An amorphous silicon TFT particularly suited for the full color liquid crystal display driver has been developed and reported here. Various fundamental factors involved in the a-Si TFT, such as the effects of structure, materials, and the method of fabrications were reviewed and investigated in terms of the field effect mobility, the threshold voltage and the reliabilities. An inverted-staggered TFT structure was employed for the purpose wherein the interface states between two layers was successfully lowered by employing the successive deposition procedures of SiNx gate insulator on the a-Si layer. Proper ohmic contacts and the blocking of hole injections were accomplished by forming a n+ layer between the a-Si layer and the source/drain metal electrodes which consists of a double layer of Al/MoSi2 in order to prevent the aluminum diffusion into the a-Si layer during the 300°C heat treatment at the succeeding fabrication processes. The degradation of display images due to the high intensity backlights was minimized by employing a light shielding layer and by making the thickness of a-Si layer 200 Å against the direct sunlight of up to 100,000 luxes. Stable actual performances of TFT for more than 4000 hours at 80 °C were confirmed. The development of a color LCD TV driven by this TFT is also reported.

The preparation and properties of hydrogenated amorphous silicon thin film transistor arrays for active matrix liquid crystal displays are reported. The effect of amorphous silicon film preparation conditions on the field effect mobility of thin film transistors was investigated. The dry etching rate of silicon nitride film was studied.The thin film transistor arrays have 408 ˜ 640 transistors on the first version and 450 ˜ 640 ˜ 3 transistors on the second version. The liquid crystal panel fabricated using the first version arrays showed good characteristics.

A new contact type linear image sensor, which has 8.5 inches in length and 300 dot/inch, has been developed. By using a thin film transistor (TFT) array as a switching device, it requires only one IC chip compared with 20 ICs in conventional sensor.

The new sensor integrates a-Si:H photodiode (PD) array, a-Si:H TFT array, multiplex circuits and an analog multiplexer on a single substrate. The TFT has an inverted staggered structure and amorphous silicon nitride (SiNx) is used as a gate insulator. The PD has a Cr/a-Si:H/ITQ sandwich structure and each PD is completely isolated by photolithography. In the multiplex circuits, a ground line is inserted between each signal line to reduce a capacitive coupling between them.

A new custom LSI has also been developed. It has 64-ch input terminals to detect small signal with amplification and 41-ch output terminals to control the gates of TFT array. The sensor with the new LSI has achieved 2MHz clock frequency operation and proved to be used as a high speed device.

In this paper we report on the dynamic characteristics of amorphous silicon thin film transistors in the time interval between ips to 1s after gate switch-on. The experimental results show a continuous change in the threshold voltage in this t'me regime. A model based on carrier thermalisation to the deep states including the spatial dependence of the thermalisation process in the band-banding region is presented to account for these results. The effect responsible for the observed time dependence of the threshold voltage is shown to be the trapping of charge in deep states in the bulk amorphous silicon away from the interface with the gate dielectric.

We have used photothermal deflection spectroscopy to examine deep gap absorption in amorphous silicon films deposited on silicon oxide and silicon nitride. Variations in the interface state density deduced from PDS correlate well with the performance characteristics of thin-film transistors. We have demonstrated processes which degrade the interfacial abruptness also increase the interface state density. In transistors this leads to a degradation in device stability. We found devices with oxide gates to be more stable and show lower interface state density than devices with nitride gates for a specific set of deposition conditions. The correspondence between deep gap absorption and transistor characteristics shows that PDS is a valuable technique for characterizing and optimizing fabrication processes.

In 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.

Native silicon-dioxide film has been grown thermally under normalpressure and low-temperature conditions, and applied to a gate insulator of amorphous-silicon thin-film transistors. Th highest field-effect mobility under room-temperature conditions was 0.7cm2/Vs. Temperature dependence of the transistor characteristics has also been studied. The transistor has been operated in a wide temperature range from -196 °C to more than 200°C with satisfactorily stable manner. Characteristic temperature of the localized state density distribution has been estimated to be about 20° C.

Amorphous silicon (a-Si:H) thin-film transistor (TFT) array technology has been developed for new applications in low-cost, high-quality electronic printing. We have fabricated page-wide arrays of low-voltage pass transistors using a-Si:H TFTs for ionographic printing, in which voltages of 0 to 15 volts applied to a line of output electrodes modulate the flow of ions charging a dielectric receptor. High-voltage a-Si:H TFTs have been used in an electrographic printer to modulate high voltages required to initiate air discharges. Combining a-Si:H photodiodes on TFT arrays, we have also designed circuits for document scanning and photosensor amplifiers. TFT performance in relation to these novel printer and sensor applications will be discussed. Issues related to process integration, circuit design, and large-area fabrication technology will be addressed.

We have studied the photoluminescence in the space charge region of field effect transistors (FETs) based on a-Si:H in the temperature range T>100K. The radiative recombination rate rises with increasing gate voltage (Vg >0) The increase is more pronounced at the high energy side of the photoluminescence spectrum and the relative changes increase with temperature. These results are compared to those obtained on doped a-Si:H films, where the defect photoluminescence band increases slightly with increasing doping level but the intrinsic band drops off very rapidly. The different influence of the Fermilevel shift in the doped films is explained by the increasing defect concentration which dominates the photoluminescence results. The results in the FETs are attributed solely to the changes in the occupation of the dangling bond states.

Amorphous silicon thin film transistors with on/off ratios >108 and on currents >100 μA for supply voltages <30 V have been manufactured. The transistors are of the structure glass/Cr/SiNx/intrinsic a-Si/n+a-Si/Al. The field effect electron mobility is typically 0.24 cm2 /Vs. Evaluation of quasistatic CV measurements yields a gap state density of 3.2*1016 cm−3eV−1 at midgap. A one-dimensional model calculation describing the influence of bulk and interface gap states and various other material and geometry parameters on transistor performance is outlined. Calculated data include space charge density, spatial distribution of band bending, band bending vs gate source voltage, static capacitance, and channel conductance. A fit calculation for the experimental sub pinch-off transfer characteristic of a thin film transistor is presented.

