Active matrix arrays

The primary requirements of the pixel circuit are that the liquid crystal capacitor charges fully during the time that each row is addressed and holds the charge for the time between refreshing the display. Consider the example of a 25 x 25 cm panel of 500 x 500 pixels. Each pixel has an area of 500 pm and a capacitance of roughly 10 pF. The panel is refreshed at the usual video rate of 30 Hz, so that each row is addressed for 60 ps (30 ms/500). Each TFT must deliver a charge of 10 C in 60 ps, in order to charge the pixel up to 10 V, which corresponds to a current of 1.5 pA. The TFT transfer characteristics in 

The pixel circuit requirements for the scanner are similar to those of the liquid crystal display. A 300 spi array scanning an image at 3 inch s must read each line in a time of 1 ms. For a matrix of 50 gate and data lines, each pixel is addressed for 20 ps. The capacitor of the sensor is about 1 pF (note that 300 spi corresponds to a pixel of about 80 pm which is smaller than in the liquid crystal example). The required TFT current of 1 pA is easily obtained. The low oflF-current of the TFT also ensures that the charge will not leak away between being read out. 

In both the liquid crystal and scanner a-Si H arrays, the time limitation of the circuit is not the intrinsic switching speed of the TFT, but is the RC time constant of the pixel circuit, comprising the capacitance of the sensor and the resistance of the TFT. Complete 

The fabrication of these one- and two-dimensional arrays follows the device processing technology developed for crystalline silicon integrated circuits. Since the feature size of the elements is of 

The metastability phenomena influence the performance of the active matrix arrays. Defect creation in the channel causes a threshold voltage shift when a TFT is held on for an extended time and results in a slow drift of the on-current. Fortunately the rate of defect creation is low at room temperature and represents a minor problem. There is a larger effect on the characteristics of the high voltage TFTs. Resistors fabricated from n a-Si H change their resistance slowly because of defect equilibration and can affect the gain of amplifier circuits. 

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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. 

Fig. 11.14. Schematic diagram of a typical pixel circuit for an active-matrix array.

Variations of the ON and OFF current of an active matrix array of 100 OTFTs on a PET substrate. (Reprinted with permission from Ref. [72]). 

Chabinye, M.L., and A. Salleo. 2004. Materials requirements and fabrication of active matrix arrays of organic thin-film transistors for displays. Chem Mater 16 4509- 521. 

The widespread use of the twisted nematic device as the display mode above an active matrix array has highlighted its shortcomings. Such displays, which command a significant price premium in the marketplace, are subject to increasing scrutiny over the quality of the image. The narrow and non-uniform viewing cone of the normal twisted nematic display is increasingly viewed as an unacceptable aspect of performance, and various approaches have been described that seek to provide a better solution [34]. 

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.

Active matrix addressing can also be used in printer heads [628] and linear sensor arrays [629]. a-Si H TFTs are used to address a linear array of output or input transducers, respectively. In the linear sensor array, u-Si H photodiodes are used. 

Thin film transistors (TFTs) based systems are much better for colour displays since they give better viewing angles, response times and resolution than STNs. Those systems in which an array of amorphous siUcon transistors is vapour-deposited onto one side of the substrate, each transistor being positioned below each pixel, are known as active matrix TFT (AMTFT) displays. 

The response time of the pressure-sensitive rubber is typically of the order of hundreds of milliseconds, and the individual sensor does not respond to the higher frequency. It should be noted, however, that the readout time of entire 16x16 sensor cell array is not the response time of individual transistors multiplied by 16 x 16, since an active matrix scheme is used. The scan speed of the entire sensor cell array is limited by the performance of individual transistors and is independent of the frequency response of the pressure sensors. The total time to access 16 x 16 transistors is about 1 s, comparable to the response time of the pressure sensor. Thus,... 

Automated droplet-based manipulation methods described in this article include electrowetting, dielectrophoretic, thermocapillary, surface acoustic wave (SAW), and pressure-driven channel-based droplet systems. Fabrication of arrays of elements to control these droplet manipulation methods typically involves the use of photolithography. The methods of addressing of the elements have become increasingly sophisticated with several efforts to utilize passive- and active-matrix control strategies. Trends and issues associated with each method are described. 

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