Fault tolerant photodiode and photogate active pixel sensors

Abstract

As digital imaging arrays increase in size and resolution, defect correction could lower costs and improve yields. A fault tolerant active pixel sensor (APS) has been designed that will operate in the presence of a single point defect. The photosensitive area of the pixel is split in half and both halves operate in parallel. The output of each half is combined using a common row select transistor. The common pixel defects are optically stuck high (bright pixel) and optically stuck low (dark pixel). Simulations showed that a non-defective pixel would function normally and if one pixel half was defective, the other half would operate normally with half the sensitivity of a non-defective pixel. Fault tolerant photodiode and photogate APS’ were designed and fabricated in CMOS 0.18-micron technology. Half stuck high and half stuck low defects were induced on the fault tolerant pixels and the sensitivity ratio of non-defective to half defective pixels was measured (ideally 2). The experimental ratios ranged from 1.89 (stuck high) and 2.02 (stuck low) for the photodiode APS to 1.73 (stuck low) and 1.77 (stuck high) for the photogate APS. Non-defective fault tolerant pixels have also shown a 2x increase in sensitivity over normal APS’.