Sandrine Martin

Thin-film organic polymer phototransistors

We have studied the electrical performance of organic polymer thin-film transistors (OP-TFTs) under steady-state white-light illumination, as well as the performance of these devices as photodetectors. The off-state drain current of the OP-TFT is significantly increased due to the illumination, while a smaller relative effect is observed on the drain current in the strong-accumulation regime. The illumination effectively decreases the threshold voltage of the device and increases the apparent subthreshold swing, while the field-effect mobility of the charge carriers in the polymer channel is unchanged. We have observed full recovery of our devices after the illumination is removed at room temperature. These observations are explained in terms of the photogeneration of excitons due to the absorbed photons. The photogenerated excitons subsequently diffuse and dissociate into free charge carriers, thereby enhancing the carrier density in the channel of the device. We have found broadband responsivities of approximately 0.7 mA/W for devices biased in the strong-accumulation regime and gate-to-source voltage-independent photosensitivities of approximately 10/sup 3/ for devices in the off-state. We also determine, for the first time, the flatband voltage of these devices to be about -2.3 V.

Top-Gate Staggered Amorphous Silicon Thin-Film Transistors: Series Resistance and Nitride Thickness Effects

Top-gate staggered hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) were fabricated over large-area glass substrates using a selective phosphorus-treatment (PT) of indium-tin-oxide (ITO) source/drain electrodes. The ohmic contact between a-Si:H and ITO had a specific contact resistivity of about 0.18 Ωcm2. For a 100-µm channel length TFT, the source/drain series resistance contributes less than 5% of the total drain-to-source resistance. This contribution increases to about 25% for a 10-µm channel length TFT. Our study also indicated that the interface quality of a-Si:H/a-SiNx:H is amorphous silicon nitride (a-SiNx:H) and a-Si:H thickness independent and dependent, respectively. Effective interface state densities of about 1.5×1012 cm-2eV-1 and 3.2×1012 cm-2eV-1 were obtained for top-gate TFTs with a 1300 and 300 A thick a-Si:H films, respectively. Channel conductance activation energy of about 0.1 eV was measured for this top-gate TFT with 300 A a-Si:H.

Influence of the amorphous silicon thickness on top gate thin-film transistor electrical performances

We have analyzed the influence of the hydrogenated amorphous silicon (a-Si:H) thickness on the electrical performances of top gate thin-film transistors (TFTs). We have observed that, when the a-Si:H thickness increases, the threshold voltage and the subthreshold slope decrease. The modification of the TFT apparent field-effect mobility has also been investigated: we have shown that it first increases with the a-Si:H thickness, and then decreases for thicker a-Si:H films. This change of electrical performances is most likely associated with both the variation of a-Si:H microstructure during the film depositions and the effect of parasitic source and drain series resistances. We have demonstrated that for a given TFT geometry, it is therefore possible to define an optimum a-Si:H thickness ensuring maximum TFT electrical performances, and that this optimum thickness increases significantly with the TFT channel length.

Angular dependence of the luminance and contrast in medical monochrome liquid crystal displays

Active-matrix liquid crystal displays (AMLCDs) are light-modulating devices that generate images by differentially transmitting a nearly uniform luminous field provided by a backlight. While emissive displays exhibit a quasi-Lambertian emission with almost constant contrast at off-normal viewing, the anisotropy of the electro-optic effect that controls light transmission in AMLCDs causes a pixel luminance that varies, sometimes strongly, with viewing angle. These variations are not identical for all gray levels and can eventually cause grayscale inversions. In this paper, we measured the luminance emission of a monochrome medical AMLCD, a medical cathode-ray tube monitor, and a color desktop AMLCD, using a collimated photopic probe positioned on a manual rotation arm, and a research radiometer with automatic readout. The probe measures luminance with a small acceptance angle and provides optical shielding from emissions at other viewing directions that contaminate the readings. We obtained luminance response curves versus angle in the vertical, horizontal and at 45 degrees diagonal directions. The display systems were calibrated to reflect the DICOM Part 3.14 standard grayscale display function (GDF) when measured using the manufacturers probe and software tools. We analyzed the measurements at different viewing directions with respect to their departure from the GDF by computing the normalized contrast (deltaL/L) as a function of the DICOM just-noticeable difference index. Although cathode-ray tubes are known to be quasi-Lambertian emitters, the luminance at normal viewing is higher than the luminance observed at large angles. This decrease in luminance is however proportionally similar for all gray levels, resulting in a relatively flat contrast response for all angles. In addition to being more pronounced, the angular variation in AMLCDs does not follow the same profile at different intensities with the subsequent variation in the achieved display contrast. The changes due to off-normal viewing are substantial at large angles in the horizontal and vertical directions, and much worse in the diagonal viewing directions.

