| Active matrix organic light-emitting-diode (AMOLED) displays using the amorphous silicon (a-Si:H) thin film transistor (TFT) as the active element offer a promising avenue for high performance displays, owing to the possibility of low temperature, low cost fabrication on flexible substrates along with low power consumption. The AMOLED display consists of an array of pixels deposited on a glass or plastic substrate, with each pixel having the OLED as the light emitting material. Since the OLED is a current controlled device, each pixel contains a driver TFT acting as a current source in order to control the pixel brightness.; While the a-Si:H TFT offers excellent matching of device properties over large areas, it suffers from a gate bias dependent threshold voltage shift (VT shift) in time. The V T shift in the TFT used in the AMOLED display causes the current sourced by the TFT, and hence the pixel brightness, to vary with time. In order to counter this problem, many compensation circuits have been designed. The purpose of the compensation circuit is to estimate the V T shift in driver TFT, and apply a correction so as to maintain a constant brightness.; However, all the compensation circuits designed to date suffer from one or more of the following problems. The first, and most common issue is that, the number of TFTs used in each pixel is more than two. This, combined with the associated passive components and signal lines, leads to increased layout area, which in turn lowers the spatial resolution and reliability of the display. Secondly, some of the pixel circuit suffer from settling time problems where the time constants associated with the error estimation and programming of the drive TFT is large, therefore lowering the temporal resolution of the display. Finally, some compensation circuits require custom made CMOS circuits thereby offseting any economical advantages provided by the low cost TFT fabrication.; In this thesis, we try to focus on building AMOLED display systems which avoid the above issues. In order to achieve this, we first study the dynamics of the threshold voltage shift in the TFT. We then develop circuit techniques for the a-Si:H TFT and design building blocks that have time-invariant transfer functions inspite of the TFT being a time varying system. Next we develop a reliable algorithm to compensate for the variation in the OLED pixel brightness, which we implement via a global compensation circuit housed outside the display array, using the TFT building blocks. Finally, we use a two TFT voltage programmed pixel circuit as our pixel element in order to achieve a high speed, high resolution, low cost and yet robust AMOLED display system. |
