Amorphous Silicon Thin Film Transistors Research Articles

Parameter Extraction of Short-Channel a-Si:H TFT Including Self-Heating Effect and Drain Current Nonsaturation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We present an extraction procedure of the above-threshold parameters of a modified level-15 model of hydrogenated amorphous-silicon thin-film transistors (a-Si:H TFTs). This procedure is useful for model parameter extraction for short-channel devices, including the self-heating effect (SHE) and the nonsaturating drain current effect via the channel length modulation (CLM). The drain current formula of the AIM-Spice level-15 model was modified to include the nonsaturating drain current and SHE in the model. This procedure is also useful for devices in which the source–drain contact resistance <formula formulatype="inline"><tex Notation="TeX">$R_{sd}$</tex></formula> is comparable to, or greater than, the channel resistance, which is common in short-channel a-Si:H TFTs with a channel length of 10 <formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> or less. We propose a modified integral function method for the extraction of contact resistance and threshold voltage. This method includes features of the ratio method and the integral function method. Using this modified integral function method, we extract both the threshold voltage <formula formulatype="inline"><tex Notation="TeX">$V_{t}$</tex></formula> and the series contact resistance <formula formulatype="inline"><tex Notation="TeX">$R_{sd}$</tex></formula> at the very beginning of the extraction process. When simulated data were used for extraction, <formula formulatype="inline"><tex Notation="TeX">$V_{t}$</tex></formula> and <formula formulatype="inline"><tex Notation="TeX">$R_{sd}$</tex></formula> extracted by our proposed method agreed with the true parameter values better than the parameters extracted by the integral function method or the ratio method. For a short-channel device with a significant SHE, the field-effect mobility <formula formulatype="inline"><tex Notation="TeX">$\mu_{ \rm FE}$</tex></formula> parameters were separately extracted for the linear and saturation regions, because <formula formulatype="inline"><tex Notation="TeX">$\mu_{\rm FE}$</tex></formula> was higher in the saturation region than that in the linear region, which is probably caused by the SHE-induced rise of channel temperature. The calculated <formula formulatype="inline"><tex Notation="TeX">$I$</tex></formula>–<formula formulatype="inline"><tex Notation="TeX">$V$</tex></formula> characteristics based on the extracted parameters fit the experimental data well in both the short- and long-channel devices. This suggests that the modified drain current model including the SHE and the nonsaturating drain current is valid; the proposed parameter extraction procedure is valid and may be implemented in the circuit simulator, such as AIM-SPICE, with some further improvement in the field-effect mobility formula when SHE is present. </para>

Analysis of effective channel length variation for thin-film transistors with edge waviness in source/drain electrodes

We analyzed the effective channel length variation of hydrogenated amorphous silicon thin-film transistors (TFTs) that have wavy edge source/drain (S/D) electrodes. Edge waviness is frequently observed when narrow electrodes are fabricated by using printing methods. We used hydrogenated amorphous silicon (a-Si:H) TFTs and photolithographically patterned wavy edge S/D electrodes for accurate analysis. From a transmission line method (TLM), we successfully related the channel current variation to the variation of current transfer length ( L T_wavy) of the wavy edge S/D electrodes originated from current spreading and geometrical edge waviness effects which can be separately extracted.

High Electrical Performance of Wet-Processed Indium Zinc Oxide Thin-Film Transistors

We developed thin-film transistors (TFTs) that use solution-processed amorphous indium zinc oxide for the channels in an all-photolithographic process. The transistors, which operate in depletion mode, have excellent transfer characteristics, including saturation mobility of 6.57 cm2/ V·s, threshold voltages of - 0.30 V, turn-on voltages of -1.50 V, on/off ratios of 109, and inverse subthreshold slopes of 0.15 V/dec. We measured the time, temperature, gate voltage, and drain-voltage dependence of the threshold voltage shift, which was 2.16 V under stress conditions. This is nearly the same as that of conventional amorphous silicon TFTs.