Characterization of a high-quality monochrome AM-LCD monitor for digital radiology

In this paper, we present results concerning the development of advanced characterization methods and their application to the evaluation of a high-end medical imaging monitor. The display is a 20.8-inch diagonal high-resolution (2048 x 1536 pixels, 123 dpi) monochrome active-matrix liquid-crystal display (AM-LCD) based on a-Si:H TFT active-matrix, dual- domain and in-plane switching technologies. We report the luminance characteristics of the AM-LCD, its grayscale performance, and the panel deviation from the DICOM standard grayscale function. The display luminance for different gray levels under both normal and off-axis viewing directions is described, together with the resulting contrast ratio. The viewing angle dependence of the luminance and contrast ratio is also studied in terms of display compliance to DICOMs grayscale function at off- normal viewing angles. Small-spot contrast ratio performances are investigated in relation to the display technology and especially parasitic phenomena such as electronic crosstalk. We also present the effect of target size and luminance on small-spot contrast ratio results, and we report the orientation dependence of this phenomenon. In addition, we present results on reflections, spatial resolution and display noise and texture and the AM-LCD performances are compared to typical parameters obtained for medical imaging CRTs.

Source / drain contacts in organic polymer thin film transistors

Organic polymer based thin-film transistors (OP-TFTs) look very promising for flexible organic electronics. In this paper, we describe devices based on a gate-planarized structure and using spin-coated organic polymer. We have analyzed the role of the device source and drain contacts and we present data indicating Schottky behavior of the contacts in OP-TFTs. In addition, we describe a quantitative evaluation of the source drain series resistances and extract the OP-TFT intrinsic electrical parameters.

Organic‐polymer thin‐film transistors for active‐matrix flat‐panel displays?

Organic-polymer-based thin-film transistors (OP-TFTs) look very promising for flexible, large-area, and low-cost organic electronics. In this paper, we describe devices based on spin-coated organic polymer that reproducibly exhibit field-effect mobility values around 5×10-3 cm2/V-sec. We also address fabrication, performance, and stability issues that are critical for the use of such devices in active-matrix flat-panel displays.

25.1: Luminance Probes for Contrast Measurements in Medical Displays

We report on a comparative study that examines four conic luminance probes in their ability to measure small-spot display contrast. We performed linear scans of a slit using a computer-controlled stage. We found that, although the probes were assembled according to the same design, small differences in their performance due to minor variations in materials and surfaces can be measured.

High-performance top-gate a-Si:H TFTs for AMLCDs

High-performance top-gate hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) structures have been fabricated over a large area from plasma-enhanced chemical vapor deposition (PECVD) materials. The electrical performances of the top-gate a-Si:H TFT (μFE≅0.75cm2/Vsec, VT≅3.5V, S≅0.55V/dec) are comparable to the electrical performances observed for an inverted-staggered bottom-gate a-Si:H TFT. We have shown that the TFT field-effect mobility first increases with the a-Si:H thickness, and then decreases for thicker a-Si:H films. This change of the electrical performances can be associated either with the variation of a-Si:H microstructure with film thickness during the PECVD processes or a large density of TFT back interface states; it also involves the source/drain parasitic access resistances, especially for thick a-Si:H layers.

Electrical Behavior of Organic Transistors and Circuits

The first studies of organic semiconductors date back nearly 100 years with the observation of photoconductivity in anthracene by Pochettino1 and the study of its dark conductivity by Koenigsberger and Schilling.2 While the physical properties of organic semiconductors have been the subject of intense study since these early findings, it has only been within the last 20 years that organic semiconductors for use as the active layer in organic thin film transistors (OTFTs) have been a topic of study. Despite the recent finding of scientific misconduct of a researcher at Bell Labs investigating single-crystal organic materials and OTFTs3 which has invalidated published results, showing unprecedented behavior in crystalline organic semiconductors (such as, high-temperature superconductivity, quantum Hall effect, ballistic transport, injection lasing, and exceedingly large carrier mobility at low temperatures), their remains tremendous interest in OTFTs, and organic electronics in general.