Sensor fill factor is one of the key pixel design requirements for high performance imaging arrays. In our conventional imaging pixel architecture with a TFT and a photodiode deposited in the same plane, the maximum area that the photodiode can occupy is limited by the size of the TFT and the surrounding metal lines. A full fill factor array design was previously proposed using a continuous sensor layer1 . Despite the benefits of 100% fill factor, when applied to large-area applications, this array design suffers from high parasitic line capacitances and, thus, high line noise. We have designed and fabricated an alternative pixel structure in which the photodiode is deposited and patterned over the TFT, but does not overlap with the lines underneath. Separating the diode from the TFT plane allows extra space for an additional TFT which can be used for pixel reset and clipping excessive charge in the photodiode developed under high illumination. This reduces memory effect by 250%. The yield and the reliability are expected to improve as well since the TFTs and lines are buried underneath the diode. With the increased fill factor, we collect 50% more electrons per pixel, thereby improving the signal to noise ratio. The maximum signal to noise ratio is achieved when the increased signal and the undesirable parasitic capacitance on the data line are best optimized. Linearity, sensitivity, leakage, and MTF characteristics of a prototype X-ray imager based on this architecture are presented.
In this paper we examine origins of electronic noise in a 127-micron pixel thin film transistor (TFT)/photodiode image sensor array. The imaging array is a 1536 data line by 1920 gate line amorphous Silicon (a-Si) sensor array connected to low noise charge amplifiers and 14-bit electronics. We measure the contributions of A/D converters, charge amplifiers, data-line resistance and capacitance, and pixel switching to the overall electronic noise of 1040 e – per pixel. Noise power spectra are evaluated for each dark offset image. When suitably filtered linear power supplies are used for supplying gate and bias voltages to the array, the noise power spectra are identical in the x and y directions, i.e., row correlated noise is negligible. Noise measurements taken under different band-pass conditions correlate well with data-line resistance generated Johnson noise. No variations in noise along the data lines and a slight reduction in noise along the gate lines suggest only small transmission line effects. Pixel noise was measured as a function of frame time, yielding a component of noise due to TFT switching and a frame time dependent component modeled as pixel leakage current shot noise. It was found that this shot noise current was three times higher than leakage currents derived from dark offset difference images.