Performance and stability of low temperature hydrogenated amorphous silicon thin film transistors fabricated on stainless steel substrate

AbstractThe key development issues in the flexible displays are TFT backplane technology, which requires competitive device performance and low temperature process compatible with flexible substrate. Here, we have fabricated low temperature hydrogenated amorphous silicon thin film transistor on a stainless steel substrate coated with organic barrier layer. Then, we have studied initial device performance by varying plasma gas and pressure conditions at a low power and a low temperature during amorphous silicon and silicon nitride deposition steps. Also, we discuss the stability characteristics of this low temperature processed thin film transistor, which reveals enough possibility for use in flexible display applications. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Flat-panel electronic displays: a triumph of physics, chemistry and engineering

This paper describes the history and science behind the development of modern flat-panel displays, and assesses future trends. Electronic displays are an important feature of modern life. For many years the cathode ray tube, an engineering marvel, was universal, but its shape was cumbersome and its operating voltage too high. The need for a flat-panel display, working at a low voltage, became imperative, and much research has been applied to this need. Any versatile flat-panel display will exploit an electro-optical effect, a transparent conductor and an addressing system to deliver data locally. The first need is to convert an electrical signal into a visible change. Two methods are available, the first giving emission of light, the second modulating ambient illumination. The most useful light-emitting media are semiconductors, historically exploiting III–V or II–VI compounds, but more recently organic or polymer semiconductors. Another possible effect uses gas plasma discharges. The modulating, or subtractive, effects that have been studied include liquid crystals, electrophoresis, electrowetting and electrochromism.A transparent conductor makes it possible to apply a voltage to an extended area while observing the results. The design is a compromise, since the free electrons that carry current also absorb light. The first materials used were metals, but some semiconductors, when heavily doped, give a better balance, with high transmission for a low resistance. Delivering data unambiguously to a million or so picture elements across the display area is no easy task. The preferred solution is an amorphous silicon thin-film transistor deposited at each cross-point in an X–Y matrix.Success in these endeavours has led to many applications for flat-panel displays, including television, flexible displays, electronic paper, electronic books and advertising signs.

Active Matrix Touch Sensor Perceiving Liquid Crystal Capacitance with Amorphous Silicon Thin Film Transistors

An active matrix touch sensor is integrated into a liquid crystal display (LCD) panel. The sensor is composed of a liquid crystal capacitor and a sensing circuit with amorphous silicon thin film transistors (a-Si TFTs). The change in sensor capacitance when the sensor is touched is converted into current change by the sensing circuit in the pixel. The proposed touch sensor is pressure-sensitive enough to work at an external push force of 10 gf. The simplified sensing circuit also gives an improved aperture ratio over that in previous work.

Self-Heating Effect on Bias-Stressed Reliability for Low-Temperature a-Si:H TFT on Flexible Substrate

Hydrogenated amorphous silicon thin-film transistors on colorless polyimide substrates were successfully fabricated at a low process temperature (160°C). The gate leakage current is as low as 10-13 A, while the field-effect mobility is 0.42 cm2V-1 · s-1, and the subthreshold swing is 0.77 V/dec. Using bias-temperature stress on devices with different channel widths, the enhancement of self-heating effect on bias-stressed reliability is investigated for the first time. Elevated temperature due to self-heating effect is estimated either by extending the bias-stressed model or by modifying the thermal equivalent circuit model. Degradation of device reliability on a bent substrate is also significant when self-heating effect is incorporated.

A-Si: H TFT SiN(下標 x)介電層製程改善

This paper is devoted to the process of dielectric layer SiN(subscript x) in amorphous silicon thin film transistor (a-Si: H TFT), which were fabricated by using the different ratio of SiH4 to NH3. The degradation effect by different SiN(subscript x) deposition condition and voltage bias stress were measured to compare a-Si: H TFT performance for optimizing the SiN(subscript x) deposition process. The degradation effect of a-Si: H TFT can been analyzed by defect poor mode, the studied results show that degradation effect of the inverted staggered structure device with voltage bias stress is defect state creation. According the performance of a-Si: H TFT, which SiN(subscript x) layer was fabricated by the ratio of gas flow in first layer of NH3/SiH4 is 4.5 with a thickness of 500Ǻ, and the ratio of gas flow in second layer of NH3/SiH4 is 4 with a thickness of 3000Ǻ, is a better deposition condition of SiN(subscript x) process.

