Hydrogenated Amorphous Silicon Thin-Film Transistors Research Articles

Long-Term Behavior of Hydrogenated Amorphous Silicon Thin-Film Transistors Covered With Color Filters for Use in Optical Sensors

This work investigates the long-term behavior of photo thin-film transistors (TFTs) that are covered with color filters and based on hydrogenated amorphous silicon (a-Si:H) technology. Based on the electrical characteristics and the optical responses of these TFTs as measured under different stress conditions, a new method for driving a photo TFT with a negative gate-source voltage is proposed to suppress the degradation of the photocurrent. The effectiveness of the newly proposed method is verified using our previously developed white-light photocurrent gating (WPCG) structure, the measurement of photocurrents, and the established models of red, green, and blue photo TFTs. An accelerated lifetime test of the fabricated circuit was carried out at 70 °C and under the illumination of ambient light for 504 hours, demonstrating that the proposed method improves the long-term reliability of optical sensors.

A Hydrogenated Amorphous Silicon Thin-Film Transistor Optical Pixel Sensor for Ameliorating Influences of Ambient Light and Reflected Light

This paper proposes a hydrogenated amorphous silicon thin-film transistorbased (a-Si:H TFT) optical pixel sensor. The proposed optical sensor compensates for variations of ambient light using photo TFTs that incorporate with three primary color filters, and a designed active load is utilized to release the photocurrent from the noise of reflected light. Measurement results reveal that the proposed sensor suppresses the effect of ambient light within the intensity of 12 560 lux and does not react to a blue light with an intensity of 588 lux, proving that the proposed optical sensor remains highly reliable under heavy ambient light and avoids the interference of reflected light.

Effect of Mechanical Strain on Hydrogenated Amorphous Silicon Thin-Film Transistors and Compensation Circuits on Flexible Substrates

Hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) compensation pixel circuits were fabricated on polyethylene naphthalate substrates at amax-imumtemperature of 170 °C. The typical a-Si:H TFTs showed a field-effect mobility ( $\mu _{\text {FE}}$ ) of 0.8–1.1 cm $^{2}$ /Vs, a threshold voltage ( ${V}_{T}$ ) of 2–3.3 V, a subthreshold swing (SS) of ~0.65 V/decade, and an ON/OFF current ratio of $10^{7}$ – $10^{8}$ . Under DC gate-bias stress without compensation, the TFT drive current decreased by ~50% withoutmechanical strain and ~60% with applied tensile strain. The TFT circuits effectively compensated for the change in the TFT drive current to within 10% of the original drive current value under mechanically strained and unstrained states. The orientation of the TFT within the circuit was found to affect the circuit compensation; TFTs having a channel length perpendicular to the mechanical strain were found to have a 50% higher threshold voltage shift ( $\Delta {V}_{T}$ ) compared to devices parallel to the applied strain.

Optical Pixel Sensor of Hydrogenated Amorphous Silicon Thin-Film Transistor Free of Variations in Ambient Illumination

This paper presents a primary color optical pixel sensor circuit that utilizes hydrogenated amorphous silicon thin-film transistors (TFTs). To minimize the effect of ambient light on the sensing result of optical sensor circuit, the proposed sensor circuit combines photo TFTs with color filters to sense a primary color optical input signal. A readout circuit, which also uses thin-film transistors, is integrated into the sensor circuit for sampling the stored charges in the pixel sensor circuit. Measurements demonstrate that the signal-to-noise ratio of the proposed sensor circuit is unaffected by ambient light under illumination up to 12 000 lux by white LEDs. Thus, the proposed optical pixel sensor circuit is suitable for receiving primary color optical input signals in large TFT-LCD panels.

Asymmetric Electrical Properties of Half Corbino Hydrogenated Amorphous Silicon Thin-Film Transistor and Its Applications to Flat Panel Displays

The half-Corbino hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) were fabricated with a five-photomask process used in the processing of the active matrix liquid-crystal displays (AM-LCD). We showed that the a-Si:H half-Corbino TFTs have the asymmetric electrical characteristics under different drain-bias conditions. In comparison to half-Corbino TFT with unpatterned gate electrode, the device parasitic capacitance can be significantly reduced by patterning the gate electrode for same device dimension, while the sub-threshold swing and threshold voltage remain identical for same bias condition. The OFF-current remains below 0.1 pA for all device configurations. Finally, we discuss their potential applications as a driving or switching TFT to various flat panel displays.

Asymmetric Electrical Properties of Half Corbino Hydrogenated Amorphous Silicon Thin-Film Transistor and Its Applications to Flat Panel Displays

The half-Corbino hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) were fabricated with a five-photomask process used in the processing of the active matrix liquid-crystal displays (AM-LCD). We showed that the a-Si:H half-Corbino TFTs have the asymmetric electrical characteristics under different drain-bias conditions. In comparison to half-Corbino TFT with unpatterned gate electrode, the device parasitic capacitance can be significantly reduced by patterning the gate electrode for same device dimension, while the sub-threshold swing and threshold voltage remain identical for same bias condition. The OFF-current remains below 0.1 pA for all device configurations. Finally, we discuss their potential applications as a driving or switching TFT to various flat panel displays.

