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High-performance emissive display device for computers, information appliances, and entertainment systemsHigh-performance emissive display device for computers, information appliances, and entertainment systems description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20050285822, High-performance emissive display device for computers, information appliances, and entertainment systems. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001] This application claims the benefit of priority under 35 U.S.C. 119 and/or 35 U.S.C. 120 to U.S. Provisional Patent Application Ser. No. 60/583,744 (Atty. Docket. No. 186051/US [474125-20]) filed Jun. 29, 2004 naming as inventors Damoder Reddy and W. Edward Naugler, Jr., and entitled High-Impedance to Low-Impedance Conversion System for Active Matrix Emission Feedback Stabilized Flat Panel Display, which application is incorporated by reference in its entirety. [0002] This application is also related to the following applications, each of which is hereby incorporated by reference: U.S. Utility application Ser. No. ______, (Atty. Docket. No. 186051/US/4 [474125-21]) filed Dec. 17, 2004 and entitled System And Method For A Long-Life Luminance Feedback Stabilized Display Panel; U.S. Utility application Ser. No. ______, (Atty. Docket. No. 186051/US/2 [474125-22]) filed Dec. 17, 2004 and entitled Feedback Control System and Method for Operating a High-Performance Stabilized Active-Matrix Emissive Display; U.S. Utility application Ser. No. ______, (Atty. Docket. No. 186051/US/3 [474125-23]) filed Dec. 17, 2004 and entitled Active-Matrix Display And Pixel Structure For Feedback Stabilized Flat Panel Display; U.S. Utility application Ser. No. ______, (Atty. Docket. No. 186051/US/5 [474125-25]) filed Dec. 17, 2004 and entitled Method For Operating And Individually Controlling The Luminance Of Each Pixel In An Emissive Active-Matrix Display Device; U.S. Utility application Ser. No. ______, (Atty. Docket. No. 186051/US/6 [474125-26]) filed Dec. 17, 2004 and entitled Device And Method For Operating A Self-Calibrating Emissive Pixel, and U.S. Utility application Ser. No. ______, (Atty. Docket. No. 186051/US/7 [474125-27]) filed Dec. 17, 2004 and entitled High-Performance-Emissive Display Device For Computers, Information Appliances, And Entertainment Systems; each of which applications is hereby incorporated by reference. FIELD OF THE INVENTION [0003] This application pertains generally to emissive flat panel displays and more particularly to systems, devices and methods for making, calibrating, and operating emissive pixel flat panel displays to provide uniform light emission level and color over the surface of the display initially and throughout its operational life and to extend the operational life of such displays. BACKGROUND [0004] Active matrix (AM) emissive displays and active matrix organic light emitting diode (AMOLED) displays in particular rely on current levels in the light emitting diode to produce luminance levels (light emission level) in a matrix of pixels (picture elements). Each pixel is a separate light emitting diode that is directly addressed and wherein each pixel has a sample and hold circuit so that a voltage can be applied to the Organic Light Emitting Diode (OLED) display driver continuously over the duration of the frame. [0005] The function of a flat panel display is to produce an image in various shades of light and dark in correspondence to voltage levels representing the original image, or an image created by computer software. These light and dark shades may form or generate colors when they are rendered as different pixel types such as in red, blue, and green through the use of different colored emissive pixels or diodes or through the use of same colored or white pixels and filters. Sometimes the set of three pixels used together to render a color by additive combination of their respective photon flux are referred to as subpixels, but in the description to follow, little distinction is made between pixels and subpixels as the subpixels are pixels in their own right and sets of pixels that are controlled as a set are merely cooperative sets of subpixels. Operation of sets of pixels or emitters to generate color are known in the art and not described in greater detail. The translation of the voltage image data into current generated OLED photon emission (flux) levels presents several complex issues involving the manufacture of the display and the aging of the display during operation and use by a user or consumer in the field. [0006] In the case of a typical conventional OLED display, an image or data voltage is placed on the gate of a power transistor (current source) in the display pixel, which feeds and controls the amount or magnitude of current to the OLED pixel. The higher the gate voltage is, the higher will be the current and therefore the brighter or more emissive will be the pixel. Typically voltages (the signal data) supplied to thin-film semiconductor transistors (TFTs) having source, drain, and gate terminals are used to control the current to the pixel emitter elements to render an appropriate gray level or pixel image luminance. [0007] The circuits, methods of control, and even materials heretofore used in conventional implementations have significant limitations so that OLED display panel performance has suffered and has limited the application of such OLED technology for larger high-performance displays at consumer acceptable price. [0008] A primary problem in such systems and devices is that it is conventionally extremely difficult if not impossible to produce uniform current from pixel-to-pixel in a display using voltage image data applied to TFTs in this manner. This problem becomes particularly acute as the displays become physically larger, have larger numbers of pixels, are driven to high current and