Amorphous Silicon Thin Film Transistors Research Articles

A new decoder-type integrated gate driver with a-Si:H TFTs for active-matrix displays

In this paper, we propose a new integrated hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) gate driver circuit based on a decoder with parallel TFTs. All a-Si:H TFTs in the proposed gate driver have duty ratios of 50% or less to suppress the threshold voltage (VTH) shift, but at the same time, the output can avoid a high-impedance state to resist against noises. The proposed gate driver also removes dead time, and reduces the circuit area and the number of TFTs compared with the previously reported decoder-type and demultiplexer-type integrated gate drivers. The simulation results show that the rising time and falling time are 2.47 and 2.43 µs, respectively, with a −5 to 30 V output voltage swing, which are suitable for full high-definition (full-HD) format active-matrix displays at a 120 Hz frame frequency.

High-reliability and low-noise amorphous-silicon gate with a novel clock-driving methodology

In this paper, for the first time, a novel amorphous-silicon thin-film transistor gate drive circuit and its successfully improved dynamic characteristics are presented. Not only was the output ripple suppressed; the rate of threshold voltage shift was also reduced by up to 20%. About 50% power-saving was also estimated. The amorphous-silicon gate driver circuit was further fabricated, and the measured results show the high practicability of the achieved design.

Introduction to the Focus Issue on Oxide Thin Film Transistors

The thin film transistor (TFT) operates on the same basic principle as the metal–oxide–semiconductor field-effect transistor (MOSFET). TFT-based liquid crystal displays (LCDs) and MOSFET-based integrated circuits (ICs) are two of the largest semiconductor products currently in production. They have a similar history of market growth, as shown in Fig. 1. The main advantages of the TFT over the MOSFET are a low fabrication temperature and the sizeand wafer-independent substrate. In addition to LCDs, TFTs have been used in a large number of products, including organic light-emitting diodes (OLEDs), other types of flat panel displays, X-ray imagers, radio-frequency identification, finger printers, sensors, etc. In spite of the successful demonstration of functional TFTs since the 1960s, the mass production of TFT products, e.g., large-area LCDs, has never been realized. The breakthrough in the field came from the report of the first functional amorphous silicon (a-Si:H) TFT with a silicon nitride gate dielectric layer in 1979. Today’s TFT LCDs are dominated by this technology, specifically large area displays. A small portion of the business, such as small mobile displays, is based on the polycrystalline silicon (poly-Si) TFT technology. The a-Si:H TFT suffers from low mobility. The poly-Si TFT is hindered by the large area, high throughput production problem. The dream of having a TFT technology that can be easily fabricated on a large-area, low-temperature substrate with a high throughput has never died. In recent years, metal oxide TFTs that can be prepared by sputter deposition at room temperature have been reported. This kind of TFT can have a mobility one order of magnitude larger than that of the a-Si:H TFT or similar to that of the poly-Si TFT. The oxide TFTs may also be useful for transparent electronics. This result attracted immediate attention from many academic and industry researchers. Many papers on oxide TFTs have been published in various types of journals and presented at conferences, such as the ECS Journal of Solid State Science and Technology, ECS Solid State Letters, and ECS Transactions volumes. The ECS TFT symposium series, which have been held regularly for more than 24 years, are popular platforms for presenting new results. Because we are still in the early production stage for oxide TFTs, there are many unsolved issue that remain under intensive study. For that reason, it was felt that a focus issue covering up-to-date research efforts would be valuable for those working in these areas. This issue of the ECS Journal of Solid State Science and Technology is devoted to the publication of scientific and technological papers

