as VON = -0.3 V, μ = 12 cm2/Vs, SS = 0.5 V/dec. The TFTs operate in depletion mode exhibiting high turn on/turn off current ratio more than 106. It is shown that the oxidative post-fabrication annealing at 250 °C in pure oxygen and next ageing in dry air for several hours provide highly stable operational characteristics under negative and positive bias stresses despite open channel backside. The prospects of using the thin-film transistor for the new type of photo detectors with a colloidal quantum dots (CQDs) sensitive layer are demonstrated. The solution-cast colloidal-quantum-dots were decorated on the nitrogen doped InZnO layer by spin-coating method. N-type CdSe/ZnS CQDs modified by the ligand (pyridine) are utilized as electron donor to inject electron to the channel layer. Higher photocurrent responsibility about 104 A/W at incident monochromatic light 405 nm is reached for hybrid phototransistor.

Optimization of CNT and TFT Parameters for Maximum Transconductance and Safe Temperature Operation of Carbon Nanotube Thin-Film Transistors (CNT-TFTs) Employed in Flat Panel Displays

Thin Film Transistors (TFTs) have lived to see its significant technological improvement for various display applications in recent years. Carbon nanotube (CNT) based TFT technologies have been found to be a promising component for next generation flexible electronics and flat panel displays in view of CNTs high carrier mobility, device stability and mechanical flexibility. However, the design of CNT-TFT is still not well established, especially with a view to achieve the best performance still protecting thermal stability. In this study, the authors had analysed the device structure and operation of transistor in which carbon nanotubes act as active channel region. CNT-TFT with different device geometrics and CNT physical parameters such as channel length, channel width, CNT tube length, network density and its orientation have been extensively studied using NanoNet simulation tool. This study has thrown new insight into the device performance characteristics of CNT-TFTs. The results show that it is essential to fix the length of the channel more than 5 µm for restricting the device temperature at 300 K and it can be brought down as low as 3 µm if the maximum operating temperature can be 400 K. Comparison with already reported experimental results show that the TFT parameters returned by the simulation experiments and presented in this paper match closely.

On the reproducibility of spray-coated ZnO thin-film transistors

Recent studies have shown spray-pyrolysis is a low-cost, simple and efficient technique for deposition of metal oxide semiconductors (such as zinc oxide) as the active layer of thin-film transistors (TFTs). However, to allow the translation from laboratory scale to industry the reproducibility of such method needs to be evaluated. We present herein a representative study concerning the reproducibility and uniformity of spray-coated ZnO TFTs using several devices, from different production batches, following the same fabrication protocol. We demonstrate that it is possible to obtain transistors with high performance and reproducibility by controlling the most relevant deposition factors, corroborated by a low deviation rate (below 10%) from the characteristic TFT parameters (mobility in saturation, threshold voltage and on/off ratio), which is compatible to commonly available commercial electronic devices.

Improvement in bias-stress and long-term stabilities for in-Ga-Zn-O thin-film transistors using solution-process-compatible polymeric gate insulator

Abstract We fabricated the indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs) employing a new coatable polymeric insulator, Zeocoat™ (produced by ZEON Corp., Japan) as gate insulators. Zeocoat film thickness was modulated by controlling the dilution ratio and spin-speed. The film showed low leakage current density (10−7 A/cm2 at 1 MV/cm for 215-nm-thick film) and no significant differences in dielectric constants depending on the film thicknesses or the measurement frequencies. Typical device parameters of the IGZO TFTs fabricated with optimum conditions exhibited the carrier mobility at saturation region of 2.4 cm2/V·s and subthreshold swing of 0.25 V/dec. While there were no marked instabilities in the negative-bias stress tests, the threshold voltage shift under the positive-bias stress was reduced as small as 1.5 V by increasing the deposition temperature of the Al2O3 protection layer from 150 °C to 200 °C. The device stability for long shelf-time was also improved through 100-nm-thick Al2O3 passivation layer deposited on top of the TFT.