Dual-Gate Characteristics of Amorphous $ \hbox{InGaZnO}_{4}$ Thin-Film Transistors as Compared to Those of Hydrogenated Amorphous Silicon Thin-Film Transistors

We compare the mutual interactions between the top-and bottom-gate fields in a dual-gate structure for amorphous InGaZnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (a-IGZO) and hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs). We find that a-IGZO TFTs show more significant mutual interactions than a-Si:H TFTs. By using a wide variety of a-IGZO TFTs with varying thicknesses of a-IGZO and gate-insulator layers, we have investigated, on the basis of a conventional fully depleted n-channel silicon-on-insulator model, the mechanism behind parallel shifts in transfer characteristics caused by negative top-gate voltages. Experimental results, even for relatively thick (over 150 nm) a-IGZO layers, are in good agreement with the fully depleted model. This fact may be related to the nonexistence of hole accumulation in a-IGZO, which is an intrinsic property of oxide semiconductors. In contrast, for the a-Si:H TFTs, there is a considerable discrepancy between the experimental results and the model. This discrepancy probably results from the limited penetration of band bending into the a-Si:H layer, as well as from the hole accumulation at the top interface. Such analysis of a-IGZO TFTs is of practical importance in device applications because many issues related to the fabrication and structure of a-IGZO TFTs may be resolved with a better understanding of dual-gate performance.

Hydrogenated Amorphous Silicon Thin-Film Transistors on Freestanding Graphite Foil

We have studied the fabrication of hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) on 125 μm thick graphite foil. The a-Si:H TFT fabricated on polyimide planarized graphite foil exhibited a field-effect mobility of 0.32 cm 2 /V s, a threshold voltage of 2.35 V, and subthreshold slope of 1.23 V/decade. The performance of the TFT on graphite foil is stable until bending it down to the radius of 5 mm, corresponding to the strain of 2.4%. The TFT on curved graphite foil can be freestanding on the table.

Cu∕CuMg Gate Electrode for the Application of Hydrogenated Amorphous Silicon Thin-Film Transistors

The feasibility of using Cu/CuMg as a gate electrode for a-Si:H thin-film transistors (TFTs) has been investigated in this work. The issue of adhesion between the Cu film and glass substrates has been overcome by introducing the Cu/CuMg alloy. Furthermore, a wet-etching process of Cu-based gate metal has been proposed by using the copper etchant in the conventional printed circuit boards. The experimental result showed superior performance of a-Si:H TFT with desired electrode taper angle and minimal loss of critical dimension. The a-Si:H TFT exhibited mobility of 0.37 cm 2 /V s, subthreshold slope of 0.83 V/dec, and V th of 2.02 V.

Modeling and Simulation of Hydrogenated Amorphous Silicon Thin-Film Transistors

A simple analytical dc model for hydrogenated amorphous silicon thin-film transistors including an accumulation region is developed. The model describes the device properties and physical phenomena of hydrogenated amorphous silicon. Current–voltage characteristics are related to basic material and device parameters. In the blocking state, drain leakage current is modeled using previously reported results. In the lower and upper threshold regimes, both deep and tail states are taken into account. The density of states in band gap is modeled by assuming an exponential distribution of the deep and tail states. The results show that the generated current–voltage characteristics show good agreement with previously reported data. The study of apparent threshold voltage, field effect mobility and ON–OFF current ratio shows that in the lower threshold regime, parameter behavior is governed by the deep states in the middle of an amorphous silicon gap while the tail state distribution controls the upper-threshold-regime behavior. The model is simplified analytically to lessen the cost of and complexity in fabricating large-area displays and optimizing their performance.

Hydrogenated Amorphous Silicon Thin-Film Transistor on Plastic With an Organic Gate Insulator

We developed a high-performance, hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) on plastic substrate using an organic gate insulator. The TFT with a silicon-nitride (SiN/sub x/) gate insulator exhibited a field-effect mobility of 0.3 cm/sup 2//Vs and a threshold voltage of 5 V. On the other hand, an a-Si:H TFT with an organic gate insulator of BCB (benzocyclobutene) has a field-effect mobility of 0.4 cm/sup 2//Vs and a threshold voltage of 0.7 V. The leakage currents through the gate insulator of an a-Si:H TFT with an organic gate insulator is about two orders of magnitude lower than that of an a-Si:H TFT with a SiN/sub x/ gate insulator.

Relationship between Leakage Current and the Type of Passivation Layer of Hydrogenated Amorphous Silicon Thin-Film Transistors

The variations in the device characteristics of hydrogenated amorphous silicon thin film transistors according to the passivation layers and manufacturing processes have been studied. If a thin SiNx layer is used as a passivation layer, leakage current is increased over one order of magnitude after the deposition of transparent conducting oxide on the passivation layer. This phenomenon is the result of an accumulation of carriers in the active channel due to the surface charging on the passivation layer. The increase in leakage current is prohibited by using an organic passivation layer. The increase in thickness, the decrease in the dielectric constant and/or changes in passivation surface state as the organic layer is used instead of a SiNx layer cause the reduction in leakage current. The knowledge acquired from this work is useful in fabricating reliable switching arrays for flat-panel displays.