In brief

PAGE 105 The transient off-state thermal emission leakage currents of hydrogenated amorphous silicon and indium gallium zinc oxide metal oxide thin-film transistors fabricated on flexible plastic substrates have been examined by researchers in the US. They have also shown that the observed transient leakage currents result in a shot noise that is twenty times larger than predicted using only conventional static leakage current measurements. PAGE 111 Researchers in the UK have presented an RF energy harvesting technique for powering small devices. Their work focuses on harvesting power from ambient radio waves, which is particularly useful in sensor networks where energy requirements are low and appeals as a solution to make them self-sustaining. The observed shot noise was 20 times greater than static leakage current measurements predicted PAGE 107 The first experimental demonstration of a continuous wave two-pump fibre optic parametric amplifier has been reported by researchers in France. Their device is of primary interest in the amplification of chirped pulses, and should allow for an all-fibre parametric amplifier operating at 1 µm with a flat gain band. The RF energy harvesting technique is aimed at allowing self-sustaining sensor networks PAGE 98 Researchers in Korea have devised a simple and cost-effective epitaxial graphene fabrication technique that results in good electric performance. This approach is expected to pave the way for a more cost-effective formation of highly reliable, low sheet resistance graphene, particularly for SiC-based electronic devices, power devices, SiC-graphene hetero-junctions and graphene-on-insulator semiconductors. The amplifier uses a dispersion stabilised microstructured optical fibre with high nonlinearity and low loss PAGE 93 Researchers in France have realised and measured an innovative microwave transmission line which can be integrated into complex multilayer systems. A multilayer structure like transmission line is essential to any telecommunication and radar power devices, such as power modules, solid state power transmitters, filters or any distribution system in that frequency range. A simpler and cheaper way to make graphene with low sheet resistance Thermal simulations suggest the structures will handle 50W average RF powers making them suitable for x-band power combination

Charge emission induced transient leakage currents of a ‐Si:H and IGZO TFTs on flexible plastic substrates

The transient off-state thermal emission leakage currents of hydrogenated amorphous silicon ( a -Si) and indium gallium zinc oxide (IGZO) metal oxide thin-film transistors (TFTs) fabricated on flexible plastic substrates have been examined for the first time. The transient leakage currents resulting from the detrapping of charge in the TFT active layer were observed to decay in a 1/ t power law behaviour from 2 to 100 ms after switching the TFT test structures off. The results demonstrated a uniform density of states for both types of thin-film devices and an ~1.3× higher transient leakage current for the IGZO TFTs. It was also shown that the observed transient leakage currents result in a shot noise at the input to the data-line charge integration amplifiers that is 20 × larger than predicted by using only the conventional static DC TFT leakage current measurements.

Flexible TFT Circuit Analyzer Considering Process Variation, Aging, and Bending Effects

This paper presents a SPICE-based simulator, FlexiAnalyzer, for flexible thin-film transistor (TFT) circuits. This simulator performs four types of analysis: yield analysis, aging analysis, performance analysis, and weak spot analysis. This simulator considers three important effects: 1) threshold voltage variation of manufacturing process; 2) threshold voltage shift due to aging effect; and 3) mobility change due to bending effect. Six different OLED pixel drivers designed in 8- μm amorphous silicon TFT technology were used in simulation results. The proposed tool provides a good solution for designers to evaluate the performance and yield of flexible TFT circuits.

A Novel Voltage-Programmed Pixel Circuit Utilizing $V_{T}$-Dependent Charge-Transfer to Improve Stability of AMOLED Display

A novel voltage-programmed pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) for active-matrix organic light-emitting diode (AMOLED) displays is proposed. The threshold voltage shift (ΔVT) of the drive TFT caused by electrical stress is compensated by an incremental gate-to-source voltage (ΔVGS) generated by utilizing the ΔVT-dependent charge transfer from the drive TFT to a TFT-based metal-insulator-semiconductor (MIS) capacitor. A second MIS capacitor is used to inject positive charge to the gate of the drive TFT to improve the OLED drive current. The non-ideality of the ΔVT-compensation, TFT overlap capacitance, programming speed, and OLED degradation are discussed. The effectiveness of the proposed pixel circuit is verified by simulation results.

High performance top-gate field-effect transistors based on poly(3-alkylthiophenes) with different alkyl chain lengths

The device characteristics of top-gate field-effect transistors (FETs) based on typical polymer semiconductor regioregular poly(3-alkylthiophenes) (P3ATs) with different alkyl chain lengths are investigated. High field-effect mobilities of ∼10−2cm2/Vs are obtained irrespective of alkyl chain length even when polymer gate insulators with different dielectric constants (2.1–3.9) are used. This is attributed to the spontaneous formation of highly ordered edge-on lamellar structures at the surface of P3AT thin films that are the channel regions in top-gate FETs. In addition, top-gate P3AT FETs containing different gate insulators exhibit high operational stability, with low threshold voltage shifts of <0.5V following prolonged gate bias stress, which is comparable to that of hydrogenated amorphous silicon thin film transistors.

