Pixel architectures for digital imaging using amorphous silicon technology

This work extends amorphous silicon (a-Si) TFT technology from traditional switching applications to on-pixel small signal amplification for diagnostic medical digital imaging applications. The developed a-Si pixel amplifier offers improved signal-to-noise ratios, lower cost, and less off-panel circuit complexity compared to its (traditional) a-Si switch counterpart. The pixel amplifier advances the state-of-the-art by offering a large area real-time imaging solution for low-noise fluoroscopic medical imaging that is not viable with current a-Si switch based pixels. More significantly, the pixel (because of its circuit gain) offers potentially reduced patient x-ray doses for other medical imaging modalities, hence improving the safety standards associated with current x-ray imaging practices.; Investigations of additional noise due to the a-Si pixel amplifier indicate that this noise is minimized for small pixel capacitance implying the need for low capacitance detectors such as amorphous selenium (a-Se) photoconductors. However, in contrast with CMOS amplified pixels, the flicker noise added by the a-Si readout circuit is comparable to the traditionally prohibitive reset noise. Measurements on in-house TFTs and pixel readout circuits indicate the feasibility of amorphous silicon APS pixels for low noise, diagnostic medical imaging applications such as digital fluoroscopy.; Appropriate bipolar bias voltages in the TFT ON and OFF states minimize the characteristic threshold voltage shift of the a-Si TFTs in the amplified pixel. Due to intrinsic feedback, the developed amplified pixel readout circuit has a compensatory effect on the pixel's gain. This gain compensation, coupled with small (typically <0.1%) duty cycles for diagnostic medical imaging applications leads to a <5% variation in circuit gain over a 10,000 hour array lifetime.; In order to obtain high fill factor, the three TFTs in the a-Si amplified pixel are assumed embedded under the sensor unlike the conventional co-planar layout of sensor and TFTs. The major challenge with vertically stacked architectures is an increase in TFT leakage current due to the presence of an overlying sensor back electrode.{09}An in-house fabricated vertically stacked pixel based on a double gate TFT illustrates how a low leakage current can be obtained even in the presence of a bias on an overlying sensor back electrode.; Lastly, the integration of an in-house amplified pixel a-Si TFT array with a continuous layer a-Se photoconductor X-ray detector is detailed including its design, fabrication and custom test setup. Results from double sampled array operation including x-ray sensitivity, gain and noise are promising and highlight the feasibility of a-Si amplified pixel arrays coupled with a-Se x-ray detectors for diagnostic medical x-ray imaging applications.; The research presented in this thesis indicates that proper circuit and pixel architecture design can overcome a-Si material shortcomings related to area, noise and metastability. The demonstrated gain, noise, and stability results indicate that a-Si on-pixel amplifiers, coupled with a low capacitance, well established x-ray detection technology such as a-Se, can meet even the stringent requirements of low noise, digital fluoroscopy (less than 1000 input referred noise electrons) without resorting to newer high x-ray absorption materials such as PbI. The results presented provide the impetus to expedite development of large area a-Si APS arrays for low noise, real-time medical imaging.