Pixels active matrix displays

Fig. 10.16. Schematic illustration of the layout of a unit cell in an active matrix display circuit (left frame). It consists of a transistor connected to row and column electrodes and a pixel electrode pad. This circuit drives an overlying layer of electronic ink. The frame on the right shows the current-voltage characteristics of a typical transistor in a large...

Active-matrix displays differ from the aforementioned displays in that they have a switch incorporated into each pixel (Tsukada 2000). This removes the limitations encountered in passive matrix displays but requires more sophisticated processing equipment to be used. The dominant pixel switch technology is the amorphous silicon thin-film transistor (TFT) on glass (Tsukada 2000), although other technology... 

Fig. 14.1. The first reported active-matrix display with an organic semiconductor. The display contains 64 by 64 pixels and is driven by 4096 polymer TFT, with solution processed PTV as the semiconductor. An image containing 256 gray levels is shown. The display is refreshed at 50 Hz.

Amorphous Si H has an effective-field drift mobility about 0.2 cm2 V-1 sec-1. To gain some perspective on how this material might fit into active matrix display applications, the requirements on a TFT designed to drive a 1-mm2 GH LCD pixel will be discussed. In the linear regime, the transistor can be modeled as a (gate-voltage-controlled) resistor, and the minimum ON resistance Rmi is approximately given by the estimate... 

Francis Gamier [3, and references given therein] fabricated the first transistor using molecules of sexithiophene. The transistor could be twisted, bent or rolled without degrading its characteristics. Computers fabricated using these devices will work at less than one thousandth of the speed of those made with amorphous Si transistors. They would be useful in video displays and liquid-crystal displays. In active matrix displays, each pixel is controlled individually by a thin film transistor. A 50 cm full color display contains more than two million pixels. Organic transistors, considerably cheaper than the amorphous Si transistors being used at present, will be a boon to the manufacturers. 

Fig. 10.8 confirm that this is well within the capability of the a-Si H devices. To prevent charge leaking from the panel before the display is refreshed, the RC time constant of the TFT and pixel capacitor must be larger than 30 ms, which translates to a requirement that the off-current of the TFT be less than 10" A. Again this is comfortably within the TFT operating characteristics. Thus active matrix displays with at least 1000 lines are possible with present a-Si H TFT technology. The low charge requirements of the liquid crystal allows the use of the a-Si H devices and the low off-current, which is inherent to the a-Si H TFT, is an essential feature. 

The ability to process the luminescent semiconducting polymer from solution enables the introduction of simple and potentially inexpensive methods for manufacturing pixilated displays. There are two general classes of pixilated displays that differ in the manner in which the individual pixels are switched on and off passive matrix displays and active matrix displays. 

Active matrix displays have a thin-film transistor (TFT) switching circuit embedded in the area of each individual pixel. Although the TFT backplanes needed for polymer emissive displays are similar to those developed for liquid crystal displays (LCDs), the TFT circuits must be capable of switching much higher currents than are required for LCDs. For active matrix displays, the luminescent semiconducting polymer and cathode, etc. are deposited directly onto the premanufactured TFT backplane (the anode for the LED pixel is built onto the TFT circuit). In an active matrix display, the pixels are held at constant brightness by the TFT circuit and the image is refreshed at video rates (e. g. 60 Hz). 

To illustrate the current capabilities of OFET technology in this application area. Figure 2.3.2 shows an optical micrograph of an A5 active matrix display demonstrator on a flexible polyethyleneterephtalate (PET) substrate made by Plastic Logic. The display was fabricated by laminating the OFET backplane with an E Ink Imaging Film. The display has a resolution of 100 pixels per inch (ppi) and displays four levels of gray. It contains 480,000 solution-processed OFETs (600 x 800 rows and columns). No substrate encapsulation is needed. The display exhibits excellent... 

Huitema, E. et ah, Polymer-based transistors used as pixel switches in active-matrix displays, J. Soc. Inf. Display, 10, 195, 2002. 

Andersson, P., D. Nilsson, P.-O. Svensson, M. Chen, A. Malmstrom, T. Remonen, T. Kugler, and M. Berggren. 2002. Active matrix displays based on all-organic electrochemical smart pixels printed on paper. Adv Mater 14 (20) 1460-1464. 