Design Considerations of an Amorphous Silicon Demultiplexer

This paper deals with the optimization of amorphous silicon gate driver circuits for active matrix array applications. It is based on static and dynamic models for hydrogenated amorphous silicon thin-film transistor operations. The models are extended to investigate amorphous silicon demultiplexer functionality. The proposed circuit is an improvement of an earlier published architecture. The circuit performance is discussed and demonstrated to be more reliable. It is shown that the response of the demultiplexer in terms of charging/discharging time is improved, the feed-through is reduced and the optimization of the circuit response is a reliable trade-off between stability and speed. This may contribute to overcome amorphous silicon technology shortcomings. DOI: http://dx.doi.org/10.5755/j01.eee.19.8.5397

A Charge-Pump-Based Current Feedback Method for AMOLED Displays

This work presents a new external driving scheme that uses the current feedback method with a 3T1C pixel circuit. The proposed circuit provides a stable OLED driving current by compensating for the threshold voltage shift of amorphous-silicon thin-film transistors (a-Si TFTs). For fabricated a-Si TFTs and the 0.35 μm CMOS process associated with the proposed driving scheme, measurements reveal that the normalized OLED current degradation is less than 5% after 60 hours of operation at 60°C. Moreover, for a data line load of 1.69 K Ω/110 pF, the relative error rates between data currents and OLED currents are less than 4% within a settling time of 19 μs.

Effects of mechanical strain on amorphous silicon thin-film transistor electrical stability

The electrical stability of hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) on flexible substrates was characterized under mechanical bending. TFT lifetime until 50% reduction in IDS under moderate constant-voltage gate bias (<2 MV/cm gate field for 104 s) was estimated to be 2.56 h and 7.43 h under uniaxial applied compression and tension, respectively, compared to 4.28 h without strain. The stretched-exponential model for VT shift in a-Si:H TFTs was fitted to measurement data and used to predict how applied strain can affect TFT lifetime in practical applications. VT relaxation showed a logarithmic time dependence as would be expected for dielectric/interface charge detrapping.

P.6: An Integrated a‐Si:H Gate Driver Circuit Design for Large‐sized TFT‐ LCD Applications

AbstractA novel hydrogenated amorphous silicon thin‐film transistor (a‐Si:H TFT) integrated gate driver circuit with 33% ac‐driving structure is presented. The proposed circuit diminishes the threshold voltage (VTH) shift by reducing the bias voltage of pull down TFTs and alternatively turning on the pull down TFTs with 33% duty ratio. The measurement result illustrated that the VTH shift of the TFTs in the proposed structure is improved by 49.93% in contrast to the realized 25% ac‐driving structure, and the gate driver can stably operate over 240‐h driving at high temperature (60 °C).

Compact Decoder-Type Gate Driver Circuits with Hydrogenated Amorphous Silicon Thin Film Transistors for Active Matrix Displays

In this paper, we propose an integrated hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) gate driver circuit with parallely connected TFTs to resolve problems of the large circuit area and large number of input signals which are founded in the previously reported decoder-type and demultiplexer-type integrated gate drivers. The proposed gate driver can alleviate the demerits of previous gate drivers while maintaining their advantages: reduction of the threshold voltage (Vth) shift of the a-Si:H TFTs with an AC-driving structure and provide a stable low-impedance output. The key idea is to construct a novel decoder based on parallely connected TFTs instead of serially connected ones that are very common for decoders. The simulation results show that the rising time and falling time are 1.27 and 1.63 µs respectively with -5 to 30 V output voltage swing which are suitable for high resolution active-matrix displays.

Optimal power consumption design of the amorphous silicon thin-film transistor gate driver circuit for 10.1-in. display panel manufacturing

In this paper, the dynamic characteristics of the novel amorphous silicon thin-film transistor gate drive circuit applied on a 10.1-in. panel are optimized by using a multi-objective optimization method. Simultaneously, considering the multiple electrical characteristics of the tested circuit, the achieved results reveal interesting properties as well as good sensitivity of the optimized design parameters. The optimal specifications not only suppress the output ripple, but also reduce the dynamic and static power consumptions.

Effects of Photon Energy Spectrum on the Photocurrent of Hydrogenated Amorphous Silicon Thin Film Transistor by Using Frequency Filters

Frequency filters with various filtering wavelengths were used in the photoelectric characterization of hydrogenated amorphous silicon thin film transistor (a-Si:H TFT) and the experimental results were described and analyzed in terms of the photon energy spectral characteristics calculated from the integration of the photon energy and the spectral intensity of transmitted backlight through the filters at each wavelength. From the comparison of the photocurrents and the calculated photon energy spectrums for the filtered ranges of wavelength, it was possible to conclude that the photocurrents are closely related to the photon energy spectrums of the backlight.