Hydrogenated amorphous silicon thin-film transistors (TFTs) were made with either n-channel or p-channel device operation. Layers of undoped amorphous silicon were deposited in the same run and appropriate contact implantation techniques were used. These devices offer the possibility of investigating unipolar conduction of either electrons or holes. In this way ambiguities in the field-effect (FE) analysis of the density of states (DOS) can be avoided. We present the DOS distribution over the entire band gap region of undoped amorphous silicon deduced from a pair of transfer characteristics (one forward and one in reverse). Both types of TFTs are subject to degradation under continuous accumulation conditions. The rate of current decay under hole accumulation appears to be larger than under electron accumulation, as is expected from the width of the respective tail-states distributions. The ON/OFF current ratios obtained for the p- and n-channel devices were 105 and 107, respectively, and both devices showed good pinchoff behaviour. Therefore, these devices are in principle attractive for application in novel amorphous silicon integrated logic circuits (IC).

Advances in materials research continue to push forward the performance of thin film photovoltaic technology and bring closer to reality the goal of low cost (less than $2/wattpeak selling price), high efficiency (greater than 15%) modules. A tandem module of thin film silicon:hydrogen alloy (TFS) above copper indium diselenide (CIS) promises to be a most effective approach to this goal. The performance of the high band gap TFS and low band gap CIS has been advanced in part by understanding the nature of the materials and by optimizing the processing techniques. Careful evaluation of various alternatives has led to the selection of suitable materials to serve as the transparent conductive electrodes and the transparent optical coupler between the TFS and CIS. This paper addresses the next steps in capturing efficiency improvements and characterizes probable real world performance.

Deposition technique of a-SiGe:H and its application to a stacked solar sell were investigated. Properties of a-SiGe:H were improved by a glow discharge method with a negatively biased substrate, and a mercury sensitized photo CVD under lower pressure. An a-SiGe:H single junction solar cell was improved by slight boron doping to an i layer and insertion of a buffer layer into a p/i interface. A conversion efficiency of more than 10 % was obtained by a triple stacked solar cell on a alass substrate.

We have shown that high efficiency solar cells can be obtained by incorporating materials of different band gaps in a multijunction configuration. This configuration gives rise to high efficiency due to spectrum splitting as well as superior stability due to thin top cells.

We have fabricated multijunction solar cells and acheived a 13% conversion efficiency using 1.7eV a-Si:F:H and 1.5 eV a-Si:Ge:F:H materials in a three-cell triple configuration. This device was deposited onto a stainless steel substrate coated with a back reflector; it also incorporates a microcrystalline p+ layer.

The physical properties of each of the solar cell layers and their role in the device physics of a high efficiency solar cell will be described.

A-Si:H p-i-n solar cells were deposited on textured transparent conducting oxide coated glass substrates to experimentally determine the enhancement in the short circuit current density. It was found that the optical thickness can be increased by a factor from 3 to 5. These results agree reasonably well with the predictions of optical enhancement theory.

The results of investigations of high energy proton radiation resistance of a-Si alloy thin film photovoltaic cells are reported. Irradiations were carried out with 200 keV and 1.00 MeV protons with fluences ranging between IEII and IEl5 cm-2. Defect generation and passivation mechanisms were studied using the AM1 conversion efficiency and isochronal anneals. It is concluded that the primary defect generation mechanism results from the knock-on of Si and Ge in the intrinsic layer of the cells. The defect passivation proceeds by the complex annealing of Si and Ge defects and not by the simple migration of hydrogen.

Results are presented showing the effects on amorphous silicon (a-Si) photovoltaic performance of the interfaces associated with a silicon carbide (a-Si:C) p+ layer. Carbon grading into the intrinsic layer from the p+ layer increases open circuit voltage (Voc) from O.7V to 0.88V. This effect is very similar to the boron profile effect reported earlier and supports the contention that Voc is being limited by an electron current at the p-i interface. The interface between the p+ a-Si:C layer and the transparent conductive oxide (TCO) is shown to be a potential source of high series resistance, with an abrupt interface showing the most serious problem. The effect is explained by electron injection from the TCO into the p+ layer being inhibited as a result of band mismatch.

Carbon grading in the buffer layer at the p/i interface increases the open circuit voltage of both p-i-n and n-i-p amorphous silicon solar cells. We propose that carbon grading enlarges the electric field and reduces the electron tunneling at the p/i interface.

The experimental phenomena associated with light-induced degradation and thermal recovery of hydrogenated amorphous silicon (a-Si:H) films are reviewed, with special emphasis on the limitations of each experimental technique. When several techniques are used in concert, a fuller picture emerges. Recent experiments suggest different positions in the band-gap of the paramagnetic-associated defect states (the dangling bonds) for doped and undopedfilms; this information can be combined with conductivity, sub-bandgap optical absorption and electron spin resonance data to yield a model for the density of gap states (DOS) in a- Si:H, including how the DOS changes upon illumination and annealing.

An expanded kinetic description of formation and anneal of metastable defects (MSDs) in a-Si:H shows that (1)the apparently “stable” defects are just a statistical subset of the total density of potential MSD sites; (2)the temperature dependence of the saturation density of MSDs has three distinct regions that have not been recognized; (3)the effects of doping in increasing both the density of “stable” defects and the sensitivity to light are readily explained by an increase in the density of total potential defect sites, not just defects; (4)the defects in good undoped material are chemical in origin, not intrinsic to the disordered state of the material.