Simulation of the effect of deep defects created by hydrogen on the performance of amorphous indium gallium zinc oxide thin film transistor (a-IGZO TFT)

Amorphous indium gallium zinc oxide (a-IGZO) are promising for developing thin film transistors (TFTs) because of their large electron mobility, small threshold voltage (Vth), and low temperature fabrication process. In this study, we have investigated the effect of near valance band defects on the output parameters of a-IGZO TFTs by using two-dimensional TCAD numerical simulation. It was found that donor defects near the valance band have no effect. The degradation of the mobility also causes degradation in the TFT performance. Acceptor defects states near valance band is the reason of positive Vth shift. It is therefore concluded that near valance band defects are not donor defects but acceptor defects with a Gaussian distribution which can also degrade the mobility.

Ultra-Short Pulsed Laser Annealing Effects on MoS2 Transistors with Asymmetric and Symmetric Contacts

The ultra-short pulsed laser annealing process enhances the performance of MoS2 thin film transistors (TFTs) without thermal damage on plastic substrates. However, there has been insufficient investigation into how much improvement can be brought about by the laser process. In this paper, we observed how the parameters of TFTs, i.e., mobility, subthreshold swing, Ion/Ioff ratio, and Vth, changed as the TFTs’ contacts were (1) not annealed, (2) annealed on one side, or (3) annealed on both sides. The results showed that the linear effective mobility (μeff_lin) increased from 13.14 [cm2/Vs] (not annealed) to 18.84 (one side annealed) to 24.91 (both sides annealed). Also, Ion/Ioff ratio increased from 2.27 × 10 5 (not annealed) to 3.14 × 10 5 (one side annealed) to 4.81 × 10 5 (both sides annealed), with Vth shifting to negative direction. Analyzing the main reason for the improvement through the Y function method (YFM), we found that both the contact resistance (Rc) and the channel interface resistance (Rch) improves after the pulsed laser annealings under different conditions. Moreover, the Rc enhances more dramatically than the Rch does. In conclusion, our picosecond laser annealing improves the performance of TFTs (especially, the Rc) in direct proportion to the number of annealings applied. The results will contribute to the investigation about correlations between the laser annealing process and the performance of devices.

Low-temperature sol-gel ZrHfO2-PMMA hybrid dielectric thin-films for metal oxide TFTs

In this work, we developed a novel inorganic-organic hybrid gate dielectric by combining zirconium and hafnium components to form zirconium hafnium oxide strongly linked with polymethyl methacrylate (ZrHfO2-PMMA) and deposited at low temperature (200 °C) by sol-gel method. The obtained 108 nm thick, high-quality hybrid gate dielectric showed an exceptionally low surface roughness (0.9-nm), a low leakage current density (7.7 × 10−6 A/cm2) and reasonable dielectric properties such as gate capacitance along with dielectric constant (77 nF/cm2 & 9.4 @1 kHz) respectively. To examine the ZrHfO2-PMMA hybrid dielectric electrical properties we constructed thin-film transistors (TFTs) with room temperature r.f sputtered n-type metal oxide semiconductors, a-IGZO and ZnO, as active channels. The bottom gate fabricated a-IGZO TFTs driving at as low as below 6 V, with extracted field effect mobility of 2.45 cm2/V. s, a low threshold voltage of 1.2 V with large ON/OFF current ratio 107 respectively. On the other hand, for comparison we employed ZnO TFTs by applying same hybrid dielectric system, the obtained parameters of bottom gate ZnO TFTs were good field effect saturation mobility of 12.8 cm2/V. s, threshold voltage of 1.8 V and ON/OFF current ratio of 103.

Effective Atmospheric-Pressure Plasma Treatment toward High-Performance Solution-Processed Oxide Thin-Film Transistors.

Solution-processed oxide semiconductors (OSs) have attracted much attention because they can simply, quickly, and cheaply produce transparent channels on flexible substrates. However, despite such advantages, in the fabrication process of OS thin-film transistors (TFTs) using the solution process, it is a fatal problem that there are hardly any ways to simply and effectively control important TFT parameters, including the turn-on voltage ( Von) and on/off current ratio. For the practical application of solution-processed OS TFT, approaches to simply and effectively control the parameters are urgently required. Here, we newly propose an atmospheric-pressure plasma (APP) treatment that can simply and effectively control the electrical properties in solution-processed InO x TFTs. Through exposure of APP, we successfully realized the changes in important TFT parameters of solution-processed InO x TFT, Von from -11.4 to -1.9 V and the on/off current ratio from ∼103 to ∼106, which still keep up the high field-effect mobility (>20 cm2 V-1 s-1). On the basis of various analyses such as X-ray-based analysis and UV-visible spectroscopy, we identified that the APP treatment can effectively control oxygen vacancy and carrier concentration in solution-processed OS.