Analysis of temperature effect on a-Si:H thin film transistors

An analytical model based on hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs) is proposed to interpret the degradation of output characteristics at different temperatures. The Poisson equation and Gauss theorem are applied to model the temperature effect on the total density of defect states. And the expression of the drain current is derived by exploiting the nonuniform distribution of surface potential. It indicates that the total density of defect states varies linearly with the temperature, whereas the drain current shows an exponential increase with the temperature. In addition, the influence of temperature on the effective mobility under different electric fields is also taken into consideration and results fit well with the experimental data.

Improving the mobility of the amorphous silicon TFT with the new stratified structure by PECVD

The amorphous silicon TFT (α-Si thin film transistor) were fabricated in a new structure, in which the ohmic contact layer (n+ layer) and the nitride silicon insulating layer for grid (G-SiNx) were stratified. Various factors which affect the electron mobility of α-Si TFT are studied using orthogonal test. With the increase in the number of n+ layer, the electronic mobility also rises. Besides, G-SiNx should be stratified into a rapid deposition film (GH) and a low-speed growing film (GL). The thickness of GL should be increased, with the thickness of GH reduced accordingly to achieve the electron mobility gradually increasing. Finally, based on the experimental results in the orthogonal combination experiments, the α-Si TFT mobility can stably reach 0.66 cm2/V·s, much higher than the traditional volume production data (0.29 cm2/V·s).

Amorphous Silicon Thin Film Transistor Biosensing System

ABSTRACTElectronic systems are a very good platform for sensing biological signals for fast point-of-care diagnostics or threat detection. One of the solutions is the lab-on-a-chip integrated circuit (IC), which is low cost and high reliability, offering the possibility for label-free detection. In recent years, similar integrated biosensors based on the conventional complementary metal oxide semiconductor (CMOS) technology have been reported. However, post-fabrication processes are essential for all classes of CMOS biochips, requiring biocompatible electrode deposition and circuit encapsulation.In this work, we present an amorphous silicon (a-Si) thin film transistor (TFT) array based sensing approach, which greatly simplifies the fabrication procedures and even decreases the cost of the biosensor. The device contains several identical sensor pixels with amplifiers to boost the sensitivity. Ring oscillator and logic circuits are also integrated to achieve different measurement methodologies, including electro-analytical methods such as amperometric and cyclic voltammetric modes. The system also supports different operational modes. For example, depending on the required detection arrangement, a sample droplet could be placed on the sensing pads or the device could be immersed into the sample solution for real time in-situ measurement. The entire system is designed and fabricated using a low temperature TFT process that is compatible to plastic substrates. No additional processing is required prior to biological measurement. A Cr/Au double layer is used for the biological-electronic interface. The success of the TFT-based system used in this work will open new avenues for flexible label-free or low-cost disposable biosensors.

High performance amorphous ZnMgO/carbon nanotube composite thin-film transistors with a tunable threshold voltage

Here we report the fabrication and characterization of high mobility amorphous ZnMgO/single-walled carbon nanotube composite thin film transistors (TFTs) with a tunable threshold voltage. By controlling the ratio of MgO, ZnO and carbon nanotubes, high performance composite TFTs can be obtained with a field-effect mobility of up to 135 cm(2) V(-1) s(-1), a low threshold voltage of 1 V and a subthreshold swing as small as 200 mV per decade, making it a promising new solution-processed material for high performance functional circuits. A low voltage inverter is demonstrated with a functional frequency exceeding 5 kHz, which is only limited by parasitic capacitance rather than the intrinsic material speed. The overall device performance of the composite TFTs greatly surpasses not only that of the solution-processed TFTs, but also that of the conventional amorphous or polycrystalline silicon TFTs. It therefore has the potential to open up a new avenue to high-performance, solution-processed flexible electronics which could significantly impact the existing applications, and enable a whole new generation of flexible, wearable, or disposable electronics.

Design of Integrated Amorphous-Silicon Thin-Film Transistor Gate Driver

A thorough study on the gate driver integrated with hydrogenated amorphous-silicon thin-film transistors (a-Si:H TFTs) for active-matrix flat-panel display (AM-FPD) is carried out in this work. The single stage circuit of the a-Si:H gate driver consists of input, pull-up, pull-down, and low-level holding units. The operation principle of the driver is described in detail. The subtle static and dynamic characteristics of the a-Si:H TFT based circuit are analyzed systematically for the first time. The long term reliability issue is also addressed. Design equations for determining the device sizes of the circuit are derived. The TFT-LCD panels integrated with the designed gate driver are fabricated to verify the design efficiency.