Amorphous Silicon Field Effect Transistors Research Articles

An ultra-lightweight design for imperceptible plastic electronics

Electronic devices have advanced from their heavy, bulky origins to become smart, mobile appliances. Nevertheless, they remain rigid, which precludes their intimate integration into everyday life. Flexible, textile and stretchable electronics are emerging research areas and may yield mainstream technologies. Rollable and unbreakable backplanes with amorphous silicon field-effect transistors on steel substrates only 3 μm thick have been demonstrated. On polymer substrates, bending radii of 0.1 mm have been achieved in flexible electronic devices. Concurrently, the need for compliant electronics that can not only be flexed but also conform to three-dimensional shapes has emerged. Approaches include the transfer of ultrathin polyimide layers encapsulating silicon CMOS circuits onto pre-stretched elastomers, the use of conductive elastomers integrated with organic field-effect transistors (OFETs) on polyimide islands, and fabrication of OFETs and gold interconnects on elastic substrates to realize pressure, temperature and optical sensors. Here we present a platform that makes electronics both virtually unbreakable and imperceptible. Fabricated directly on ultrathin (1 μm) polymer foils, our electronic circuits are light (3 g m(-2)) and ultraflexible and conform to their ambient, dynamic environment. Organic transistors with an ultra-dense oxide gate dielectric a few nanometres thick formed at room temperature enable sophisticated large-area electronic foils with unprecedented mechanical and environmental stability: they withstand repeated bending to radii of 5 μm and less, can be crumpled like paper, accommodate stretching up to 230% on prestrained elastomers, and can be operated at high temperatures and in aqueous environments. Because manufacturing costs of organic electronics are potentially low, imperceptible electronic foils may be as common in the future as plastic wrap is today. Applications include matrix-addressed tactile sensor foils for health care and monitoring, thin-film heaters, temperature and infrared sensors, displays, and organic solar cells.

Dynamics of Threshold Voltage Shifts in Organic and Amorphous Silicon Field‐Effect Transistors

Progress in environmental stability and processability, and the increase of the field-effect mobility of organic semiconductors has triggered their use as the active element in microelectronic devices. The advantages of their application are the easy processing, for example, spin-coating and ink-jet printing, without a temperature hierarchy, and their mechanical flexibility. Applications are foreseen in the field of large-area electronics where numerous devices are integrated on low-cost substrates such as plastics. The first flexible, even rollable, quarter video graphics array (QVGA) active matrix displays based on organic semiconductors have already been reported.[1]

Amorphous silicon junction field-effect transistor for digital and analog applications

A hydrogenated amorphous silicon field-effect transistor suitable for digital and analog applications is described. The device consists of a p+-i-n− junction, with the drain and source contacts connected to the n− layer and the control gate electrode to the p+ layer. As in the corresponding crystalline junction field-effect transistor, the channel resistivity is modulated by the gate voltage. The selection of the thickness of the intrinsic layer and the doping and thickness of the n layer is critical for correct operation of the device. A device with a well defined triode and saturation region was manufactured. A transconductance of 1.5×10−6 A/V at VDS=25 V was achieved. This value is comparable to that of state-of-the-art thin-film transistors.

Megavoltage imaging with a large-area, flat-panel, amorphous silicon imager

Purpose : The creation of the first large-area, amorphous silicon megavoltage imager is reported. The imager is an engineering prototype built to serve as a stepping stone toward the creation of a future clinical prototype. The engineering prototype is described and various images demonstrating its properties are shown including the first reported patient image acquired with such an amorphous silicon imaging device. Specific limitations in the engineering prototype are reviewed and potential advantages of future, more optimized imagers of this type are presented. Methods and Materials : The imager is based on two-dimensional, pixelated array containing amorphous silicon field-effect transistors and photodiode sensors which are deposited on a thin glass substrate. The array has a 512 × 560-pixel format and a pixel pitch of 450 μm giving an imaging area of ∼ 23 × 25 cm 2. The array is used in conjunction with an overlying metal plate/phosphor screen converter as well as an electronic acquisition system. Images were acquired fluoroscopically using a megavoltage treatment machine. Results : Array and digitized film images of a variety of anthropomorphic phantoms and of a human subject are presented and compared. The information content of the array images generally appears to be at least as great as that of the digitized film images. Conclusions : Despite a variety of severe limitations in the engineering prototype, including many array defects, a relatively slow and noisy acquisition system, and the lack of a means of generate images in a radiographic manner, the prototype nevertheless generated clinically useful information. The general properties of these amorphous silicon arrays, along with the quality of the images provided by the engineering prototype, strongly suggest that such arrays could eventually form the basis of a new imaing technology for radiotherapy localization and verification. The development of a clinically useful prototype offering high-quality images, ultimately with an ∼52 × 52-cm 2 detection surface, is anticipated.