An Analytical Surface-Potential-Based Drain Current Model of Full Depleted Polysilicon TFTs

We propose an explicit surface-potential-based direct current (dc) model for fully depleted polysilicon (poly-Si) thin-film transistors (TFTs). Being analytical and explicit, the proposed model is a solid base for compact models of these devices. The essential for our model is the explicit calculation of the surface potential, using the Lambert W function. Subsequently, using the surface potential, we also develop the expressions for charge and current, employing well established approaches, which include integration of carrier’s charge for drain current. Furthermore, the solutions of surface potential and drain current were validated by numerical results and experimental data, respectively. Finally, we discussed in detail about the effects on the surface potential and dc properties from the full depleted poly-Si TFT parameters. As a result, such a model is suitable to be implemented into computer-aided simulations of both device and circuit designs, due to its high computational accuracy and efficiency.

5.8‐inch QHD flexible AMOLED display with enhanced bendability of LTPS TFTs

AbstractBy applying the curve‐type thin film transistor (TFT) with longitudinal strain, TFT parameters do change little down to the 2R bending. The mobility variation range reduces down to 4% compared with 28% of the line‐type channel with transverse strain. The smaller variation is preferred for a high quality display. We clarified that majority carrier's effective mass and scattering rate are dominant factors influencing the bended TFT's performance, which can be controlled by the strain orientation and channel shape. This understanding and improvement was embedded in the 5.8″ flexible QHD active matrix organic light emitting diode panel with multi edge curvature of Galaxy S8. Through this achievement, we made our flexible premium active matrix organic light emitting diode panels more performable, reliable, and highly productive in small R bending circumstance.

Modulating Thin Film Transistor Characteristics by Texturing the Gate Metal

The development of reliable, high performance integrated circuits based on thin film transistors (TFTs) is of interest for the development of flexible electronic circuits. In this work we illustrate the modulation of TFT transconductance via the texturing of the gate metal created by the addition of a conductive pattern on top of a planar gate. Texturing results in the semiconductor-insulator interface acquiring a non-planar geometry with local variations in the radius of curvature. This influences various TFT parameters such as the subthreshold slope, gate voltage at the onset of conduction, contact resistance and gate capacitance. Specific studies are performed on textures based on periodic striations oriented along different directions. Textured TFTs showed upto ±40% variation in transconductance depending on the texture orientation as compared to conventional planar gate TFTs. Analytical models are developed and compared with experiments. Gain boosting in common source amplifiers based on textured TFTs as compared to conventional TFTs is demonstrated.

Zinc tin oxide thin film transistor: Effect on channel isolation and channel width

The n-channel field-effect thin-film transistors (TFTs) with amorphous zinc tin oxide as an active layer has been investigated. The transparent ZTO thin films deposited at room temperature using rf magnetron sputtering have an optical bandgap of 3.3 eV. Before the fabrication of TFT postdeposition annealing of the film was carried out 300°C for 1 hour. The channel layer on Si/SiO2 substrate was deposited in two different structures, isolating each device from the neighboring devices and without isolation. The isolation of active material improves TFT performance by reducing the gate leakage current and parasitic bias stress. The on-off ratio and subthreshold swing were significantly changed from 2.3x103 and 0.73 V/dec to 2.55x106 and 0.23 V/dec respectively on isolation. The effect of channel width on the TFT parameters such as saturated mobility, subthreshold swing and threshold voltage are also studied. The threshold voltage decreases with increasing channel width but mobility and subthreshold swing are relatively constant. The threshold voltage decreased from 19 V to 11 V when the channel width increased. It may be due to the increasing parasitic resistance between the source/drain contacts and the ZTO channel layer.