Noise analysis of fault tolerant active pixel sensors with and without defects

Abstract

As the sizes of imaging arrays become larger both in pixel count and area, the possibility of pixel defects increases during manufacturing and packaging, and over the lifetime of the sensor. To correct for these possible pixel defects, a Fault Tolerant Active Pixel Sensor (FTAPS) with redundancy at the pixel level has been designed and fabricated with only a small cost in area. The noise of the standard Active Pixel Sensor (APS) and FTAPS, under normal operating conditions as well as under the presence of optically stuck high and low faults, is analyzed and compared. The analysis shows that under typical illumination conditions the total noise of both the standard APS and FTAPS is dominated by the photocurrent shot noise. In the worst case (no illumination) the total mean squared noise of the FTAPS is only 15.5% larger than for the standard APS, while under typical illumination conditions the FTAPS noise increases by less than 0.1%. In the presence of half stuck faults the noise of the FTAPS compared to the standard APS stays the same as for the FTAPS without defects. However, simulation and experimental results have shown that the FTAPS sensitivity is greater than two times that of the standard APS leading to an increased SNR by more than twice for the FTAPS with no defects. Moreover, the SNR of a faulty standard APS is zero whereas the SNR of the FTAPS is reduced by less than half.