Density of bulk and surface states and static characteristics of amorphous silicon field-effect transistors

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Use of a Field Effect Transistor to Study Phototransport Properties of a-Si:H

We report new measurements of the diffusion length of minority photocarriers in thin film amorphous silicon field effect transistor structures. We are able to vary the majority carrier photoconductance by more than four orders of magnitude while monitoring the effective ambipolar diffusion length using the photocarrier grating technique.

Theoretical analysis of channel-doped amorphous silicon field-effect transistors

The electrical characteristics of the amorphous silicon (a-Si:H) field-effect transistors have been analyzed theoretically. It is found that if no interface states exist at the SiO2 and a-Si:H interface, the transistor with undoped channel gives the best subthreshold swing and on/off current ratio. However, the existence of the interface states shifts the threshold voltage and reduces the transconductance of the transistor. It is found from numerical analysis that the problem caused by the interface states could be compensated for by doping the channel slightly with phosphorus. The physical reason for the improved performance is discussed in detail.

Low Temperature Characteristics of Hydrogenated Amorphous Silicon Field Effect Transistors

ABSTRACTWe investigated the temperature dependent characteristics of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT's) at temperatures down to 20 K. With decreasing temperature, the threshold voltage increased, the field effect mobility and the on-current decreased. The measured on-currents versus inverse temperature above 80 K are represented as the sum of two exponentially varied currents. It is concluded that on-current is nearest-neighbour hopping between 120 K and 80 K. Below this temperature, the temperature dependence of on-current is explained by variable range hopping and below about 30 K on-current becomes nearly independent of temperature. At very low temperature hopping probability may be governed not by temperature but by temperature independent tunneling, depending on the overlap of the wave function. The explanation of threshold voltage increase at low temperature is given.

Hypoxanthine sensor based on an amorphous silicon field-effect transistor

The pH response of an a-ISFET with xanthine oxidase immobilized on a ca. 20-μm thick poly(vinyl butyral) membrane over the gate insulator, is used to detect the uric acid produced by enzyme-catalyzed oxidation of hypoxanthine. The pH sensitivity between pH 5.0 and 10.0 is ca 48 mV/pH at 32°C in 10 mM phosphate buffer. The change in the output gate voltage 1 min after sample injection, is linearly related to the logarithm of the hypoxanthine concentration in the range 0.02–0.1 mM. The optimum buffer pH is 7.5. The system can be used for 2 weeks with 30% loss of enzymatic activity.

Physics of Novel Amorphous Silicon High-Voltage Transistor

AbstractIn this paper we describe the operation of a novel amorphous silicon high voltage transistor. Its attractive feature is that it can operate at source-drain voltages in excess of 400 volts but its characteristics are controlled by applying only a low bias (0-10 volts) to a gate electrode covering a small portion of the sourcedrain channel near to the source. The portion of the device over this gate electrode operates as a conventional amorphous silicon Field-Effect Transistor which injects electrons into the intrinsic amorphous silicon between this region and the drain electrode. We present experimental data showing the current-voltage characteristics of this new transistor as a function of geometry and demonstrate that the above model realistically describes its operation.

Double-injection field-effect transistor: A new type of solid-state device

We propose a new general principle of operation for solid-state devices, and demonstrate a novel transistor which we call a double-injection field-effect transistor, based on this principle. We have fabricated amorphous silicon alloy double-injection transistors operating on the modulation of a double-injection current by a gate field covering the complete path of the current channel. Using these amorphous silicon alloy double-injection transistors, we have achieved currents over 20 times those theoretically possible for conventional amorphous silicon field-effect transistors operating under similar conditions. This new principle, applicable to both thin-film amorphous and crystalline devices, offers the potential of high-current, high-speed field-effect transistors with modulated optical emission.

Double Injection Field Effect Transistor A New Type of Solid State Device

ABSTRACTWe propose a new general principle of operation for solid state devices, and demonstrate a novel transistor which we call a Double Injection Field Effect Transistor, based on this principle. We have fabricated amorphous silicon alloy double injection transistors, operating on the modulation of a double injection current by a gate field covering the complete path of the current channel.Using these amorphous silicon alloy double injection transistors, we have achieved currents over fourteen times those theoretically possible for conventional amorphous silicon field effect transistors operating under similar conditions. This new principle, applicable to both thin film amorphous and crystalline devices, offers the potential of high current, high speed field effect transistors.