Ultraflexible amorphous silicon transistors made with a resilient insulator

The conventional, brittle, silicon nitride barrier layer and gate insulator in amorphous silicon thin-film transistors (a-Si:H TFTs) on 50 μm thick polyimide foil was replaced by a resilient, homogeneous, hybrid of silicon dioxide and silicone polymer. The transistor structures can be bent down to 0.5 mm radius (5% strain) in tension and down to 1 mm radius (2.5% strain) in compression. This pronounced flexibility shifts the criterion for reversible bending away from a-Si:H TFT backplanes and toward the materials for substrate and encapsulation. It qualifies a-Si:H TFTs for pull-out display screens in handheld devices.

Temperature influence on photo-leakage-current characteristics of a-Si:H thin-film transistor

Abstract The electrical characteristics of hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs), in the dark or under front-side illumination, were investigated at different temperatures. Temperatures ranging from 100 K to 300 K were applied in this study, and experimental results showed the degradation of on-state current, mobility, and threshold voltage at lower temperatures. Furthermore, different photo-leakage-current trends were found in this work. Accordingly, we provide the indirect recombination rate and the parasitic resistance ( R p ) to explain the photo-leakage-current of a-Si:H TFTs under varied temperature operations.

Low voltage and high transmittance blue-phase liquid crystal displays with corrugated electrodes

A low voltage (&amp;lt;10 V) and high transmittance (∼85.6%) polymer-stabilized blue-phase liquid crystal (BPLC) display is proposed. The periodic corrugated electrodes generate a strong horizontal field component to induce isotropic-to-anisotropic transition in the BPLC medium through Kerr effect. Moreover, this field is uniformly distributed across the entire LC layer so that the accumulated phase retardation along the beam path is large, resulting in low voltage and high transmittance. This approach enables BPLC to be addressed by amorphous-silicon thin film transistors, which would accelerate its emergence as next-wave display technology.

Hysteresis of Transfer Characteristics of Floating-Gate a-Si:H Thin Film Transistor Nonvolatile Memories

The mechanism controlling the memory function of the floating-gate amorphous silicon thin film transistor, i.e., the hysteresis behavior of the transfer characteristics, has been identified. The magnitudes of the forward and backward gate voltages determine the charge trapping and detrapping processes including the curve of the transfer characteristics, the hysteresis direction, and the memory window. The thickness of the channel-contact layer also controls the charge transport distance and the memory function. The memory window can be well defined and optimized by avoiding excessive trapping of holes and enhancing the trap of electrons.

High Mobility a-Si:H TFT Fabricated by Hot Wire Chemical Vapor Deposition

AbstractThe hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) having a very high field-effect mobility of 1.76 cm2/V-s and a low threshold voltage of 2.43 V have been fabricated successfully using the hot wire chemical vapor deposition (HWCVD).

Threshold Voltage Shift Variation of a-Si:H TFTs With Anneal Time

AbstractHydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) are widely used in many areas and the most important application is in active matrix liquid crystal display. However, the instability of the a-Si:H TFTs constrains their usability. These TFTs have been annealed at higher temperatures in hope of improving their electrical performance. But, higher anneal temperatures become a constraint when the TFTs are grown on polymer-based flexible substrates. This study investigates the effect of anneal time on the performance of the a-Si:H TFTs on PEN. Thin-film transistors are annealed at different anneal times (4 h, 24 h, and 48 h) and were stressed under different bias conditions. Sub-threshold slope and the off-current improved with anneal time. Off-current was reduced by two orders of magnitude for 48 hours annealed TFT and sub-threshold slope became steeper with longer annealing. At positive gate-bias-stress (20 V), threshold voltage shift (∆Vt) values are positive and exhibit a power-law time dependence. High temperature measurements indicate that longer annealed TFTs show improved performance and stability compared to unannealed TFTs. This improvement is due to reduction of interface trap density and good a-Si:H/insulator interface quality with anneal time.

Electrical Stability of Hexagonal a-Si:H TFTs

In this letter, we study the current-temperature-stress-induced electrical instability of single and multiple hexagonal (HEX) hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) connected in parallel. The influence of the threshold voltage shift of a single HEX TFT on the overall electrical performance of multiple HEX TFTs is discussed. The results indicate that a-Si:H HEX TFTs have an improved electrical stability and a threshold voltage shift linear dependence on a number of connected HEX-TFT units.