luminance levels, and/or are operated either continuously or intermittently for longer periods of time (they age). This problem arises at least in part because the current delivered by a TFT at a particular gate voltage depends on many parameters, such as for example the TFT threshold voltage, the effective electron mobility, and current gain of the TFT device (which may vary from TFT device to TFT device as a result of manufacturing variations, environmental exposure during operation, and/or operational history. These three parameters (threshold voltage, effective electron mobility, and current gain) may in turn depend, for example, on inter-grain and intra-grain trap densities, semiconductor thickness, and semiconductor-to-gate dielectric trap densities. Other factors include: gate dielectric thickness, dielectric constant of the insulators, the TFT geometry, electron/hole mobilities, and other factors alone and in combination. [0009] Among the problems at issue are how voltages (e.g. TFT voltages) to be applied are determined and how that voltage is placed on the power TFT to give the right current level to produce the correct gray level. Some studies have suggested a particular way or ways to use a particular luminance of a pixel to correct the voltage supplied to the pixel power TFT (See for example, U.S. Pat. No. 6,518,962 B2 by Kimura and assigned Seiko-Epson; U.S. Pat. Nos. 6,542,138B1 and 6,489,631B2 assigned to Philips, and the paper by Eko T. Lisuwandi at MIT (See "Feedback Circuit for Organic LED Active-Matrix Display Drivers, by Eko T. Lisuwandi submitted to the Department of Electrical and Computer Science in Partial Fulfillment of the Requirements for Degrees of Master of Engineering in Electrical Engineering and Computer Science at the Massachusetts Institute of Technology, May 10, 2002). However, these conventional attempts to improve OLED (or indeed other active emission display technologies) have not been entirely effective and are in one way or another flawed. [0010] For example, U.S. Pat. No. 6,542,138 B1 (assigned to Philips) describes a method that at most attempts to make pixels tend to be uniform to some extent over a frame duration but does not describe or suggest that exact emission levels corresponding to a series of gray levels can be controlled. This invention described in this patent for example, uses a light sensitive discharge device across the signal hold capacitor that maintains the gate voltage on the OLED current driving TFT during the frame time. The photon emission from the OLED causes the light sensitive discharge device to discharge the voltage on the holding capacitor thus turning off the current driving TFT and thus extinguishing the OLED. The rate of extinguishment is dependent on the level of photon emission; therefore if the pixel over-produces photon emission the OLED will be extinguished faster than were the pixel to under-produce photon emission. As a further refinement of such a system, the photosensitive discharge device is a photo-transistor, the gate of which is controlled by the current passing through the OLED. The circuit is designed so that at high current through the OLED the photo-transistor is in the off condition because the voltage to the gate of the photo-transistor is close to ground due to the high OLED current, but the photo-transistor while in the off condition acts like a reverse biased photo-diode and the charge on the holding capacitor is slowly leaked to ground, causing the current through the OLED to be reduced as the current is reduced. Due to the declining voltage on the storage capacitor the voltage rises on the gate of the photo-transistor. When the current decrease to a certain point the threshold voltage of the photo-transistor is exceeded causing the photo-transistor to turn on and dump the remaining charge in the storage capacitor and thus shut off the OLED. The rapidity, and thus, the perceived luminance of the OLED is determined by the luminance level of the OLED. The higher the luminance of the OLED the faster is the OLED shut off. [0011] There are several objections to this approach. Firstly, the turning on of the photo-transistor to shut off the OLED depends on the threshold voltage of the photo-transistor. One of the problems that this approach is supposed to correct is the variable threshold voltages of the TFTs used in the pixel circuitry. This means that the time when the OLED is shut off will vary from pixel to pixel and thus actually contribute to the nonuniformity between different pixels of the display. Secondly, at low emission values the voltage applied to the gate of the photo-transistor will be close to the threshold voltage at the beginning of the frame time. Any variations in threshold voltage are therefore greatly magnified and the uncertainty of the actual luminance values is not well controlled at all. Thirdly, the actual brightness perceived by the viewer depends on the total photon emission during the frame. The total photon emission during the frame depends at least in part on the initial value of the data voltage supplied to the storage capacitor, the rate of discharge of the storage capacitor during the off time of the photo-transistor (which is dependent on the emission level of the OLED caused by the initial voltage), the threshold voltage of the current controlling TFT whose gate is controlled by the voltage stored on the storage capacitor, current gain of the current controlling TFT, the effective electron mobility of the current controlling TFT, the age point of the OLED materials, the color spectrum of the OLED materials and the threshold voltage of the photo-transistor. All these mentioned controlling parameters are not well controlled in the manufacturing process and therefore the pixel uniformity is not well controlled using the structures and methods of described or inferred by the U.S. Pat. No. 6,542,138 B1 (Philips) reference. [0012] U.S. Pat. No. 6,518,962 B2 by Kimura (assigned to Seiko-Epson) describes circuits in which current levels are obtained by certain pixel associated sensors in the short address time allocated for making a measurement. These are essentially instantaneous measurements and the measurement time is too short to give a practically acceptable signal-to-noise ratio so that useful information for determining the voltage or current to be supplied to the TFT (or OLED pixel) can be extracted from the measurement. The signal extracted is expected to be on the order of a few nano-volts (10.sup.