Self-alignment a-Si FET by using a lift-off technique

New self-alignment amorphous-silicon field-effect transistors (a-Si FETs) have been proposed and demonstrated. This method uses lift-off and ion-implantation techniques, and no voltage offset was observed. Simple analysis indicates that this structure may be used to produce devices with channel lengths down to 5 μm.

Self-alignment processed amorphous silicon ring oscillators

Amorphous silicon field-effect transistors (a-Si FET's) have been integrated into eleven-stage ring oscillators. The inverter consists of an n-channel driver a-Si FET and an n+ a-Si load resistor. a-Si FET's were fabricated by the self-alignment process. Ring oscillators with a gate length of 5 µm and a gate width of 120 µm achieved a propagation delay time of 210 ns, the lowest reported to date. Those with a length and width of 5 and 1200 µm, respectively, achieved 540 ns. These propagation delay times are about two orders of magnitude faster than those attained previously.

Proposed vertical-type amorphous-silicon field-effect transistors

Novel amorphous-silicon field-effect transistors (a-Si FET's) with a vertical channel have been proposed and demonstrated for the first time. The channel length of the new FET's is not limited by the photoetching process and thus can be reduced a great deal. Prototype FET's with a channel length of 1 µm had an on-off current ratio of more than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> and the on-resistance was proportional to the channel length, so far as it was longer than 1 µm.

Anomalous surface photoconductivity in hydrogenated amorphous silicon

The effects of field inhomogeneities on photoconductivity were investigated by measuring the current of amorphous silicon field effect transistors during illumination for various gate voltages. The photocurrent was nearly independent of light intensity, and the decay time increases to ∼1 sec for large band bending. The photocurrent characteristics of a gap electrode sample are the same as a transistor with a 1 V gate voltage. The results suggest that the gap cell photocurrent is affected by surface fields.

The characteristics and properties of optimised amorphous silicon field effect transistors

A series of experiments aimed at improving the performance of amorphous silicon field effect transistors has been carried out. The dc and dynamic characteristics of the optimised devices are described. Stable devices capable of ON-currents of the order of 100 μA with OFF-currents ≃10−11 A can be fabricated which could, in principle, be used to address more than 1000 lines of a liquid crystal display. The properties of the highly conducting ON-state channel have also been studied. The field effect mobility, 0.3 cm2 V−1 s−1 at room temperature, has an activation energy of 0.1 eV at the higher gate voltages. The possible reasons for the improvement in performance over earlier devices are discussed.

The effect of?-irradiation on amorphous silicon field effect transistors

The paper deals with the effect ofγ-radiation from a Co60 source on the electronic properties of amorphous silicon field effect transistors. These thin film devices, deposited by the glow discharge technique, are being developed for addressable liquid crystal displays, logic circuits and other applications. 1 Mrad (Si) and 5 Mrad (Si) doses were used and the transistors were held at gate voltages between −8V and +8V during irradiation. Measurements on irradiated specimens showed shifts in threshold voltage of less than 3 V and a change in transconductance below 10%, both of which could be removed by annealing above 130 °C. These results are compared with presently available “radiation hardened” crystalline silicon device structures and it is concluded that in spite of the thicker gate insulation layer (0.3 μm of silicon nitride) of the amorphous devices, the latter are remarkably radiation tolerant, with little degradation in performance. Measurements on irradiatedα-Si films deposited on glass show pronounced conductivity changes, not observed in the transistors. It is suggested that these effects arise at the Si/glass interface, and are prevented by the presence of the silicon nitride film in the devices.

Theoretical Analysis of Amorphous-Silicon Field-Effect-Transistors

Static characteristics of amorphous-silicon field-effect transistors have been analyzed under the assumption that the localized state density distribution (LSDD) in amorphous-silicon with respect to energy takes on an exponential or uniform form. In the case of an exponential LSDD, logarithmic drain current ID vs logarithmic gate voltage VG characteristics of the FET for large VG is found to be linear, the slope of which yields the characteristic temperature of the exponential LSDD. While, in the case of a uniform LSDD, log (IDVG)–VG curves for large VG are found to be linear. The experimental data is qualitatively in good agreement with the theoretical results of the exponential LSDD.

An amorphous silicon integrated inverter

Integration of hydrogenated amorphous silicon field effect transistors (a-Si FET's) has been investigated. It is shown that the new inverter which consists of an n-channel enhancement mode driver FET and a p-channel depletion mode load FET has excellent steplike transfer characteristics. With this integrated inverter, a small-signal gain of more than ten was measured and an integrated ring-oscillator of nine-stages operated for the first time. The power delay product and the minimum propagation delay time were about 480 pJ and 110 µs, respectively.