Low voltage blue-phase liquid crystal displays

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.

비정질 실리콘 박막 트랜지스터의 광특성 분석을 위한 백라이트의 광자 에너지 스펙트럼에 대한 연구

For the investigation of the mechanism of photoelectric characteristics of a hydrogenated amorphous silicon thin film transistor(a-Si:H TFT), spectral characteristics of various backlights were analyzed in terms of the photon energy at each wavelength. Photon energy spectral characteristics were obtained through the multiplication of each photon energy and spectral intensities of backlights at each wavelength and the total photon energies were obtained by the integration of the photon energy spectrums. From the comparison of the experimental photo leakage current and the calculated photon energy, it was possible to conclude that the absorption of illuminated backlight to a-Si:H layer and the generation of electrons and holes are mainly carried out at the wavelength less than 500 nm as described in previous reports.

Hemispherical thin-film transistor passive pixel sensors

Hemispherical image sensors are very promising technology for cameras, surveillance systems and artificial vision. We report on the electrical performance of the hydrogenated amorphous silicon thin-film transistor passive pixel image sensor (PPS) circuits fabricated on a hemispherical substrate using maskless laser-write lithography (LWL). The level-to-level registration and alignment over the curved surface with a high accuracy are demonstrated for the LWL in this work. The obtained results clearly show that it is possible to realize active-matrix PPS with a 150 μm pixel pitch and a dynamic range of about 40 dB that is suitable for hemispherical image sensors.

Effect of Delta Doping in a-SiN:H Thin Film on the Mobility of Thin Film Transistors

Amorphous silicon modulation-doped field effect transistors using the heterostructure of phosphorous-contained a-SiN:H and intrinsic a-Si:H films have been fabricated by plasma enhanced chemical vapor deposition of SiH4, PH3, Ar and NH3 gases. The characteristics of the devices have been investigated and compared with conventional amorphous silicon thin film transistors. Various studies have been carried out on the properties of the materials of a-SiN:H films that were deposited with a varied delta-doping thickness and it was found that the Vth shifted from 2.5 to 4.5 V with an increase in the delta-doping thickness from 0 to 200 Å. The μn changed also from 0.19 to 0.38 cm2/V·sec.

Scan equalization digital radiography (SEDR) implemented with an amorphous selenium flat-panel detector: initial experience

It is well recognized in projection radiography that low-contrast detectability suffers in heavily attenuating regions due to excessively low x-ray fluence to the image receptor and higher noise levels. Exposure equalization can improve image quality by increasing the x-ray exposure to heavily attenuating regions, resulting in a more uniform distribution of exposure to the detector. Image quality is also expected to be improved by using the slot-scan geometry to reject scattered radiation effectively without degrading primary x-rays. This paper describes the design of a prototype scan equalization digital radiography (SEDR) system implemented with an amorphous silicon (a-Si) thin-film transistor (TFT) array-based flat-panel detector. With this system, slot-scan geometry with alternate line erasure and readout (ALER) technique was used to achieve scatter rejection. A seven-segment beam height modulator assembly was mounted onto the fore collimator to regulate exposure regionally for chest radiography. The beam modulator assembly, consisting of micro linear motors, lead screw cartridge with lead (Pb) beam blockers attached, position feedback sensors and motor driver circuitry, has been tested and found to have an acceptable response for exposure equalization in chest radiography. An anthropomorphic chest phantom was imaged in the posterior–anterior (PA) view under clinical conditions. Scatter component, primary x-rays, scatter-to-primary ratios (SPRs) and primary signal-to-noise ratios (PSNRs) were measured in the SEDR images to evaluate the rejection and redistribution of scattered radiation, and compared with those for conventional full-field imaging with and without anti-scatter grid methods. SPR reduction ratios (SPRRRs, defined as the differences between the non-grid full-field SPRs and the reduced SPRs divided by the former) yielded approximately 59% for the full-field imaging with grid and 82% for the SEDR technique in the lungs, and 77% for the full-field imaging with grid and 95% for the SEDR technique in the subdiaphragm. The SEDR technique demonstrated a substantial improvement in PSNRs over the anti-scatter grid technique. The improvements of PSNRs varied with the regions and are more pronounced in heavily attenuating regions.