-9 volts) and the noise is expected to be on the order of several volts due to the long conductor line terminated essentially by an open circuit for a signal-to-noise ration (SNR) of less than about 0.1 percent Furthermore, it is also expected that different noise characteristics may arise for different regions of a display owing to the different localized electromagnetic fields and to the same pixels at different times. [0013] Another limitation of Kimura et al (U.S. Pat. No. 6,518,962 B2) is that the system and method as described appears to apply a predetermined signal to the signal data line and it then alters this signal by the voltage control unit to make the light level come close to the reference value. The predetermined data signal therefore appears to cause a luminance that is an incorrect luminance because it varies from the reference and is subsequently altered by the voltage-adjusting unit to produce luminance that is only "close" to the reference. Kimura therefore does not appear to actually match the reference or any other target luminance. [0014] The work of Lisuwandi et al., which is generically and conceptually similar to U.S. Pat. No. 6,518,962 B2 has too long a feedback settling time (greater than 150 ms) and thus, is not practical, especially for displays that have dynamic content that changes from frame to frame for normal computer screen, television, and similar applications. [0015] Conventional systems and methods that have attempted to control pixel luminance, have by-and-large attempted to measure instantaneous light or luminance levels that have been too small and too noisy to accurately and precisely provide such control. They have therefore been ineffective and their limitations will be even more severe as the size and performance expectations of OLED displays increases. [0016] These performance problems may likely be even more severe when amorphous silicon (a-Si) is used for the display electronics. Amorphous silicon is the semiconductor used by the LCD industry and has billions of dollars invested in the infrastructure. It is, therefore, desirable for the major display manufacturers to use amorphous silicon. Early development of OLED active-matrix displays has employed the use of poly-silicon due to its higher speed and better stability. There is very little investment in poly-silicon infrastructure and the costs are high as opposed to amorphous silicon. [0017] Recall that there are three forms of silicon conventionally used in electrical integrated circuits. Crystalline silicon used in monolithic integrated circuits (ICs). This type of silicon has no grain boundaries since the material is a solid crystal. This type of silicon (x-Si) has only one area for electrical charge to accumulate, and that area is at the interface between the gate dielectric and the silicon surface contacted by the dielectric. The area of this interface is just the width and length of the gate dimensions. [0018] Poly-silicon (p-Si) is made up of course grains of silicon having more or less intimate contact with each other. In order for electrons to go from grain to grain and thus, travel through a p-Si channel in a field effect transistor (FET), a certain amount of energy must be added. Also, the interface between grains can collect stray charges (both positive (holes) and negative (electrons) stray charges) just like the interface between the dielectric and the silicon crystal in the x-Si material, but now the area has greatly expanded. The intergranular area in the p-Si is inversely proportional to the grain size. Therefore, the smaller the grain size, the greater the interfacing area will be and the greater the chance for stray charges to build up. [0019] In the case of amorphous silicon (a-Si) the grain boundary area is magnitudes greater than for p-Si. Trapped charge is normally the dominant characteristic that determines electron mobility and threshold voltage for a-Si devices and therefore any changes in the charge density at the inter-grain boundaries causes fluctuation in the electron mobility and threshold voltage with much greater effect in the amorphous silicon (a-Si) as compared to the poly-Silicon (p-Si) or crystalline silicon (x-Si). [0020] As display size increases, there is great desirability to use amorphous silicon rather than poly-silicon or crystalline silicon. However, due to the differences and fluctuations in electron and hole mobility characteristics, stray electrical charge accumulation characteristics, and threshold voltage characteristics, it is increasingly difficult to maintain a desired and uniform display luminance characteristics over a large display surface at any single moment in time and as the display device is used with amorphous silicon. [0021] Various attempts have been made to overcome the uniformity problem in emissive displays, including some that have involved circuit-based, some of which are still in use today. These attempts have not been entirely successful and do not meet the needs and application requirements of the current and next generation of emissive display applications, particularly OLED display applications. Continue reading about High-performance emissive display device for computers, information appliances, and entertainment systems... 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