Abstract
This paper presents a novel pixel design for activematrix organic light emitting diode (AM-OLED) displays usingnanocrystalline silicon thin-film transistors (TFTs). The proposedpixel design can effectively reduce the variation of its storeddisplay data caused by the leakage current of nanocrystallinesilicon TFTs, which can in turn increase the contrast resolutionof AM-OLED displays. With a proper setting of its capacitors,the proposed pixel design can achieve a 5.55 reduction on itsdisplay-data variation while requiring only a 1.15 write timewhen compared to the typical pixel design. The aperture ratioresulting from the layout of the proposed pixel design can alsobe maintained above 40%, which satisfies the specification ofmost AM-OLED displays. A series of simulations as well as measurementresults are provided to validate the effectiveness of theproposed pixel design.Index Terms—active matrix organic light emitting diode(AM-OLED), coupling effect, microcrystalline thin-film transistor(TFT), nanocrystalline TFT.
I. INTRODUCTION
DURING the past few years, major display manufacturershave put a lot research efforts into the area of active matrixorganic light emitting diode (AM-OLED) displays, whichuse self-light-emitting devices, require no backlight elements,and in turn are lighter, thinner, and less power-consumed comparedto the current display mainstream, active matrix liquidcrystal display (AM-LCD displays) [1], [2]. AM-OLED displaycan also provide faster response time, wider viewing angle, andbetter front-of-screen (FOS) quality, and hence is considered asthe best candidate for next-generation displays [1], [2]. In orderto build a robust backplane which can reliably control OLED’scurrent and thus its lightness, a stable and high-quality thin-filmtransistor (TFT) technology is required. However, the availableTFT technologies in current industry are not yet cost-effectiveor stable enough, which has been the biggest barrier for developingquality AM-OLED displays today [1], [3].Table I compares the three major prospects of the TFT technologyused for building the backplane of AM-OLED displays,which are: 1) amorphous silicon ( -Si) TFT; 2) polycrystallinesilicon (poly-Si) TFT; and 3) nanocrystalline silicon (nc-Si)TFT (or microcrystalline silicon TFT). The first prospect -Si TFT, which is currently used for building the backplane ofAM-LCD displays, has the advantages of low leakage, lowprocess temperature, better electric uniformity, and easy deposition,resulting a relatively low manufacturing cost [1], [2], [4].However, -Si TFT suffers from its low mobility, which mayresult in a low aperture ratio and in turn low illumination efficiency.-Si TFT also suffers from its large threshold-voltageshift over time, which significantly reduces the reliability ofAM-OLED displays since a large current needs to pass throughthe TFTs which drive the OLEDs. Such an OLED-driving TFTis not required in AM-LCD displays, whose light source comesfrom a stable backlight element.In contrast with -Si TFT, the second prospect poly-Si TFT,which is also a mature technology in current industry, can providestable threshold voltage over time and high mobility fordriving OLED due to its larger silicon grain size [1], [2]. However,by using the technique of excimer laser annealing for crystallization,poly-Si TFT has the disadvantages of low electricuniformity, which limits the application of poly-Si TFT only onsmall-size panels [1], [2]. Also, the overall manufacturing costof poly-Si TFT is high because of its high process complexity,the required laser equipments, and its high process temperature.The third prospect nc-Si TFT, whose silicon grain size isbetween the other two TFT technologies, can overcome thedrawbacks of low mobility and poor reliability when comparedto -Si TFT, and at the same time overcome the drawbacksof high manufacturing cost and poor electric uniformity whencompared to poly-Si TFT. As in [1], [4]–[6], nc-Si TFTcan be manufactured by using the conventional 13.56 MHzplasma-enhanced chemical vapor deposition (PECVD), whosemanufacturing infrastructures are already well established incurrent industry for -Si TFT technologies. Furthermore, if thenc-Si TFT is constructed by using the bottom-gate structure[1], [3] instead of the top-gate structure [5], [6], the processflow can match the one used for the current manufacturingof AM-LCD displays. In this condition, the technology ofnc-Si TFT can reuse the current production lines of AM-LCDdisplays and, thus, can be brought to mass production soonerand more economically.


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