The organic field effect transistor (OFET) acts essentially as an electronic valve by modulating the semiconductor channel conductance via the gate field. This device is essential in all electronic applications, including integrated circuits for memories and sensors and also to drive individual pixels in active matrix displays. Probably one of the most exciting applications of organic electronic circuits is in the supply chain area, where radiofrequency-powered elements (e.g. RFID tag) may replace ID barcodes for identification and be applicable as a backplane drive for displays. 

The essential principle of an active matrix display is that each pixel has associated with it a semiconductor device that is used to control the operation of that pixel. It is this rectangular array of semiconductor devices (the active matrix) that is addressed by the drive circuitry. The devices, which are fabricated by thin-film techniques on the inner surface of a substrate (usually glass) forming one wall of the LCD cell, may be either two-terminal devices (Fig. 6) or three terminal devices (Fig. 7). Various two-terminal devices have been proposed ZnO varistors, MIM devices, and several structures involving one or more a-Si diodes. Much of the research effort, however, has concentrated on the three-terminal devices, namely thin-film, insulated-gate, field-effect transistors. The subject of thin-film transistors (TFTs) is considered elsewhere in this volume suffice it to say that of the various materials that have been suggested for the semiconductor, only a-Si and poly-Si appear to have serious prospects of commercial exploitation. 

Active matrix displays using thin film transistors (TFTs) as electrical switches to control the transmission state of liquid crystal pixels offer excellent image quality and are commonly employed for direct-view displays [5,6]. Figure 10.7 shows the device structure of a... 

Figure 13.12. Thin film transistor active matrix display pixel.

Increasing the display size I requires larger active matrices with a higher number of pixels, which results in an exponential growth of a number of defects, such as broken dots and lines, disconnections, shortages, etc. At the same time the operation characteristics of each nonlinear element must be more stable and reliable. All this leads to a sharp increase in the price of the large area active matrix displays. 

Figure 5.27 Schematic of an element of a thin-film transistor active matrix display. Each liquid crystal pixel is addressed directly by a transistor element in a matrix...

Figure 88. The first version of Displaytech s miniature display (ChronoColor), which went into production in early 1997, in comparison with a conventional active matrix display. This microdisplay utilizes sequential color to produce full color on each pixel, resulting in a brighter, crisper image than that of an AMLCD, which uses a triad of red, green, and blue pixels to form a color. In the insets, the pixels of the ChronoColor display and AMLCD are magnified to show the difference in fill factors. The color is what makes the image that you see, and it occupies 75% of the display on the left, but only 35% of the corresponding active matrix display. (Courtesy of Displaytech, Inc.)...

OLED flat and flexible panel display technology is advancing rapidly and full color displays are currently being used in cell phones. Sony recently annoimced a 2.5 in. flexible screen TV that is only 0.3 mm thick and now has an 11 in. OLED TV in production. Samsimg also annoimced a prototype 17 in. high definition (1600 x 1200 pixels) active matrix OLED display panel. 

Figure 13-18. Diagram of a simple pixel circuit for active matrix addressing of an OLED array. For a color display of N lows and M columns, this circuit must be reproduced Ny.My.7t limes.

The fifth of the color methods places the three emitting structures in a stack one on top of the other, rather than side by side ]20l ]. Clearly there is a requirement here that the two electrodes in the middle of the structure must be transparent. The advantages are that the display can be made much brighter with up to three times the luminance from each pixel, and the requirements for high resolution patterning are relaxed by a factor of three. The disadvantages are that three times as many layers must be coated (without defects) over the area of the display and electrical driving circuitry must make contact with four sets of elec- trades. It will be extremely difficult to incorporate a stacked OLED into a active matrix array. 

Many LCDs are based on active-matrix addressing, in which an active device circuit containing one or more TFTs is connected to each pixel. The TFT circuit at each pixel effectively acts as an individual electrical switch that provides the means to store display information on a storage capacitor for the entire frame time, such that the pixel can remain emitting during this entire time rather than for a small fraction of time, as is the case in passive addressing. 

Y He, R Hattori, and J Kanicki, Four-thin film transistor pixel electrode circuits for active-matrix organic light-emitting displays, Jpn. J. Appl. Phys., 40 (Part 1) 1199—1208, 2001. 

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